Any quantitative data on how much difference does a couple thou shoulder bump make?

[little_scrapper]

I was thinking about how much difference neck length on tension/velocity makes. All my keepers shoulders are bumped to 1.622 (+- .001"). I culled everything outside that range. However, my OAL on keepers is from 2.000" to 2.005". I trimmed the few longer ones to 2.005" and left everything under that as is.

Asked myself how much difference would .005" make in regards to neck tension/velocity. Divided the max difference, .005" by the neck length roughly measured at .285" = 1.7%. And if I just "say" my OAL's are 2.0025" then my variance is only +- .8% from that center. Makes me feel better about them.

Now for shoulders? I culled any cases that got bumped shorter than 1.621". I have quite a few and some as low as 1.619" These will end up in a different ladder as the number of cases that length grows. So along the same lines as neck length vs neck tension as it relates to velocity? How much pressure/velocity difference does a few thou in shoulder bump make?

Like my 1% neck area I am wondering if I really need to bother culling that tight for a gasser. I get it! for loading precision EVERYTHING should be the same. I get that. I do have a chrono now and I am making detailed notes of what I did. Eventually I will get my answer but in the mean time..... What do we know about it? Do we have an idea of how much difference in velocity a few thou in shoulder bump makes in 308 win?

P.S. Sorry for the long post.

 

[Dthomas3523]

In a very basic sense, a case that is using energy to blow the shoulders out more will be a bit slower than a case not blowing shoulders as much.

I’m sure you will be able to see a quantifiable velocity difference (as long as you keep everything else static except the shoulder).

At that point, the individual discipline and distance being shot will be the determining factors in whether or not it matters enough to care about.

 

[Supersubes]

I dont think you’ll see any variance in speed or accuracy. There are other factors that affect the very things you’re trying to account for, like varying depth of the inside neck chamfer, or varying primer seating depth/pocket depth. None of which make really any difference, even in a bolt gun.

 

[little_scrapper]

From what I have read same amount of powder in a smaller volume, shoulders bumped back farther, creates higher pressures and higher velocities. I did find one thread here where a guy was chronoing loads with shoulder variances and the shortest cases head to shoulders were producing the higher velocities.

 

[little_scrapper]

I have a couple charge weights that look promising but now I picked up an Atlas CAL. Was shooting off bags. That will probably throw everything out the window. LOL I may have to run a whole new ladder

 

[Supersubes]

From what I have read same amount of powder in a smaller volume, shoulders bumped back farther, creates higher pressures and higher velocities. I did find one thread here where a guy was chronoing loads with shoulder variances and the shortest cases head to shoulders were producing the higher velocities.

Usually it’s the opposite. It’s counterintuitive, but the smaller brass is somehow changing the pressure curve, softening the peak pressure by acting like a buffer perhaps. Virgin brass for example is always slower in my experience.

 

[Dthomas3523]

Usually it’s the opposite. It’s counterintuitive, but the smaller brass is somehow changing the pressure curve, softening the peak pressure by acting like a buffer perhaps. Virgin brass for example is always slower in my experience.

Typically because with virgin or smaller brass, some of the energy is being used to push out the shoulders. A lot of times I see about .005 movement from virgin to fired brass.

Bump it back .001-.003 and that’s anywhere from 20%-80% more difference in movement in the shoulder between virgin and fired brass.

So, once it’s fired, the shoulders may only move .002 now, so they blow out faster and the energy it was using on virgin brass shoulders is now pushing the bullet.

Then case capacity comes into play. But you’d likely have to bump the shoulders pretty far to start messing with capacity in a meaningful way.

 

[Supersubes]

It would be neat to make some comparisons with pressure trace gear.

 

[SPAK]

Interested in this as well.

For the first time I’ve encountered a significant mismatch in my chamber and full length sizer where I need to bump shoulders back .007” in order to chamber properly. Neck sizing should take care of this for a few firings but that leads to the issue of this topic. What will be the impact on my velocity and on target performance between neck sized vs shoulders bumped .007”?



My initial thought was no impact since brass has to expand to fill the chamber and build up pressure before the bullet even starts to travel. But some of the comments here actually have me thinking twice about it.



I may actually have to test this over the chrono and on target.



(Before anyone starts telling me I’m sizing my brass wrong, I’ve been reloading for several precison rifles for almost 11 years in 8 different calibers. My die has been inspected by Redding and compared with my fired brass. It’s a definite mismatch at the body shoulder junction by about .001” until I push the shoulders back .007”)

 

[47guy]

It would be neat to make some comparisons with pressure trace gear.

this right here^^^i think pressure trace equipment will tell you more than a chrory will...i also think that a shoulder will have to be pushed back quite a bit before it would really matter...i also think that in order to run a test like this you would have to do ALL the brass prep to get meaningful results.

 

[Dthomas3523]

Interested in this as well.

For the first time I’ve encountered a significant mismatch in my chamber and full length sizer where I need to bump shoulders back .007” in order to chamber properly. Neck sizing should take care of this for a few firings but that leads to the issue of this topic. What will be the impact on my velocity and on target performance between neck sized vs shoulders bumped .007”?



My initial thought was no impact since brass has to expand to fill the chamber and build up pressure before the bullet even starts to travel. But some of the comments here actually have me thinking twice about it.



I may actually have to test this over the chrono and on target.



(Before anyone starts telling me I’m sizing my brass wrong, I’ve been reloading for several precison rifles for almost 11 years in 8 different calibers. My die has been inspected by Redding and compared with my fired brass. It’s a definite mismatch at the body shoulder junction by about .001” until I push the shoulders back .007”)

I‘d bet that you’ll get slower chrono speed with the .007.

 

[spife7980]

Ill bet it’s faster.

 

[Milo 2.5]

It would be neat to make some comparisons with pressure trace gear.

Do you feel the trace equipment available at reasonable costs to the majority of us will reveal much? Seems like more hassle than it is worth to me.
There are so many variables in this shit, even talk of testing is getting to be redundant. And in this case, uniformity of brass stand alone, plus most of us never get brass to full chamber size before messing with it, so it takes a full 3 firings before expecting same dimension of brass. Then add, how good was the load to start with, are variables even going to be visible.
Unless we are willing to go through a lengthy culling of the brass we choose to shoot, there is too much built into brass to cause discrepancies on it's own.

 

[918v]

From what I have read same amount of powder in a smaller volume, shoulders bumped back farther, creates higher pressures and higher velocities. I did find one thread here where a guy was chronoing loads with shoulder variances and the shortest cases head to shoulders were producing the higher velocities.

There are other reasons to explain it. Case hardness, for one.

 

[little_scrapper]

Not only that but I am not bumping my brass past .005". 90% of my brass comes out at 1.626 measured on a comparator. 1.621 is an absolute hard stop for me. Nothing shorter goes into my rifle. I try to keep it 1.623" strong. That being said I only use NATO M118LR brass so it can probably handle the extra expansion.

 

[Rlandry]

No where in this thread do I see any mention of head space. Using shoulder bump the get more velocity ???
Do you even know what your head space measurement is?

 

[Supersubes]

Do you feel the trace equipment available at reasonable costs to the majority of us will reveal much? Seems like more hassle than it is worth to me.
There are so many variables in this shit, even talk of testing is getting to be redundant. And in this case, uniformity of brass stand alone, plus most of us never get brass to full chamber size before messing with it, so it takes a full 3 firings before expecting same dimension of brass. Then add, how good was the load to start with, are variables even going to be visible.
Unless we are willing to go through a lengthy culling of the brass we choose to shoot, there is too much built into brass to cause discrepancies on it's own.

If you have a laptop, the gear and software is about $700(strain gauge type). There are definitely some Interesting things going on, which I’d like to put instrumentation to. You’d only need small samples of brass though, which you could start at virgin, and progressively work through full truing.

 

[SPAK]

Here’s what I do know: pulling harder into my shoulder influences velocity over the chrono... how much shoulder bump in the case influences it I’m not sure. I’m still leaning towards the difference is negligible but I am curious.

 

[47guy]

Here’s what I do know: pulling harder into my shoulder influences velocity over the chrono... how much shoulder bump in the case influences it I’m not sure. I’m still leaning towards the difference is negligible but I am curious.

how much does shoulder pull vary velocity?

 

[little_scrapper]

I know on my AR10 the difference between a snug hold into my shoulder, and a harder hold leads to a 1" drop in my POI at 100 yards. That shoulder tension makes a real difference. Maybe not as much on a tighter and heavier bolt gun but I sure can see the difference.

 

[47guy]

I know on my AR10 the difference between a snug hold into my shoulder, and a harder hold leads to a 1" drop in my POI at 100 yards. That shoulder tension makes a real difference. Maybe not as much on a tighter and heavier bolt gun but I sure can see the difference.

i do not shoot gas guns but that sounds like a pretty big difference to me and the opposite way id think it would be...the only time i see that kind of difference is if im laying on the gun and impacts high....interested to hear if others see the same thing as you.

 

[Supersubes]

I know on my AR10 the difference between a snug hold into my shoulder, and a harder hold leads to a 1" drop in my POI at 100 yards. That shoulder tension makes a real difference. Maybe not as much on a tighter and heavier bolt gun but I sure can see the difference.

That’s Newton’s 3rd law screwing with you, and not velocity. You’re affecting how the gun moves during the shot. An additional 1” drop at 100 yds would probably take 150-200 fps, and there’s no way you’re losing that much. The tests I’ve seen between free recoil and a hard hold velocity variation range from zero change, to a few FPS.

 

[j-dubya]

I know on my AR10 the difference between a snug hold into my shoulder, and a harder hold leads to a 1" drop in my POI at 100 yards. That shoulder tension makes a real difference. Maybe not as much on a tighter and heavier bolt gun but I sure can see the difference.

Same for me, if I’m loading hard through the bipod. Keeping the bipod load consistent but then a “harder” pull into the shoulder or or somewhat shrugging my shoulder tighter around the stock (if that makes sense) results in 1” high at 100m. This is with a 8.6 lb gas gun. Heavier guns aren’t quite as temperamental, but will still follow the trend for me.

 

[Rocketmandb]

That’s Newton’s 3rd law screwing with you, and not velocity. You’re affecting how the gun moves during the shot. An additional 1” drop at 100 yds would probably take 150-200 fps, and there’s no way you’re losing that much. The tests I’ve seen between free recoil and a hard hold velocity variation range from zero change, to a few FPS.

This, this, and more this.

 

[47guy]

That’s Newton’s 3rd law screwing with you, and not velocity. You’re affecting how the gun moves during the shot. An additional 1” drop at 100 yds would probably take 150-200 fps, and there’s no way you’re losing that much. The tests I’ve seen between free recoil and a hard hold velocity variation range from zero change, to a few FPS.

this is what ive found playing with different holds.

 

[6brshooter]

I can say that rds that chamber harder will shoot our of the group. Saw that in br.

 

[little_scrapper]

I assumed that holding tighter, I was preventing rearward motion and some lift. Because when I hold tight vs just snug I hit an inch lower.

 

[j-dubya]

I should have mentioned above that all the POI shift with different loading/holding pressures has shown nothing quantifiable velocity-wise. Everyone one of my shots when playing with that stuff goes across the Labradar.

 

[Rocketmandb]

I assumed that holding tighter, I was preventing rearward motion and some lift. Because when I hold tight vs just snug I hit an inch lower.

I did three YouTube videos relating to this. First one:



Basically, a hard shoulder reflects more recoil back into the rifle more quickly - meaning more while the round is still in the barrel. The third one depicts how it can alter POI.

 

[SPAK]

how much does shoulder pull vary velocity?

from what I remember in my notes years ago 30 fps on avg over the chrono.. nothing I was really worried about.

also agree that varying shoulder tension and muscling the rifle tended to produce different poi, but not from velocity.

 

[abn31c]

I agree with @SPAK on this. I did just enough chrono testing on my 6.5 cm to prove to me that free recoil is 28 fps slower than my normally shouldered bipod loaded shot. But more important group shifted about .7 high on the free recoil shots.

 

[little_scrapper]

1.621 is an absolute hard stop for me. Nothing shorter goes into my rifle. I try to keep it 1.623" strong.

I retract this statement. I just measured about 10x of my M118LR and they were all between 1.620" adn 1.622". I then measured ten Federal GM Bergers 185gr and all were in the exact same range from 1.620 - 1.622" with most on the lower end.

I think my shortest M118LR shoulder bump was 1.619 so I think those are still ok. I was getting ready to trash 20-30 prepped cases because they were all1.619 to 1.621".

 

[Supersubes]

I retract this statement. I just measured about 10x of my M118LR and they were all between 1.620" adn 1.622". I then measured ten Federal GM Bergers 185gr and all were in the exact same range from 1.620 - 1.622" with most on the lower end.

I think my shortest M118LR shoulder bump was 1.619 so I think those are still ok. I was getting ready to trash 20-30 prepped cases because they were all1.619 to 1.621".

Of course you can shoot them that way, you just don’t want to do that repeatedly. One to two thousandths bump will do.

 

[little_scrapper]

I am about to order a set of Redding competition shell holders, S type FL sizing die and a competition micrometer seating die.

Probably just sell my basic 3 die set. I need to figure out what bushings to order also.