Not sure what the muzzle velocity of this type of ammo might be when shot from a pistol, but is bound to be much less effective at penetrating body armor than when fired by a rifle. Has anyone actually tested this?
The "armor piercing" designation has nothing to do with ability to pierce armor.
It only is a matter of having a "core" made out of certain materials.
And what's at issue here is what constitutes a "core", which is undefined in law, and which the BATFE is straining to apply to the steel "tip" of M855.
The M-4gery's 16 inch barrels give higher velocity that the M-4.