I've read that the problem with the 5.56x45 ammo is when it's being shot out of barrels less than 20 inches, cause they lose velocity
the shorter the barrel, the more velocity is lost....but the military uses 20" and 16" barrels.......and really the velocity drop between them is not enough to matter...something like 50fps maybe give or take......and this slight alteration in velocity isn't going to have a significant effect on a rounds "knock-down power"