Re: AR 15 barrel length question?
In my hi-power service rifle days, 16 was standard (and the reg) for AR's and were plenty capable and very accurate, even at 1000yd matches. No 'external' mods were permitted, probably the same still. Colt with Douglas SS match barrels were pretty popular then. I was an M14 guy so didn't mess around with mouse guns very much until the military.
Some of the space gun guys had bloop tubes so I have no idea what their true barrel lengths were.
"Loud noises don't end gunfights.... well placed shots do."