Velocity drop vs barrel length

[gemini1]

At Hodgdon's online, Barrel Length: 24" Twist: 1:12 55 Grain SPR SP
Benchmark Max load 25.6 gr Velocity 3264.

I read that velocity drops approximately 50 fps/inch, as the barrel gets shorter. So for a 16" barrel, its about 400 fps. or about 2864 velocity.

For my 16" 1/7 twist middy, my three test loads at 25.3/25.5 and 25.7 has a chrono average of 3013/3028/3059
So is the velocity drop of 50 fps/inch accurate? and is there any safety issue on my barrel getting that high a reading?
A fellow shooter at the range commented that I was getting a pretty high reading.

[blockfort]

Any high pressure signs? If not you should be ok, but super hot/fast rounds may still wear out your barrel faster.

[ar15barrels]

So is the velocity drop of 50 fps/inch accurate?

No.
There is no such thing as a fixed velocity loss/gain per inch of barrel.
That loss/gain per inch of barrel is different depending on length.

For example, you will lose/gain much more velocity between 11" and 12" than you will between 19" and 20".
It's a matter of pressure and velocity and acceleration rate to determine how much velocity will change with a change in barrel length.

Quickload can do these calculations to a great level of accuracy.

[JMP]

That rule of thumb would tend to overstate, especially in this example. 50fps/inch can be roughly applied as it is about 200fps difference going from a 16" to a 20" barrel on an AR. As the barrel gets longer, you generally see diminishing returns. You are comparing to a hypothetical 24" barrel, where results could be higher or lower depending on your actually 24" barrel. However, the change from 16" to 17" may get you 50 fps, but the change from 23" to 24" is not likely to get you 50fps.

And, double check your chronograph with the dope of your actual shots. Chronographs can lie in suboptimal lighting conditions.

ETA: ar15barrels beat me to it.

[stilly]

No.
There is no such thing as a fixed velocity loss/gain per inch of barrel.
That loss/gain per inch of barrel is different depending on length.

For example, you will lose/gain much more velocity between 11" and 12" than you will between 19" and 20".
It's a matter of pressure and velocity and acceleration rate to determine how much velocity will change with a change in barrel length.

Quickload can do these calculations to a great level of accuracy.

Yeah I gotta agree with this.

I tested several rounds out of my ak pistol and the rifle and saw no real difference like was suspected to happen... :\

[gemini1]

Thank you guys. Yeah it does look like there is/are some difference.

On my other test loads, this time for the 77 HPBT pills. There's two datas I looked at.
Nosler online has for a 20" barrel, a max of 24gr Varget, velocity at 2658.
And on Lyman's 49th their max is at 25 grains vel 2788.

My 3 test loads at 24.1 gr showed an AV 2588 (HI 2592). A difference of only 70 fps from Nosler's.
So as long as I dont see any pressure signs, and just go between 24 to 25 grains, it looks like I can match Nosler's 20" barrel velocity, and still be within published data (that of Lyman's), safely on my 16" barrel?
I dont plan on mass reloading my 77 pills, at such hot a load, I already found my sweet spot at a lower load. But since the result I got, seems to show that I can get a carbine size AR, shoot a round that can match the distance a standard rifle length AR would produce?
Then maybe its not a bad idea to keep a few hundred rounds, for that purpose?