Velocity- Accuracy Relationship Question

[Harronek]

I’m a reloader who has never really dived deeper than producing accurate hunting ammo .

I have a question to ask about the velocity - accuracy relationship of loads .

For example
You have a chosen load you have developed that is accurate and safe with brand X powder/projectile/primer/brass
You chronograph that load and get it’s average speed , lets say 2700fps for argument’s sake.

You then change powders to brand Y but change nothing else ( same seating depth ) and find a powder charge weight with that new powder that duplicates the 2700fps average.

Will that load match the first load in regards to accuracy/ performance or will there be a difference?
Does the burn characteristics of a powder / case fill percentage / pressure characteristics etc effect the accuracy / barrel harmonics of the load even if the projectile is leaving the muzzle at the exact same velocity ?

Just thought I’d ask those who are way more knowledgeable than me on these subjects.

 

[straightshooter1]

I’m a reloader who has never really dived deeper than producing accurate hunting ammo .

I have a question to ask about the velocity - accuracy relationship of loads .

For example
You have a chosen load you have developed that is accurate and safe with brand X powder/projectile/primer/brass
You chronograph that load and get it’s average speed , lets say 2700fps for argument’s sake.

You then change powders to brand Y but change nothing else ( same seating depth ) and find a powder charge weight with that new powder that duplicates the 2700fps average.

Will that load match the first load in regards to accuracy/ performance or will there be a difference?
Does the burn characteristics of a powder / case fill percentage / pressure characteristics etc effect the accuracy / barrel harmonics of the load even if the projectile is leaving the muzzle at the exact same velocity ?

Just thought I’d ask those who are way more knowledgeable than me on these subjects.

When I use just a different powder (even a different lot of the same powder that's significantly different) I do try and match the same velocity (or "Barrel Time" if you will). Because different powder's burn different having different internal ballistic characteristics, just having the same MV has not given me the same accuracy. But, it does give me a good starting point where I can make a small adjustment in powder weight and/or adjustments in seating depth to bring back the accuracy. I will use the app QuickLoad first to help me determine what adjustment(s) I might need to make first, then go to the range and get the actual MV's and make adjustments accordingly.

 

[Yondering]

I’m a reloader who has never really dived deeper than producing accurate hunting ammo .

I have a question to ask about the velocity - accuracy relationship of loads .

For example
You have a chosen load you have developed that is accurate and safe with brand X powder/projectile/primer/brass
You chronograph that load and get it’s average speed , lets say 2700fps for argument’s sake.

You then change powders to brand Y but change nothing else ( same seating depth ) and find a powder charge weight with that new powder that duplicates the 2700fps average.

Will that load match the first load in regards to accuracy/ performance or will there be a difference?
Does the burn characteristics of a powder / case fill percentage / pressure characteristics etc effect the accuracy / barrel harmonics of the load even if the projectile is leaving the muzzle at the exact same velocity ?

Just thought I’d ask those who are way more knowledgeable than me on these subjects.

If you're changing a major component, be it powder, bullet, primer, etc, then no, you can't really assume that the velocity of an accuracy load with powder A will be the same velocity as the accuracy load for powder B.

There are times when you can adjust to a specific velocity and expect accuracy to follow, within a narrow window. The main example of this IME is when dealing with temp stability of your powder. If your accuracy load in the winter is 2,700 fps, reducing the powder charge (leaving all other components and load details the same) to match that 2,700 fps in the summer seems to work for me, in most cases. But in that case we're only talking about a minor change in charge weight and burn characteristics. If you use a completely different powder, I don't think you can expect the same effect unless powder A is very similar to powder B.

Hope that helps.

 

[Doom]

I’m a reloader who has never really dived deeper than producing accurate hunting ammo .

I have a question to ask about the velocity - accuracy relationship of loads .

For example
You have a chosen load you have developed that is accurate and safe with brand X powder/projectile/primer/brass
You chronograph that load and get it’s average speed , lets say 2700fps for argument’s sake.

You then change powders to brand Y but change nothing else ( same seating depth ) and find a powder charge weight with that new powder that duplicates the 2700fps average.

Will that load match the first load in regards to accuracy/ performance or will there be a difference?
Does the burn characteristics of a powder / case fill percentage / pressure characteristics etc effect the accuracy / barrel harmonics of the load even if the projectile is leaving the muzzle at the exact same velocity ?

Just thought I’d ask those who are way more knowledgeable than me on these subjects.

You've asked this question elsewhere and received responses with justification. You got similar answers here as above. The answer is it may or may not be the same or similar velocity that works well with the new powder. The variables of powder case fill (density and heat of explosion) pressure characteristic (burn rate), case fill (related to density and heat of explosion), burn percentage (burn rate), barrel harmonics (barrel time) are all related to accuracy. The bullet itself is not sensitive to velocity as long as stability is sufficient and not extreme.

 

[Doom]

@Harronek,
My post above may be a little harsh. It wasn't meant to be. It sounds like you are trying to use a chronograph as a crutch to bypass load development. It is not advisable to take that approach. You may spend a lot of time and components trying to match velocity and find that effort successful but not find accuracy. However, it's not totally unreasonable to start in that area for similar powders, like Varget (ADI 2208) and IMR 4064 in 308. They are very similar in burn rate and density. Trying the same comparison between IMR 4064 and Alliant 2000MR isn't going to work in all probability because the powders are different burn rates and actually burn differently. One is progressive and one is digressive.

Putting it another way is to say that vetocity doesn't determine accuracy/precision. The proper combination of all of the factors you mention and a few others determine the resulting velocity.

Hope this helps.

 

[alamo5000]

You chronograph that load and get it’s average speed , lets say 2700fps for argument’s sake.

You then change powders to brand Y but change nothing else ( same seating depth ) and find a powder charge weight with that new powder that duplicates the 2700fps average.

Will that load match the first load in regards to accuracy/ performance or will there be a difference?
Does the burn characteristics of a powder / case fill percentage / pressure characteristics etc effect the accuracy / barrel harmonics of the load even if the projectile is leaving the muzzle at the exact same velocity ?

There will be a difference in about 99.8% of the situations. Although you may match velocity in some cases, but different powders have drastically different characteristics. There is a lot of sciency voodoo that goes beyond my individual knowledge base when it comes to how powders are made and what characteristics they have.

The people that create powders know about the relationships to the point that they can tune powder for a cartridge and even a specific bullet weight range.

For example powders not only burn at different rates, but they also have differences in density. Some might be more dense and others might be a lot more fluffy. This has a huge impact on a given cartridge and how consistent it might be with any number of variables. Then add in different burn rates and a whole lot of other factors and odds are better than average that you will not get the same results.

THEN on top of that--the rifle itself--which is a whole other animal will behave differently to a given combination. Getting super 'accurate' is basically the science of getting a rifle in tune. Barrels have harmonics and they essentially vibrate throughout the shooting sequence. "Tuning" is trying to make sure that the bullet exits the bore at the same point every time during the harmonic sequence.

If you just change one thing--say you get a slightly slower or faster powder-- and even though you get the same velocity---HOW you get to that velocity is a little bit different.

 

[Rust]

Even if your velocity is the "same" your ES (extreme spread) and SD (standard deviation) may differ enough to be problematic as range increases. In developing one match load several powders in the same range safely achieved a decent velocity which was my target velocity but only one was best for 1,000 yard competition. All other components were the same. At a couple of hundred yards the difference would have been a moot point.

The more accuracy and consistency you want, the more crucial minute differences become.

 

[bax]

"You then change powders to brand Y but change nothing else ( same seating depth ) and find a powder charge weight with that new powder that duplicates the 2700fps average."

The reason a given load shoots well in a given rifle (ignoring such things as the shooter, rest, wind, etc) is the timing of the bullet exiting the barrel and where and how the rifle, but mostly the barrel, responds to the shock of firing. Using the new load, if the same bullet "model" (for example, Hornady, 30 caliber, 230 grain, A-Tip or Sierra Match King, 30 caliber, 175 grain) exits the barrel at the same time and with the same velocity in both cases, they should hit in the same place - assuming that the barrel vibrates the same way for both powders. If the vibrations are different, the bullets will go to different places. They may both be accurate (that is, shoot small groups) but with different points of impact.

Those assumptions with duplicate timing, velocity, vibration pattern and amplitude are almost certainly not achievable. If you change only powder, you need to tune that brass, primer, powder, and bullet combination for that gun. At the very least, you need to check. Most likely, the barrel vibration patterns will be different and point of impact and accuracy will change.

Lately, following conversations here, I have been playing with seating depth. It matters. With my 308 rifle and changing only seating depth, the best seating depth groups are just under half an inch, the worst are about 1.25 inches. This is repeatable with only seating depth changes.

 

[Bbracken667]

If you're changing a major component, be it powder, bullet, primer, etc, then no, you can't really assume that the velocity of an accuracy load with powder A will be the same velocity as the accuracy load for powder B.

There are times when you can adjust to a specific velocity and expect accuracy to follow, within a narrow window. The main example of this IME is when dealing with temp stability of your powder. If your accuracy load in the winter is 2,700 fps, reducing the powder charge (leaving all other components and load details the same) to match that 2,700 fps in the summer seems to work for me, in most cases. But in that case we're only talking about a minor change in charge weight and burn characteristics. If you use a completely different powder, I don't think you can expect the same effect unless powder A is very similar to powder B.

Hope that helps.

I do the same thing. I load very small batches that are designed for the ambient temps I expect. Fairly simple to adjust using a temp sensitivity chart.
In those situations where I loaded and did not use the ammo immediately (Covid era) I made note of the temp range loaded for and then placed the ammo in a cooler to chill it down before using in the summer lol. Eliminates any worries that may arise from overcharge due to heat.

 

[Rogerthatout]

Hey guys, I’m seeing some pretty wonky things with my .223, shooting 75ELDs in hot temperature. Temp is 85 outside, sent about 10 or 15 rounds down range, barrel felt pretty hot to the touch , and at 600 yards I was seeing POI about .5 mil high.

This bullet is a sensitive bullet, and I know the caliber to pretty finicky too, just wondering if this is normal or should I be scratching my head on this?

FYI, shooting a .223 BARTLEIN MTU contour chambered by SAC so pretty confident we can rule out poor quality barrel or hunting profiles.
Shooting 24.25 gr varget, 75 ELDS and I was trued out to 1000 yards before running hot.

I’m going back out in the morning to see if my cold bore settles back in at 600 and 1000.

Do I need to collect data on barrels temperature to be able to predict DOPE going forward? Ie, barrel temperate should be collected for powder sensitivity in my kestrel calculations ? I don’t care if I run hot and high as long as I know why and if it’s predictable.

 

[straightshooter1]

Hey guys, I’m seeing some pretty wonky things with my .223, shooting 75ELDs in hot temperature. Temp is 85 outside, sent about 10 or 15 rounds down range, barrel felt pretty hot to the touch , and at 600 yards I was seeing POI about .5 mil high.

This bullet is a sensitive bullet, and I know the caliber to pretty finicky too, just wondering if this is normal or should I be scratching my head on this?

FYI, shooting a .223 BARTLEIN MTU contour chambered by SAC so pretty confident we can rule out poor quality barrel or hunting profiles.
Shooting 24.25 gr varget, 75 ELDS and I was trued out to 1000 yards before running hot.

I’m going back out in the morning to see if my cold bore settles back in at 600 and 1000.

Do I need to collect data on barrels temperature to be able to predict DOPE going forward? Ie, barrel temperate should be collected for powder sensitivity in my kestrel calculations ? I don’t care if I run hot and high as long as I know why and if it’s predictable.

Temperature can certainly change you're POI as powder burn rate will increase with temperature and how much it may increase will depend on the particular powder you're using. Some powders are pretty temperature stable while others are very temperature sensitive. For temperature sensitive powder, increased temperature increases velocity which will cause POI to shift upwards. You can see this phenomenon when using a chrono during shooting.

 

[Doom]

Not knowing the actual temperature that you trued your ballistic dope to I can only speculate that you are seeing predominately the ambient temperature effect of the higher temperature on your drop. Higher temperature, less density in the air and less bullet drag.

 

[mrcheesemoose]

From my experience, the biggest factor when it comes to accuracy is the barrel, powder, bullet in that order. So if you change powder brands, even with the same velocity, you are very likely going to have a different accuracy

 

[TSULLI]

Will that load match the first load in regards to accuracy/ performance or will there be a difference?

Not necessarily. There can be differences. Velocity is only 1 metric used to determine the success of a load. Just because your velocity is similar or even exactly the same does not mean that everything else is the same. Testing and re testing is crucial when developing a new load especially if used in a hunting application. You owe it to that animal to know for a fact that your bullet will hit exactly where it should at any distance.

Does the burn characteristics of a powder / case fill percentage / pressure characteristics etc effect the accuracy / barrel harmonics of the load even if the projectile is leaving the muzzle at the exact same velocity ?

Yes. While everything might be relatively close, the fact remains that you have fundamentally changed that load. There is no way to know if that new load will work in your rifle without testing it.

 

[Yondering]

From my experience, the biggest factor when it comes to accuracy is the barrel, powder, bullet in that order. So if you change powder brands, even with the same velocity, you are very likely going to have a different accuracy

3 B's: Barrel, bullets, and bedding.

Powder choice and other load details are secondary to those primary 3. Occasionally a particular combination will show a large change from a different powder or primer choice, but that's more the exception than the rule IME.

But that doesn't mean you'll get the same accuracy from any powder at a given velocity of course.