It doesn't make a difference to my point
I'll break this down for you and try to be as simple as possible; the AR-15 is the semi-auto version of the military M-16 rifle, although most AR-15s now more closely resemble the M-4 carbine in features.
What this means is, that the AR-15 will fire one round each time the trigger is depressed, after the weapon has the intial round manually chambered.
M-16s and variants are more of a long-range weapon, where as the M-4 carbine is a more close-quarters weapon designed primarily for urban combat situations. The military versions will fire in either semi-auto, 3-round burst or full-auto, although the full-auto versions have been mostly phased out.
(I'm ex-military, former weapons instructor and maintainer for Army, Navy and USAF personnel.)