Velocities - Max Loads - Barrel Lengths

[IDT]

Ok - my coffee hasn't kicked in yet, so I don't know if I can adequately explain my question:

95% of all the published load data I can find or have uses a 24" barrel for their testing of .223/5.56 I have a 16" RECCE barrel chambered in 5.56 with a 1:8 twist.

I'm not a novice re-loader...just a novice in rifle case reloading. I've been doing this for years with pistols - but I've never delved into the the technical aspects of this until recently; so I have a few questions:

1st Question:
On Alliant's Powder Reload Recipes it calls for 25.6gr of AR-COMP to achieve 3,359ft/s with a Sierra 55gr HPBT - but this is tested from a 24" barrel.
Using a 16" barrel, and knowing that the 25.6gr of powder is the MAX load - (without using a chrony) what can I expect the ft/s drop to be?
Is there a formula to calculate the ft/s drop when using a barrel that is 8 inches shorter than a test barrel?

2nd Question:
Because the internal pressures would be less due to the shortened barrel length (16" v. 24") can the load be increased past the 25.6gr of powder -- or is that considered to be the MAX load regardless of barrel length?

3rd Question:
If a 52gr Sierra HPBTfrom a 24" barrel will MAX at 3,434ft/s with 25.2gr of AR-COMP powder - then knowing that a smaller grain bullet normally takes more powder to reach the same velocities, what would be an expected powder charge to push a 50gr bullet at the same velocity from a 16" barrel; or will that velocity be impossible to reach before maxing out the powder load due to the shortened barrel length?

Also, this is for the sake of math on paper. I fully intend on starting 10% lower than the published rates, using the chronometer and working up...however, I want to see if my achieved results are on par with expected results.

[AllenJ]

what can I expect the ft/s drop to be?
Is there a formula to calculate the ft/s drop when using a barrel that is 8 inches shorter than a test barrel?

There is no formula that is always right, but figure 15-25 fps for every inch of barrel as a general rule of thumb.

Because the internal pressures would be less due to the shortened barrel length (16" v. 24") can the load be increased past the 25.6gr of powder -- or is that considered to be the MAX load regardless of barrel length?

It is considered to be a MAX load in their test gun, not your rifle. Following safe reloading procedures you may well be able to exceed their max, and you may not, only your gun is going to be able to tell us. Pick up 3 different reloading manuals and you'll see 3 different max loads for the same bullet weight.

or will that velocity be impossible to reach before maxing out the powder load due to the shortened barrel length?

My money is that you'll reach your guns max load way before reaching the same velocity as you'd get out of a 24" barrel.

[Brian Pfleuger]

Originally Posted by IDT

1st Question:
On Alliant's Powder Reload Recipes it calls for 25.6gr of AR-COMP to achieve 3,359ft/s with a Sierra 55gr HPBT - but this is tested from a 24" barrel.
Using a 16" barrel, and knowing that the 25.6gr of powder is the MAX load - (without using a chrony) what can I expect the ft/s drop to be?
Is there a formula to calculate the ft/s drop when using a barrel that is 8 inches shorter than a test barrel?

2nd Question:
Because the internal pressures would be less due to the shortened barrel length (16" v. 24") can the load be increased past the 25.6gr of powder -- or is that considered to be the MAX load regardless of barrel length?

3rd Question:
If a 52gr Sierra HPBTfrom a 24" barrel will MAX at 3,434ft/s with 25.2gr of AR-COMP powder - then knowing that a smaller grain bullet normally takes more powder to reach the same velocities, what would be an expected powder charge to push a 50gr bullet at the same velocity from a 16" barrel; or will that velocity be impossible to reach before maxing out the powder load due to the shortened barrel length?

Also, this is for the sake of math on paper. I fully intend on starting 10% lower than the published rates, using the chronometer and working up...however, I want to see if my achieved results are on par with expected results.

1)The bullet is accelerating more in the early part of the barrel than it is later... It loses more fps per inch, the more you cut. In other words, going from 24 to 23 will generally be less than going from 17 to 16. Still 1 inch but the bullet is going faster and accelerating less at 23 than it was at 16. I would expect the average to be around 20fps per inch for 223.

2)Barrel length has ZERO impact on peak pressure. None. Peak pressure occurs in the first inch or two or three of bullet travel. So, if your barrel is longer than that, peak pressure is unaffected.

3)The difference between 50 and 52gr is irrelevant. You will never, ever, ever get close to 24" velocity in that 16" barrel... except maybe for ONE shot and by pulling the trigger with a string from a distance.... the gun will be no more.

[math teacher]

+1 Well stated Brian.

[SL1]

Using QuickLOAD to get in idea of what to expect:

My QuickLOAD data base does not include AR-Comp powder. But, by using a similar weight max charge of Vectran SP 10 under a 52 grain Sierra bullet in a 24" barrel, I can get 3,360 fps. Dropping the barrel length to 16" results in 3052 fps.

Just for giggles, here is how the decrease progressed:

24" 3360 fps
22" 3297 fps
20" 3227 fps
18" 3146 fps
16" 3052 fps

So, while the decrease is only about 32 fps per inch at 24" barrel length, but it has gone up to about 47 fps per inch at 16" barrel length.

SL1

[Boomer58cal]

I loose 180 fps with my loads when going from my 24" to my 16.5" 5.56 AR's.


Boomer