Military round comparisons

[BjornF16]

I think the curve starts to take a dip between 16" and 14.5", but not so much between 14.5" and 12.5". Just like a 16" and 18" are hard to distinguish from each other, just like a 20" and 24" are indistinguishable when looking at velocities. 14.5" velocities for a 123gr AMAX are 2350fps, while they are 2450fps from a 16". The best velocity Hornady lists with the 123gr AMAX is 2500fps from an 18". The average loss I have seen with identical loads between a 24" and 16" was 18fps.

My suspicion is that this is a factor of gas system length, more than just barrel length. You'll notice that the guns that use rifle-length gas systems produce the same velocities (20", 22", and 24"), just like the mid-lengths (18" & 16"), and carbine lengths (14.5" & 12.5"). If true, the argument can be made that the shortest barrel length practical with the gas system used is most efficient.

What about a mid-length 14.5" like the AA Ultra-light?

I'm still awaiting parts or I'd be able to answer.