Re: AR 15 barrel length question?
Albtraum, you're on track with me in general but I should have clarified a bit more, I'm talking from the target rifle stance:
17" was the well accepted number thrown around the smallbore target shooting arena in my younger days, I have no facts to back that up. However, even a 100 fps difference means there is a difference. Like you said and others here, it may be a small difference so technically.... but 17 was a technical reference to where the bullet stopped gaining performance in both accuracy and velocity.
Velocity was not the only factor for that 17" theory; it was all things related to smallbore target shooting. velocity was one but also bullet stability. And I should have clarified standard velocity ammo for this as well, no target shooter is using Hi-vel stuff, not sure how much difference there is there as far as max barrel length.
Black powder guns generally need more barrel for the relatively slow burning 2 or 3F powder to develop good velocities. I've no personal experience with the 'modern' BP stuff.
but you are correct in saying that extra barrel length will also increase accuracy because of weight/balance. but only when you are talking position shooting; put it on a bench and that factor is gone completely, just as you stated. And remember, we're assuming the shooter is a 400-40x capable (potentially anyway) so I'm talking only about the gun's capability, not the shooter or his technique.
Once you reach a certain point, (say 14-16" in the AR), we're talking a much bigger emphasis on length for the job vs. extra accuracy or velocity for that 2-4" difference. If you're clearing rooms, stay short; if you're needing a long range performer, stay on the longer side to maximize performance. Just realize it is often a small difference.
"Loud noises don't end gunfights.... well placed shots do."