What seating depth is actually doing...actual node or just pass/fail?

[morganlamprecht]

Not sure where to begin...but jump testing is something we all see a lot, and it typically the discussion goes back to bullet timing/barrel vibrations/etc...along those lines.

one thing that has always hung in my mind is "forgiving" bullets...how are bullets "forgiving" if the whole idea of load dev, is based upon timing up when the bullet exits the muzzle with some movement/vibration pattern...do forgiving bullets not make the barrel move/vibrate? (obviously not the case) do forgiving bullets not deflect the same when exiting a moving barrel? self correct? i dont have an answer, just questions...

is it possible that instead of barrel timing/muzzle exit, all seating depth needs to be is "good enough to hand off the bullet straight"? ie; Bullet A, gets a clean/straight hand off due to bullet shape and chamber spec relationship from .005" to .150" off...basically it gets a good hand off anywhere having the bearing surface out of the neck because that "forgiving" design allows it so...while maybe Bullet B, has less agreeable relationship with chamber design, and it doesnt get a clean/straight hand off from .005" to .020" off...gets straight from .025 to .050"...then its iffy again past that...again, just illustrative examples

all of this also assumes the reloading practices and chambers have the correct relationships...bad reloading practices or bad chamber specs/chamber jobs is another thing...basically sound reloading practices (consistent doesnt always equal sound...if youre making burs in brass, deforming bullets in seating, etc...even if you do it every time...its not good for results)

im sure anyone who has done much reloading/shooting knows certain bullets just shoot in a lot of barrels...its almost hard to get them not to...SMKs, Berger BT Target (better than hybrids in my experience)...Hybrids and Hornady ELDs are way better than most also

For actual shooting scenarios where this has shown up for me...i had a 223 barrel a while back...shooting 80 VLDs. I did powder tests, i did the Berger jump tests (the off the lands version i think from .020 to .120 or whatever it was)...and everything was basically 3/4" @ 100...some groups around 1/2", some closer to 1", but everything in a decent sample size...3/4", which is not a pretty looking group shooting small .224 bullets

Buddy of mine said "try them .005" off"...so i did, groups instantly cut in approx. 1/2...went to shooting a lot of ragged holes groups on the regular in the 1/4-5/8" range...and it did this for a pretty long time, until the lands wore away...it basically became a 3/4moa system again over time

at the same time...ive had A LOT of barrels over the years...easily north of 20 since 2015...where i shot forgiving bullets...SMKs, Berger BT target/juggernauts/hybrids...and in those barrels, i cant find a REAL seating depth difference...i can seat a Berger BT Target (what i use most the last few years), anywhere from .020 to .100 off, and every barrel will shoot them basically the same for relevant sample sizes...i cant shoot the difference day to day, but i know everyone on the internet shoots better so maybe they can lol

so my question goes back to...if jump (and powder charge) have to do with barrel timing/vibrations/muzzle movement...why do certain bullet designs seem to ignore this? barrel has still got to be moving

is it possible that the amount of jump is just a pass/fail scenario? in that it gets handed off clean/straight or it doesnt in relation to how the case/bullet reacts with the chamber geometry

of course, it could also be possible that shooting from a bipod/rear bag...the barrel vibration/movement variation is so small, i cant reliably/consistently see it in the results...and bullets being handed off crooked is all i can shoot accurately enough to see on target...maybe?

and yes...ive seen peoples small sample testing that show "clear winners"...i have stacks of my own also. I also have stacks of load dev where i shot "forgiving" bullets across 2-5 grain charge windows and .100" jump windows that didnt move significantly at all...possible can barrels be oscillating within a similar window at all times, and the variation seen on target is just how straight the bullet was handed off and leaves the bore?

ive also seen the barrel time info/papers/data....but i still hang up when that doesnt apply or show up on target...

curious to hear some other thoughts and experiences...lots of nuance and noise to weed out in these topics so hopefully i got my thoughts across clearly

id also like skip over any "i shot 5 rounds of A and 5 rounds of B, and A was .1" smaller so A will always be better..." ...we all know how that would hold up lol

Side Note: outside of the "unforgiving" VLD design in that 223 that showed a clear difference...i dont see the large shifts/changes a lot of people seem to see from small jump variations, or powder for that matter...its not like .020 is 1/3moa for 10 shots, and .030 is 1.25moa for 10 shots...if i do see large variations, when retested, they dont hold up, typically attributed to an outlier or i shanked one...my variations are always very small, with lots of overlap, and larger samples blend the differences together...simply put, my barrels/rifles dont consistently (aka the vast majority of the time) shoot better than 1/2" or worse than 3/4" for large sample sizes...no matter what i change in my loads with the "forgiving" bullets (within reason, using good components and known reliable combos)...yes yes, ive shot a ton of ragged hole groups like everyone else, but theyre small outliers, not the overall average everyday

 

[Aftermath]

Yes, like most everything, it is possible.
Now, talk probable.

 

[Evintos]

https://bulletin.accurateshooter.co...ant-hybrid-bullet-geometry-explained-by-litz/

 

[morganlamprecht]

doesnt answer the question...i know the differences in bullets...i shoot the ones i shoot for a reason

how do bullets with more forgiving designs at the throat...also account for various muzzle exit timing?

unless litz is implying similar to my posed question, that how the bullet is handed off is what matters...but that doesnt align with most everyones talks of muzzle exit timing...thats the rub

 

[Precision Underground]

I didn’t read the entire post but I’m very skeptical that changing .002 in seating depth is doing anything to time a bullet. I think that’s folklore. My guess is that it’s about how smooth/straight the bullet is inserted into the rifling to engrave it cleanly and evenly.

 

[morganlamprecht]

I didn’t read the entire post but I’m very skeptical that changing .002 in seating depth is doing anything to time a bullet. I think that’s folklore. My guess is that it’s about how smooth/straight the bullet is inserted into the rifling to engrave it cleanly and evenly.

my experience leans the same

 

[Cascade Hemi]

I didn’t read the entire post but I’m very skeptical that changing .002 in seating depth is doing anything to time a bullet. I think that’s folklore. My guess is that it’s about how smooth/straight the bullet is inserted into the rifling to engrave it cleanly and evenly.

I read on SH if you seat bullets .100" or more off the lands you barrel will never wear out. This threat must be true. Everyone knows you need the bullet to leave when the barrel is wiggling, not waggling, and for sure at the top, not the bottom, of the target.

I believe the science. Sleepy bullets and wave patterns!

 

[Evintos]

doesnt answer the question...i know the differences in bullets...i shoot the ones i shoot for a reason

how do bullets with more forgiving designs at the throat...also account for various muzzle exit timing?

unless litz is implying similar to my posed question, that how the bullet is handed off is what matters...but that doesnt align with most everyones talks of muzzle exit timing...thats the rub

That's what it's implying. The juncture between ogive and bearing surface of a bullet design may or may not lend itself well for bullet alignment into the lands and it's more about alignment than it is timing.

 

[straightshooter1]

Not sure where to begin...but jump testing is something we all see a lot, and it typically the discussion goes back to bullet timing/barrel vibrations/etc...along those lines.

one thing that has always hung in my mind is "forgiving" bullets...how are bullets "forgiving" if the whole idea of load dev, is based upon timing up when the bullet exits the muzzle with some movement/vibration pattern...do forgiving bullets not make the barrel move/vibrate? (obviously not the case) do forgiving bullets not deflect the same when exiting a moving barrel? self correct? i dont have an answer, just questions...

is it possible that instead of barrel timing/muzzle exit, all seating depth needs to be is "good enough to hand off the bullet straight"? ie; Bullet A, gets a clean/straight hand off due to bullet shape and chamber spec relationship from .005" to .150" off...basically it gets a good hand off anywhere having the bearing surface out of the neck because that "forgiving" design allows it so...while maybe Bullet B, has less agreeable relationship with chamber design, and it doesnt get a clean/straight hand off from .005" to .020" off...gets straight from .025 to .050"...then its iffy again past that...again, just illustrative examples

all of this also assumes the reloading practices and chambers have the correct relationships...bad reloading practices or bad chamber specs/chamber jobs is another thing...basically sound reloading practices (consistent doesnt always equal sound...if youre making burs in brass, deforming bullets in seating, etc...even if you do it every time...its not good for results)

im sure anyone who has done much reloading/shooting knows certain bullets just shoot in a lot of barrels...its almost hard to get them not to...SMKs, Berger BT Target (better than hybrids in my experience)...Hybrids and Hornady ELDs are way better than most also

For actual shooting scenarios where this has shown up for me...i had a 223 barrel a while back...shooting 80 VLDs. I did powder tests, i did the Berger jump tests (the off the lands version i think from .020 to .120 or whatever it was)...and everything was basically 3/4" @ 100...some groups around 1/2", some closer to 1", but everything in a decent sample size...3/4", which is not a pretty looking group shooting small .224 bullets

Buddy of mine said "try them .005" off"...so i did, groups instantly cut in approx. 1/2...went to shooting a lot of ragged holes groups on the regular in the 1/4-5/8" range...and it did this for a pretty long time, until the lands wore away...it basically became a 3/4moa system again over time

at the same time...ive had A LOT of barrels over the years...easily north of 20 since 2015...where i shot forgiving bullets...SMKs, Berger BT target/juggernauts/hybrids...and in those barrels, i cant find a REAL seating depth difference...i can seat a Berger BT Target (what i use most the last few years), anywhere from .020 to .100 off, and every barrel will shoot them basically the same for relevant sample sizes...i cant shoot the difference day to day, but i know everyone on the internet shoots better so maybe they can lol

so my question goes back to...if jump (and powder charge) have to do with barrel timing/vibrations/muzzle movement...why do certain bullet designs seem to ignore this? barrel has still got to be moving

is it possible that the amount of jump is just a pass/fail scenario? in that it gets handed off clean/straight or it doesnt in relation to how the case/bullet reacts with the chamber geometry

of course, it could also be possible that shooting from a bipod/rear bag...the barrel vibration/movement variation is so small, i cant reliably/consistently see it in the results...and bullets being handed off crooked is all i can shoot accurately enough to see on target...maybe?

and yes...ive seen peoples small sample testing that show "clear winners"...i have stacks of my own also. I also have stacks of load dev where i shot "forgiving" bullets across 2-5 grain charge windows and .100" jump windows that didnt move significantly at all...possible can barrels be oscillating within a similar window at all times, and the variation seen on target is just how straight the bullet was handed off and leaves the bore?

ive also seen the barrel time info/papers/data....but i still hang up when that doesnt apply or show up on target...

curious to hear some other thoughts and experiences...lots of nuance and noise to weed out in these topics so hopefully i got my thoughts across clearly

id also like skip over any "i shot 5 rounds of A and 5 rounds of B, and A was .1" smaller so A will always be better..." ...we all know how that would hold up lol

Side Note: outside of the "unforgiving" VLD design in that 223 that showed a clear difference...i dont see the large shifts/changes a lot of people seem to see from small jump variations, or powder for that matter...its not like .020 is 1/3moa for 10 shots, and .030 is 1.25moa for 10 shots...if i do see large variations, when retested, they dont hold up, typically attributed to an outlier or i shanked one...my variations are always very small, with lots of overlap, and larger samples blend the differences together...simply put, my barrels/rifles dont consistently (aka the vast majority of the time) shoot better than 1/2" or worse than 3/4" for large sample sizes...no matter what i change in my loads with the "forgiving" bullets (within reason, using good components and known reliable combos)...yes yes, ive shot a ton of ragged hole groups like everyone else, but theyre small outliers, not the overall average everyday

When I started precision reloading I read a lot about and references for jump and distance to the lands. So, I focused on that in trying to keep the jump the same for a good performing load as the throat eroded. In chasing the lands like that, I was constantly having to make powder adjustments to get back to the performance I had gotten before. It was like chasing your tail and wasting a lot of time and components.

Then I started listening to several top competitive shooters who were saying that once they found their load that worked well, they didn't change anything in putting together their cartridges for the life of their barrel, even though the throat was eroding. I decided to test this on my .308 and not change my seating depth as the throat eroded until I could see a difference on paper. After 2,000 rounds and my throat having eroded .033, I was still getting nice tight sub .5 MOA groups; and it was more consistent than when I was chasing the lands.

Based on what those shooters were saying and now my own experience, I've come to see that the emphasis on jump is incorrect and the emphasis should be on seating depth. Seating depth is way more important than the distance to the lands. However, when it comes to bullets with a secant ogive, they can be very finicky with regards to jump as they tend to like being very close to the lands or touching or jammed.

Now if one is loading to touch or jam bullets, then keeping track and making sure the bullet stays very close to the same touch or jam specs is necessary to maintain good performance.

The only other time distance to the lands is important is for determining a starting point for a seating depth. Once the starting point is established, distance to the lands can be ignored . . . unless one just likes to track throat erosion (as I do ).

Bullet performance is subject to their particular design and how well and consistent manufacturers produce them to a spec as well as how a shooter intends to use them. In my limited experience, I find how "forgiving" a bullet might be has more to do with the barrel than the bullet; though there seems to be some correlation with the size of the bearing surface. One day, I'll start looking at that closer. Long heavy barrels will exhibit much more "forgiveness" than thin light barrels. And that appears to be largely do to the lower barrel frequency.

Internal ballistics is so complex making for a lot to learn and easy to be wrong about what we reloaders understand. I feel I've only scratched the surface towards getting a good handle on it all.

 

[Aftermath]

Based on what those shooters were saying and now my own experience, I've come to see that the emphasis on jump is incorrect and the emphasis should be on seating depth. Seating depth is way more important than the distance to the lands. However, when it comes to bullets with a secant ogive, they can be very finicky with regards to jump as they tend to like being very close to the lands or touching or jammed.

Now if one is loading to touch or jam bullets, then keeping track and making sure the bullet stays very close to the same touch or jam specs is necessary to maintain good performance.

The only other time distance to the lands is important is for determining a starting point for a seating depth. Once the starting point is established, distance to the lands can be ignored . . . unless one just likes to track throat erosion (as I do ).

I see seating depth and distance to the lands as being BERY BERY related. Please explain the difference.

EDIT: I have never chased erosion. Once I find the combination for my barrel and set of components, I shoot it.

 

[918v]

“so my question goes back to...if jump (and powder charge) have to do with barrel timing/vibrations/muzzle movement...why do certain bullet designs seem to ignore this? barrel has still got to be moving”

Maybe it’s not a timing issue alone. Maybe it’s a timing issue qualified by a bullet engraving issue.

For example, if a tangent or hybrid bullet engraves more gradually then timing becomes less critical. A secant hits the lands abruptly vs a tangent hitting the lands gradually which softens the blow sort of and makes the transition easier.

Does a secant stop at the lands and then begin to move again? Does a tangent simply slow down a bit?

 

[straightshooter1]

I see seating depth and distance to the lands as being BERY BERY related. Please explain the difference.

EDIT: I have never chased erosion. Once I find the combination for my barrel and set of components, I shoot it.

Seating Depth: The distance from the base of the bullet to the base of the case, usually measured by the distance in CBTO.

Distance To The Lands (DTTL): The distance from the ogive to touching the lands, usually measured by the difference between the two CBTO's of the different lengths.

Since you don't chase erosion, I assume you keep the same seating depth as the distance between the CBTO and the lands increases. Certainly, when you change the seating depth, you change the distance to the lands by the same amount at that particular moment wherever the lands happens to be.

When someone states that they load with a particular distance from the lands (jump), it tells us nothing about the seating depth because we don't know how long the freebore is. Freebores are different from one barrel to another and often it's a substantial difference . . . like it could be as much as .100" difference. To get a better idea of someone's seating depth load, it's better to know their COAL, even knowing there's typically a good difference due to the variations in bullet's OAL's. Even knowing one's CBTO can be problematic in figuring what the seating depth is because comparator insert diameters can vary a lot (like Sinclair's vs Hornady's).

It's well known how changing seating depth during load development will change velocity and often adjustments are made in .003" increments. But when the throat erodes, one doesn't find much if any change in velocities if any at all. Seating depth effects velocity way more than any changes in distance to the lands.

Too often, people talk in terms of jump when they're really talking about seating depth.

 

[just browsing]

This is more or less the basis for Litz’s whole “load development doesn’t really matter” theory.

Assuming you follow good loading practices with good components, the thought is that it the actual values don’t matter so much as the consistency from round to round does.

Ex. If you shoot 100 5-shot groups jumping .010” and then another 100 5-shot groups jumping .100”… assuming everything else is the same - brass, barrel wear, charge etc. - those samples will average out to have the same spreads, group size, and consistency over the duration of the test.

I also think a lot of stuff is overly-accredited to specific components of a system… like cartridge selection, bullets, or gunsmiths, but that’s an entirely different discussion for another time.

 

[Aftermath]

Seating Depth: The distance from the base of the bullet to the base of the case, usually measured by the difference in CBTO.

Distance To The Lands (DTTL): The distance from the ogive to touching the lands, usually measured by the difference between the two CBTO's of the different lengths.

Since you don't chase erosion, I assume you keep the same seating depth as the distance between the CBTO and the lands increases. Certainly, when you change the seating depth, you change the distance to the lands by the same amount at that particular moment wherever the lands happens to be.

When someone states that they load with a particular distance from the lands (jump), it tells us nothing about the seating depth because we don't know how long the freebore is. Freebores are different from one barrel to another and often it's a substantial difference . . . like it could be as much as .100" difference. To get a better idea of someone's seating depth load, it's better to know their COAL, even knowing there's typically a good difference due to the variations in bullet's OAL's. Even knowing one's CBTO can be problematic in figuring what the seating depth is because comparator insert diameters can vary a lot (like Sinclair's vs Hornady's).

It's well known how changing seating depth during load development will change velocity and often adjustments are made in .003" increments. But when the throat erodes, one doesn't find much if any change in velocities if any at all. Seating depth effects velocity way more than any changes in distance to the lands.

Too often, people talk in terms of jump when they're really talking about seating depth.

When I develop a load, I find the amount of distance to/from the lands that provides (arguably) the best group sizes. That determines the CBTO for that set of components. The seating depth is inherently involved.

 

[Aftermath]

Now....if you want to discuss how seating depth changes the size (and shape) of the combustion chamber...there probably is an optimum seating depth for a given set of components.
Does THAT optimum jive with the optimum distance to/from the lands as determined by the target?
Does all of that jive with barrel harmonics?
Is it more about barrel dwell time? Because more "jump" equates to less dwell time.
Is there an optimum velocity for a given bullet weight and shape (to include diameter)?

 

[Aftermath]

When someone states that they load with a particular distance from the lands (jump), it tells us nothing about the seating depth because we don't know how long the freebore is. Freebores are different from one barrel to another and often it's a substantial difference . . . like it could be as much as .100" difference. To get a better idea of someone's seating depth load, it's better to know their COAL, even knowing there's typically a good difference due to the variations in bullet's OAL's. Even knowing one's CBTO can be problematic in figuring what the seating depth is because comparator insert diameters can vary a lot (like Sinclair's vs Hornady's).

This is where I feel you are missing it...
Let's say 2 shooters use the same exact everything to load with but use 2 different rifles.
When shooter A states that their load development gave them a certain distance from the lands, no matter the freebore, that is the distance from the lands. Shooter B can duplicate that amount of distance to the lands in his own rifle no matter if there is any difference in freebore.
Same scenario but CBTO...shooter A says I got this CBTO to shoot great. Is that because he has some unknown amount of jump to the lands? Shooter B loads up the same CBTO and it shoots like shit. is that because there is some different amount of freebore?
I'm not trying to be argumentative...

 

[straightshooter1]

When I develop a load, I find the amount of distance to/from the lands that provides (arguably) the best group sizes. That determines the CBTO for that set of components. The seating depth is inherently involved.

I do understand. You're using the distance to the lands to measure you're seating depth. And once you determine the best group, you keep that seating depth; as do I. Once I determine a starting point (a distance from the lands, like maybe .010"), I ignore the distance to the lands and simply focus on maintaining a consistent seating depth in order to keep my best group sizes going.

Now....if you want to discuss how seating depth changes the size (and shape) of the combustion chamber...there probably is an optimum seating depth for a given set of components.
Does THAT optimum jive with the optimum distance to/from the lands as determined by the target?
Does all of that jive with barrel harmonics?
Is it more about barrel dwell time? Because more "jump" equates to less dwell time.
Is there an optimum velocity for a given bullet weight and shape (to include diameter)?

Those are whole different discussions . . . and long ones at that.

 

[Precision Underground]

When I started precision reloading I read a lot about and references for jump and distance to the lands. So, I focused on that in trying to keep the jump the same for a good performing load as the throat eroded. In chasing the lands like that, I was constantly having to make powder adjustments to get back to the performance I had gotten before. It was like chasing your tail and wasting a lot of time and components.

Then I started listening to several top competitive shooters who were saying that once they found their load that worked well, they didn't change anything in putting together their cartridges for the life of their barrel, even though the throat was eroding. I decided to test this on my .308 and not change my seating depth as the throat eroded until I could see a difference on paper. After 2,000 rounds and my throat having eroded .033, I was still getting nice tight sub .5 MOA groups; and it was more consistent than when I was chasing the lands.

Based on what those shooters were saying and now my own experience, I've come to see that the emphasis on jump is incorrect and the emphasis should be on seating depth. Seating depth is way more important than the distance to the lands. However, when it comes to bullets with a secant ogive, they can be very finicky with regards to jump as they tend to like being very close to the lands or touching or jammed.

Now if one is loading to touch or jam bullets, then keeping track and making sure the bullet stays very close to the same touch or jam specs is necessary to maintain good performance.

The only other time distance to the lands is important is for determining a starting point for a seating depth. Once the starting point is established, distance to the lands can be ignored . . . unless one just likes to track throat erosion (as I do ).

Bullet performance is subject to their particular design and how well and consistent manufacturers produce them to a spec as well as how a shooter intends to use them. In my limited experience, I find how "forgiving" a bullet might be has more to do with the barrel than the bullet; though there seems to be some correlation with the size of the bearing surface. One day, I'll start looking at that closer. Long heavy barrels will exhibit much more "forgiveness" than thin light barrels. And that appears to be largely do to the lower barrel frequency.

Internal ballistics is so complex making for a lot to learn and easy to be wrong about what we reloaders understand. I feel I've only scratched the surface towards getting a good handle on it all.

This seems the most logical to me and is the type of thing I ramble in my head about when I should be doing something productive. I think it’s less about timing a bullet or barrel whip(these are on way too large of a scale for .005 seating depth to matter IMO), and it’s more about each barrel/cartridge combo having a happy window where ignition, bullet movement, harmonics(on a microscopic scale- not barrel whip)and pressure leave a large margin for error to allow the bullet to start, proceed, and be engraved consistently. I have no way to verify this it’s just a conclusion of thought experiments lol.

 

[Aftermath]

I agree!! HAHA!! I spend way too much time thinking about this stupid shit when I should be thinking about or doing other stuff!
It's the engineer downfall.
I don't have access to the sort of equipment required to measure the things I would like to measure and the books don't include this stuff either. If they did, there would be so many combinations as to make the work cumbersome to perform and the data extremely voluminous.
When it comes right down to it, it's just magic. This particular combination of stuff put together in this particular configuration magically performs to my expectations. I should just leave it there, but it's difficult to do so.

 

[Aftermath]

Does a secant stop at the lands and then begin to move again? Does a tangent simply slow down a bit?

Neither bullet stops once ignition happens, it is nothing but acceleration until the bullet exits. There is very likely a difference in the acceleration once they hit the lands but I bet that only JPL could determine exactly what that is.

 

[straightshooter1]

This is where I feel you are missing it...
Let's say 2 shooters use the same exact everything to load with but use 2 different rifles.
When shooter A states that their load development gave them a certain distance from the lands, no matter the freebore, that is the distance from the lands. Shooter B can duplicate that amount of distance to the lands in his own rifle no matter if there is any difference in freebore.

If shooter A's freebore is .135" and shooter B's freebore is .225", shooter B is going to have to load his cartridge .090" longer to have the same distance to the lands.

Wouldn't you say that as seating depth increases the interior case volume decrease, which raises the pressure upon ignition and that pressure raises the velocity? And if you were to seat a bullet .090" longer, there's gong to be a significant difference in velocity that effects how the load performs?

As the throat erodes there's very little effect on pressure change, though there is some and isn't that why we don't see much velocity change as the throat erodes?

Same scenario but CBTO...shooter A says I got this CBTO to shoot great. Is that because he has some unknown amount of jump to the lands? Shooter B loads up the same CBTO and it shoots like shit. is that because there is some different amount of freebore?

Most likely, not.

Internal ballistics is too complex to be able to say just why shooter B's load performed worse than A's since they're loaded identically identical. Was the bullet specs exactly the same, was the powder the same lot with the same burn rate, was the barrel the same (e.g. heavy high end vs factory light pencell barrel). Any number of things could be a play. Consider for example, a lot has to do with the barrel itself as it too could have the same everything along with the same cartridge specs and still get different results. This is why many top competitive shooters buy and test several different barrels for competition, all supposedly cut to the same specs but some just shoot better than others with ammo that's all been loaded the same.

 

[straightshooter1]

I agree!! HAHA!! I spend way too much time thinking about this stupid shit when I should be thinking about or doing other stuff!
It's the engineer downfall.
I don't have access to the sort of equipment required to measure the things I would like to measure and the books don't include this stuff either. If they did, there would be so many combinations as to make the work cumbersome to perform and the data extremely voluminous.
When it comes right down to it, it's just magic. This particular combination of stuff put together in this particular configuration magically performs to my expectations. I should just leave it there, but it's difficult to do so.

LOL

I know a lot about that "engineer downfall"

 

[Aftermath]

My point is that the distance to the lands can be duplicated regardless of freebore and the "jump" is then also duplicated. It does seem to be, especially with the VLD stuff, that distance to the lands is worth finding up front for an accurate load.
I start my load development on the lands and change charge weight to find pressure. I back off to no pressure signs and start seating deeper looking for that magic. Then I keep that CBTO and do some more charge weight testing and usually, about 75% of the time, discover that the original charge weight still yields the desired magic.
I most definitely agree that altering the combustion chamber ever so minutely has profound impact, primarily to pressure. We have all seen first hand or know of someone that has used a given charge in different cases (of different thicknesses, altering the combustion chamber dimension) with extreme differences in pressure...blown primers in one (or worse) while the other shoots without pressure indications. But also, increasing jump tends to reduce pressure.
You bring up the point of not much velocity change as erosion happens. This, coupled with what I have observed as minimal change in accuracy, is why I don't chase the erosion.
As to "identically identical".....I mean, just as we often negate things like whether we are at sea level to seek a simplified example...let's say everything (except the rifle, but let's only say that they are different rifles as can be purchased OTC, same make, same model, same length of barrel...but not "identically identical') is EXACTLY the same.
Even the CBTO being the same probably has different results. Yes, maybe one has a shit barrel. Maybe it's the shooter. I will venture to say it is jump that is unknown.

 

[918v]

Neither bullet stops once ignition happens, it is nothing but acceleration until the bullet exits. There is very likely a difference in the acceleration once they hit the lands but I bet that only JPL could determine exactly what that is.

Well something happens differently that improves our quality of life.

 

[Dave62677]

I jump my dasher / 338nm / 33xc 0.060~0.100. All shoot great with Fmj and solid. I stop chasing the land for long time after I read SAC article.

 

[Aftermath]

Well something happens differently that improves our quality of life.

I do know from personal experience that some bullets are less sensitive to jump than others. The VLD's that I use in several different caliber rifles all like to be near the lands at about 0.006" off. The Sierra Game Kings in my 243AI prefer to be 0.01" off but they shoot pretty much lights out seated from 0.05" to 0.005", so I started with them at 0.005" to allow for the erosion. Same weight VLD in the same rifle...0.006". When I did the load development for that one, the difference was very noticeable.
Back to the load development I mentioned on previous post where I find the pressure, back off, seat deeper, change the charge, only to find the the original charge to be the best...well, duh! That was already determined. I have been thinking I might change charge weights and look for pressure at the new CBTO, back off and look for another seating depth for magic. If components were more readily available, I would.

 

[RegionRat]

Long heavy barrels will exhibit much more "forgiveness" than thin light barrels. And that appears to be largely do to the lower barrel frequency.

Because of their section thickness, these long heavy match barrels are stiffer than light ones and have a higher frequency and lower amplitude.

Long light barrels are less stiff, and give a lower note thus lower frequency and higher amplitude.

ETA: You are on the right track, just for a different reason... folks need to realize that while frequencies are easy to calculate, amplitudes are not, and amplitudes matter much more than frequency when it comes to the difference between where the barrel was pointing when the shot was ignited versus where it is when the bullet exits. Carry on.

 

[morganlamprecht]

When I started precision reloading I read a lot about and references for jump and distance to the lands. So, I focused on that in trying to keep the jump the same for a good performing load as the throat eroded. In chasing the lands like that, I was constantly having to make powder adjustments to get back to the performance I had gotten before. It was like chasing your tail and wasting a lot of time and components.

Then I started listening to several top competitive shooters who were saying that once they found their load that worked well, they didn't change anything in putting together their cartridges for the life of their barrel, even though the throat was eroding. I decided to test this on my .308 and not change my seating depth as the throat eroded until I could see a difference on paper. After 2,000 rounds and my throat having eroded .033, I was still getting nice tight sub .5 MOA groups; and it was more consistent than when I was chasing the lands.

Based on what those shooters were saying and now my own experience, I've come to see that the emphasis on jump is incorrect and the emphasis should be on seating depth. Seating depth is way more important than the distance to the lands. However, when it comes to bullets with a secant ogive, they can be very finicky with regards to jump as they tend to like being very close to the lands or touching or jammed.

Now if one is loading to touch or jam bullets, then keeping track and making sure the bullet stays very close to the same touch or jam specs is necessary to maintain good performance.

The only other time distance to the lands is important is for determining a starting point for a seating depth. Once the starting point is established, distance to the lands can be ignored . . . unless one just likes to track throat erosion (as I do ).

Bullet performance is subject to their particular design and how well and consistent manufacturers produce them to a spec as well as how a shooter intends to use them. In my limited experience, I find how "forgiving" a bullet might be has more to do with the barrel than the bullet; though there seems to be some correlation with the size of the bearing surface. One day, I'll start looking at that closer. Long heavy barrels will exhibit much more "forgiveness" than thin light barrels. And that appears to be largely do to the lower barrel frequency.

Internal ballistics is so complex making for a lot to learn and easy to be wrong about what we reloaders understand. I feel I've only scratched the surface towards getting a good handle on it all.

we had similar paths...i started off worrying about always having the jump correct. was always tweaking loads. Eventually got around to loading it at a spot and shooting it until it quits...and on those barrels that i shot 1 load until it quit, i was never able to bring them back by redoing any load work...they were toast best i could tell.

i only reference the jump as a starting reference now also...that last line is too true lol

 

[straightshooter1]

Because of their section thickness, these long heavy match barrels are stiffer than light ones and have a higher frequency and lower amplitude.

Long light barrels are less stiff, and give a lower note thus lower frequency and higher amplitude.

ETA: You are on the right track, just for a different reason... folks need to realize that while frequencies are easy to calculate, amplitudes are not, and amplitudes matter much more than frequency when it comes to the difference between where the barrel was pointing when the shot was ignited versus where it is when the bullet exits. Carry on.

Hmmm??? OK I guess there's something about some harmonics I haven't quite grasped yet.

Since I don't have the electronic instruments to use to see what the frequencies or amplitudes look like, I just use a rubber mallet to strike my barrels and the long heavy barrels have a lower tone than my light ones. So, I figured that lower tone was a lower frequency. . . like we have with tuning forks.

Oh, I did figure that amplitude does matter in that timing.

PS: My thinking was also in terms of a string in that a barrel is like a string, only way shorter, where things like length, thickness, density and tension effect frequency.

 

[CK1.0]

Assuming you follow good loading practices with good components, the thought is that it the actual values don’t matter so much as the consistency from round to round does.

^^^ this.

The longer I do this, and the more I test things for myself, the more it seems like consistency and repeatability matter more than any of the other shit.

Somehow, even though I ignore most, if not all, of the "legacy theories" (like "nodes", supposed OCW, etc that have never made any sense or have been explained completely despite the ~zilllion threads), miraculously, my stuff always seems to shoot. The limiting factor always seems to end up being the monkey pulling the trigger more so than anything else.

Like the OP, lately, I'm also starting to wonder about how my pathetically small sample sizes impact my results (something I've come to accept when it comes to powder charges), even when it comes to jump testing and seating depth.

Chances are it has way more to do with the small sample size versus one seating depth being better than another.

FWIW, all my barrels have shot .3's now and then, .75" when I'm half-assing it, and stay generally ~.5" and/or 1/2 MOA from 100 out to 1000+... seemingly whether I'm .020" off the lands or .120" off the lands.

I tend to jump more these days (~.100" off) because it means I can shoot the same load over the life of a barrel without it changing since the throat erodes more slowly (my last couple of barrels died from fire-cracking more so than erosion), and to me, "forgiving" means no surprises...

 

[supercorndogs]

When these question are posed. It usually seems the people who say it doest make a difference, actually mean they can't shoot the difference. A lot of people do a lot of things in their reloading process they aren't seeing a practical advantage from. As a guy I know used to say, "its you weiner, you can wash it how you want."

 

[morganlamprecht]

When these question are posed. It usually seems the people who say it doest make a difference, actually mean they can't shoot the difference. A lot of people do a lot of things in their reloading process they aren't seeing a practical advantage from. As a guy I know used to say, "its you weiner, you can wash it how you want."

ive hounded people on this for years...they follow load work up instructions from BR experts or f class shooters using joystick rests and special bags...all the while shooting from a bipod and rear squeeze bag or a hunting rifle off sand bags

theyre not going to be able to resolve the same variations in their testing, period....but we see them every day on the internet claiming they do

personally, i have a good idea where i stand....shot lots of competitions next to various prs and f class guys with known track records

id be placed on the not a big difference side id imagine...and i still have a safe of rifles that shoot just like everyone else who claims it all makes big differences, when comparing similar rifle builds...obviously im not taking my prs rigs to an f class match and mopping up...but when i take my prs rig with little load effort and put it next to an f class shooter on a similar prs rig with all the load effort...i know how that shakes out even just shooting prone...ive never seen where all their time and effort showed up on target

 

[Chickentoast]

Seating depth test for a 6.5 Max. Based on all of the load development I've done, I believe seating depth is more important than powder charge.

 

[Haney]

ive hounded people on this for years...they follow load work up instructions from BR experts or f class shooters using joystick rests and special bags...all the while shooting from a bipod and rear squeeze bag or a hunting rifle off sand bags

theyre not going to be able to resolve the same variations in their testing, period....but we see them every day on the internet claiming they do

personally, i have a good idea where i stand....shot lots of competitions next to various prs and f class guys with known track records

id be placed on the not a big difference side id imagine...and i still have a safe of rifles that shoot just like everyone else who claims it all makes big differences, when comparing similar rifle builds...obviously im not taking my prs rigs to an f class match and mopping up...but when i take my prs rig with little load effort and put it next to an f class shooter on a similar prs rig with all the load effort...i know how that shakes out even just shooting prone...ive never seen where all their time and effort showed up on target

Hound people ? Says it all .

 

[morganlamprecht]

Seating depth test for a 6.5 Max. Based on all of the load development I've done, I believe seating depth is more important than powder charge.
View attachment 8051926

id be curious to see what a handful of 10 shot groups of 15 off vs 75 off looked like

ive never shot any badlands, maybe they respond to how my VLDs did...that looks similar to what i saw with my VLDs

 

[morganlamprecht]

Hound people ? Says it all .

figure of speech...the same people are hounding me about their load work up...we have on going discussions back and forth over the years...ill use a better word next time for you

 

[supercorndogs]

You is gonna get accused of not having a large enough sample.

id be curious to see what a handful of 10 shot groups of 15 off vs 75 off looked like

ive never shot any badlands, maybe they respond to how my VLDs did...that looks similar to what i saw with my VLDs

I had that typed up under his post and didn't post it.

I convinced myself once that seating depth made a big difference. IT was with a barrel that i finally figured out wouldn't shoot tight for the first 10 or 15 rounds. It would always take a lot of rounds to get it to foul in and calm down also.

 

[brianf]

I think I read it all, but until there is actual verified data/video etc of a barrels movement and possible bore dimensional changes it’s all lost in the noise.

Everything I’ve seen and researched is not detailed enough to give a definitive answer

Most likely because the money used can not be recovered. It’s either true or not..big deal. It can’t be sold or marketed.

No one can say they are timing a load to a barrel if the barrel is a variable as well.

Can’t have 2 variables and be definitive, not the way it works.

 

[impactaddict]

id be curious to see what a handful of 10 shot groups of 15 off vs 75 off looked like

I would bet (based on the people that test this stuff for a living) the groups would look a lot like each other... and this is the entire point of your OP.

 

[Chickentoast]

It was just one target that I happened to have a pic of on this computer - the correlations hold for all of my rifles shooting projectiles that are very high BC for caliber. And for those projectiles, seating depth is more important than powder charge.

 

[Namekagon]

My personal suspicion is this: That the custom rifles the serious shooters use have gotten so damn good that, for many of them, it really doesn't matter what you feed them. The just plain shoot everything 0.5moa or better, and its almost impossible statistically to see differences in load variables in groups that small.

This leads those shooters to conclude load development doesn't matter. But if they went back to the days of trying to wring 0.5" groups out of a factory Savage model 10, they'll suddenly remember where all those "wives-tales" came from.

To me, it is indisputable that you can take any factory rifle and feed it factory ammo, and you'll discover there are certain ammo options that a rifle "likes" and other ones that shoot like shit. Ive also seen too many factory rifles shooting 1.5" groups with factory ammo, that I was able to get down below 0.75 or lower by handloading.

Edit to add: I highly doubt, in those factory rifle examples, that my handloading techniques were twice as consistant as factory ammo. So consistancy of handloads isn't enough of an explanation.

 

[acudaowner]

I still find this a great read , but wonder how it explains once you squeeze the trigger and the very second your firing pin makes contact with the primer and the primer ignites the powder inside the case one kernel or something pretty close to it after another and the very second your bullet leaves the case and in that tiny space of free travel how long and or short does anything after the bullet makes it into the barrel and gets pushed through the barrel how can anything that happened at the case still be affecting the flight changes on an object inside the barrel and once it leaves the barrel the twist , and where its going are imparted through the barrel and what you are pointing it at ish . if the bullet did not have to come in contact with the barrel then I might believe something way back here affects something later whose had it entire being shoved by pressure through a tube smaller than it's original shape . I may not be saying it totally correctly I am not a really scientific minded person but from shooting factory loads ( short) or my hand loads ( longer) and getting the same type of groups I do not get how long or short are really different I have even taken myself completely out of the picture and noticed zero difference ( except the gun shot a hell of a lot straighter without me ) than with me .

 

[morganlamprecht]

My personal suspicion is this: That the custom rifles the serious shooters use have gotten so damn good that, for many of them, it really doesn't matter what you feed them. The just plain shoot everything 0.5moa or better, and its almost impossible statistically to see differences in load variables in groups that small.

This leads those shooters to conclude load development doesn't matter. But if they went back to the days of trying to wring 0.5" groups out of a factory Savage model 10, they'll suddenly remember where all those "wives-tales" came from.

To me, it is indisputable that you can take any factory rifle and feed it factory ammo, and you'll discover there are certain ammo options that a rifle "likes" and other ones that shoot like shit. Ive also seen too many factory rifles shooting 1.5" groups with factory ammo, that I was able to get down below 0.75 or lower by handloading.

Edit to add: I highly doubt, in those factory rifle examples, that my handloading techniques were twice as consistant as factory ammo. So consistancy of handloads isn't enough of an explanation.

i agree with all that...ill be the first to admit, i left factory rifles behind long ago...i still have some, but they are more for looks than their performance (lever guns, single shot falling blocks, etc)

in those factory rifles that shot certain factory ammo bad...did the reloaded components match the factory ammo you were trying to shoot by any chance? at least as much as possible...obviously you cant match the factory powder or primers exactly...but shooting same bullet at least? same brass brand?...like factory 6.5creed 140 eld ammo shot 1.5", and then you also used 140 elds in your reloads?

id definitely not be surprised that changing bullets all together changes group sizes in factory or reload ammo...changing components is usually the biggest difference in actual group size ive seen

 

[Namekagon]

i agree with all that...ill be the first to admit, i left factory rifles behind long ago...i still have some, but they are more for looks than their performance (lever guns, single shot falling blocks, etc)

in those factory rifles that shot certain factory ammo bad...did the reloaded components match the factory ammo you were trying to shoot by any chance? at least as much as possible...obviously you cant match the factory powder or primers exactly...but shooting same bullet at least? same brass brand?...like factory 6.5creed 140 eld ammo shot 1.5", and then you also used 140 elds in your reloads?

id definitely not be surprised that changing bullets all together changes group sizes in factory or reload ammo...changing components is usually the biggest difference in actual group size ive seen

I can't say I ever duplicated the components of a factory load with handloads that I can think of. Though I am planning try just that with my valkyrie when the weather warms up. It shoots the 60gr Federal well, and I want to try to duplicate that load with handloads cause the factory is hard to find sometimes. Will be interesting to see what comes of that.

I have had different lot number of the same bullet shoot different groups. They were 105 VLD hunting bullets I think, and the ogive was in a different place in reference to the tip on the new lot. Once I adjusted the seating die to put the ogive back into the correct position relative to the rifling I had before, precision improved. Groups went from around .6-.7moa to around 1 moa when I changed lots and back when I adjusted seating depth.

 

[morganlamprecht]

Gotcha, I’d bet the Valkyrie thing works out for you

I’ve duplicated a lot of hornady 6/6.5 factory ammo and if shot the factory well, it shot my duplicate even a little better…I can hold specs on my small scale production way better than hornady lol

Long time ago I shot factory hornady 75 amax and 75 bthp in my 223…75 amax was terrible, lucky to avg 1”…and the 75 bthp shot bug holes. I tried to reload 75 amax’s and never could get them to shoot consistently…so I switched to 75 bthp for reloads and it shot lights out just like the factory…same barrel stacked sierra 77 smk/Berger 77 otm/Berger 80.5 fullbore also…just couldn’t stand them amax’s

 

[bax]

I still find this a great read , but wonder how it explains once you squeeze the trigger and the very second your firing pin makes contact with the primer and the primer ignites the powder inside the case one kernel or something pretty close to it after another and the very second your bullet leaves the case and in that tiny space of free travel how long and or short does anything after the bullet makes it into the barrel and gets pushed through the barrel how can anything that happened at the case still be affecting the flight changes on an object inside the barrel and once it leaves the barrel the twist , and where its going are imparted through the barrel and what you are pointing it at ish . if the bullet did not have to come in contact with the barrel then I might believe something way back here affects something later whose had it entire being shoved by pressure through a tube smaller than it's original shape . I may not be saying it totally correctly I am not a really scientific minded person but from shooting factory loads ( short) or my hand loads ( longer) and getting the same type of groups I do not get how long or short are really different I have even taken myself completely out of the picture and noticed zero difference ( except the gun shot a hell of a lot straighter without me ) than with me .

I'm going to make a comment that is about drawing a parallel only it will be obvious that the two things cannot be compared - sort of.

In a nuclear weapon, all of the stuff that makes all of that energy happens within about 570 to 590 nanoseconds - about two thirds of a microsecond, about 0.00057 milliseconds, about .00000057 seconds. When that amount of time has passed, all of the energy that is going to be liberated has been liberated and everything that happens after that is just consequences, not generation. For comparison, this about 1,000 times faster that the chemical that we see with smokeless powder.

According to Gordon's, in our rifles the powder burn is all over in about 1.3 milliseconds. By that time, the bullet is almost out of the barrel. When the firing pin dropped on the primer a shock wave started in the barrel steel. That shock wave traveled down the barrel, bounced off the muzzle, and reflected back to the breach. As the bullet leaves the barrel, that shock-wave cycle has happened about five and a half times. Inside the barrel, the pressure curve doesn't peak until about 400 microseconds. The initial impetus that forces the bullet into the barrel must happen in some small number of microseconds - something like 50, maybe less.

Like you, after a millisecond I think that the case contribution to ballistics is long over but those initial few microseconds must matter else all of our case prep work would make no difference. And because of vast amounts of testing, we know that case prep does make a difference. It is my guess that the combination of case shape versus chamber shape, powder ignition, pressure generation as the powder flashes, and bullet shape being forced into the lands is critical. I leave it to smart people to figure it out but I think that it is fascinating to contemplate.

 

[Scottnoidea]

I have done seating depth tests on many new rifles and saw real improvements as I adjusted, shot groups, adjusted, repeat until accuracy requirements met.

My latest new rifle is a Lithgow LA105 6.5 Creedmoor and due to the cost and availability of reloading components in Australia I just duplicated the Hornady Match ammo using a chronograph and shot a few club matches. I shot a few groups before and after matches and was pretty surprised to find that the initial (disappointing) accuracy kept improving with no load changes until at about 120 rounds down the tube it is shooting very well.

I now suspect that my previous load development practices were not "improving" accuracy. I think that if I had just loaded the most consistent ammo I can produce at safe pressure and magazine length that I'd have ended up with the same result as the barrel shot in.

When I shoot the barrel out and replace it I will repeat the "experiment" of no load development once a safe pressure is established.

 

[RegionRat]

Hmmm??? OK I guess there's something about some harmonics I haven't quite grasped yet.

Since I don't have the electronic instruments to use to see what the frequencies or amplitudes look like, I just use a rubber mallet to strike my barrels and the long heavy barrels have a lower tone than my light ones. So, I figured that lower tone was a lower frequency. . . like we have with tuning forks.

Oh, I did figure that amplitude does matter in that timing.

PS: My thinking was also in terms of a string in that a barrel is like a string, only way shorter, where things like length, thickness, density and tension effect frequency.

You are on the right track. You don't miss the point about the first mode being lower with a heavy barrel. However, I am pointing out the modes that count more in this load tuning discussion are the ones that cause us to resort to heavy barrel sections when we want accuracy and how those affect tuning.

No background in structural dynamics is required to know you would use a heavy barrel for a test rig or target rifle, but why is that if they lower some modes? Why are heavier sections not worse when they lower certain frequencies and how do they relate to being able to see or not see dramatic effects from load tuning?

The low note you are sensing is the first mode shape of the barrel and or it's support which is also very low frequency compared to other modes. When all that weight is a cantilever on the receiver, the simple view is not deceptive. Add weight on a cantilever, the frequency goes down. The first mode of the barrel also goes down.

You were not wrong, but some frequencies (modes) go up with stiffness from increased section (weight), and they can also move down if the weight isn't distributed correctly. This is why barrel flute cuts may or may not help with accuracy and why some barrel contours become popular in the accuracy crowds and others don't. It is also why your first statements were not wrong, heavier barrels lower some mode shapes, especially the low mode ones. The point being, why do high quality match guns seem to be insensitive to load tune variations?

The barrel first mode is usually low enough that the bullet exits within a fraction of that oscillation, however the second mode has enough time to oscillate many cycles.

Oversimplification in the thread means folks are missing the point of load tuning affects and why they are or are not easy to see in some guns and very obvious in others. I am not good at making this simple to explain or easy to understand, sorry. I'm dyslexic and admit I am a horrible writer....

It takes many words, diagrams, and math, to explain ballistic structural dynamics. I know what happens if there are this many words and then we add equations, especially the kind where you have to jump back and forth from frequency domain to time domain in order to have a good discussion.

Once a presentation starts up with phase diagrams, Eigen values, decibels, and Bode plots, folks start getting bleary eyed and head for dream land. Then I pulled the lanyard and the demonstrations got more interesting.... Who doesn't like to see ordinance plowing into targets or would they rather hear a math class on how a barrel and load were tuned.... Most folks just want to see the targets and I don't blame them.

BTW, there were several folks asking for a physical reason about why or even if the barrel mode shapes are mostly vertical. The reason most sporting and competition bolt guns have a strong vertical pattern, is due to two main reasons, 1) the low mode where the CG of the system is below the centerline during recoil, and, 2) the pressure shock inflation of the barrel against the gravity sag.

This description discounts the situation where your barrel touches a stock. All bets are off when that happens for this discussion.

The gun barrel is already sagging down due to gravity. If you consider the shock of the pressure extending the barrel and exciting the first modes of barrel/stock oscillations, the motion of the barrel inflating due to pressure counter acts the barrel sag and that first reaction becomes mostly vertical. The recoil lug and receiver are not symmetrical in bolt guns because they are underneath but even guns with symmetrical barrel supports move vertical on the shot due to that sag and barrel inflation. However, that said... The low mode shapes are not always just a straight line up and down, but many are nearly oval or ellipse with the long axis vertical. Get your tune exactly wrong (or have sloppy charges), and you will see a horizontal dispersion in many systems.

When weight is cantilevered over the length of the stock fore end, with the back of the stock touching a rear bag for example, there is a very low frequency mode associated with the whole gun and the weight of the barrel and receiver on the stock. The barrel first mode can be on the order of roughly 100 Hz.

A bullet is out of the muzzle before this mode gets even a half cycle. You can't really tune out this mode. Not saying we can ignore this mode, but for this discussion you can't do much about it since it is cast into the system. You can't go slow enough to take advantage of it is a different way of saying it.

The response of the first mode of the barrel on the stock doesn't play a strong role with respect to OBT tuning, but it does play into which effects are important on the whole because it is always there and potentially very large. The recoil response of the system on the whole is a huge effect on the difference between the point of aim and the motion of the muzzle as the bullet exits because the whole system is moving before the bullet exits.

It is not what folks want to hear, but it is why shooting technique matters. Rest or hold the gun wrong, and watch what happens next, regardless of how carefully your ammo is crafted.

The same ammo in a heavy system is more likely to be more accurate than that same ammo in a light system, mainly due to the interaction of the first mode with recoil. By the same token, get the load tune right in a light flimsy system (or more importantly get it wrong), and you can see bigger differences on the target. The potential changes here in the light barrel are larger, but the two baselines probably tip in favor of the heavy section to make it a moot point. You pick the heavier gun and barrel if you want accuracy... but nobody like to hump a heavy gun....

With respect to load development and component selection, those low mode frequencies and their effects are not always load tunable. Think about an AR-15 where the barrel extension sits in the upper, and then the whole upper rattles around on pins. As a mental exercise now imagine taking a pencil barrel profile versus a varmint profile in that same gun, and doing a modal analysis.

You will certainly find lots of the mode shapes shifted left on the PSD plot due to the weight of the heavier barrel, however, now think about what you have observed with different AR rigs between light weight pencil barrels versus heavy barrel varmint rigs and match barrels. No structural dynamics background required to guess which mode shapes matter more. Even when we lower some of the frequencies due to the weight of the barrel section, we are jacking other important ones with respect to load tuning. All while sitting in a sloppy pin interface between the upper and the lower.

With the little pencil barrels, the support frequencies shift up but many of those are a fraction of their oscillation compared to the bullet exit time. Keep in mind that some of the frequencies are very low and the others are in multiples of those and some are even in the kilohertz range. Both the thin light flimsy barrels and the thick heavy stiff ones have low first modes. It is those second modes and higher that are different between them, and you don't need to know much about structural dynamics to know that heavy section bull barrels and light section pencil barrels don't do the same things even if they both have a low first mode.

Some choices we make in the gun system set the tone for what the potential group dispersion will be, and as a result only some things can be load tuned. The reason for confusion is the share of the ones that are affected by barrel exit timing and load tune may be very dead or low amplitude on match guns because the barrels are heavy and stiff. The heavy choice lowers some frequencies, but improves recoil issues and stiffens important modes.

If you could hypothetically make a magic barrel that was infinitely rigid and weighed little or nothing but still had a moment inertia, you would still have a low frequency issue from the receiver, stock, and recoil. The recoil modes would force you to add weight to the system to reduce recoil displacement. But think about the share of the target dispersion that could be affected due to the load tuning. Only certain frequencies are within the reach of the load tuning. A key concept is to remember that amplitude is just as important as frequency when all is said and done. YMMV.

 

[357Max]

I'm going to make a comment that is about drawing a parallel only it will be obvious that the two things cannot be compared - sort of.

In a nuclear weapon, all of the stuff that makes all of that energy happens within about 570 to 590 nanoseconds - about two thirds of a microsecond, about 0.00057 milliseconds, about .00000057 seconds. When that amount of time has passed, all of the energy that is going to be liberated has been liberated and everything that happens after that is just consequences, not generation. For comparison, this about 1,000 times faster that the chemical that we see with smokeless powder.

According to Gordon's, in our rifles the powder burn is all over in about 1.3 milliseconds. By that time, the bullet is almost out of the barrel. When the firing pin dropped on the primer a shock wave started in the barrel steel. That shock wave traveled down the barrel, bounced off the muzzle, and reflected back to the breach. As the bullet leaves the barrel, that shock-wave cycle has happened about five and a half times. Inside the barrel, the pressure curve doesn't peak until about 400 microseconds. The initial impetus that forces the bullet into the barrel must happen in some small number of microseconds - something like 50, maybe less.

Like you, after a millisecond I think that the case contribution to ballistics is long over but those initial few microseconds must matter else all of our case prep work would make no difference. And because of vast amounts of testing, we know that case prep does make a difference. It is my guess that the combination of case shape versus chamber shape, powder ignition, pressure generation as the powder flashes, and bullet shape being forced into the lands is critical. I leave it to smart people to figure it out but I think that it is fascinating to contemplate.

Kind of a semi related side tangent that gets my brain twisted.

At about .44 milliseconds or 3.2" bullet travel the projectile breaks the sound barrier creating yet another shock wave. That shock wave cannot clear the bullet like it does in the air. That sonic shock wave is probably traveling 1.3 mack so again the accelerating bullet out runs it stacking shock waves in the barrel. What affect does this have??? Any of these shock waves that beat the bullet to the muzzle change signs @ muzzle and return to smack the bullet (still in the bore). I have no answers, but this is a question I've never really seen much if any research on.

What got me thinking about the above. I have a rifle 6.5 Creedmoor that shot factory Berger 140 Hybrid ammo like crap as in 1.5-2 moa. Really had me scratching my head. Hornady factory 147 ELDM shot .75 moa. I switched brakes from an Area 419 Hellfire to a Precision Armament and the 140 Berger is now an honest .5 - .75moa. Could it be the weight, sure I suppose but these breaks are very close in weight. Got me thinking it may have more to do with how the port geometry deals with the shock wave in front of the bullet vs behind.

Edit to add: How much of barrel timing has to do with getting the bullet to exit between sign wave changes of the shock wave in front of the bullet????

Hell let me tag someone that might know. @Jim Boatright

 

[Precision Underground]

My personal suspicion is this: That the custom rifles the serious shooters use have gotten so damn good that, for many of them, it really doesn't matter what you feed them. The just plain shoot everything 0.5moa or better, and its almost impossible statistically to see differences in load variables in groups that small.

This leads those shooters to conclude load development doesn't matter. But if they went back to the days of trying to wring 0.5" groups out of a factory Savage model 10, they'll suddenly remember where all those "wives-tales" came from.

To me, it is indisputable that you can take any factory rifle and feed it factory ammo, and you'll discover there are certain ammo options that a rifle "likes" and other ones that shoot like shit. Ive also seen too many factory rifles shooting 1.5" groups with factory ammo, that I was able to get down below 0.75 or lower by handloading.

Edit to add: I highly doubt, in those factory rifle examples, that my handloading techniques were twice as consistant as factory ammo. So consistancy of handloads isn't enough of an explanation.

This.