The AR-15 is the semiautomatic version of the M16 series of assault rifles, designed by the great designer Eugene Stoner, and it is most commonly used by the US armed forces. I would get into the history of this gun, but it is very longwinded but thoroughly interesting. AR-15s are usually chambered in 5.56 x 45mm (.223 Remington) caliber and have either 24", 20", 16", or 11.5" barrels and have either collapsible stocks or full stocks and may either have a NATO railed flattop or a carrt handle with an adjustable rear sight. This weapon has been the basis of numerous high-accuracy rifles.
The Mac-10 is a submachine gun developed in the mid-1960s by Gordon B. Ingram for use by urban operators. It fires either the .45 ACP or the 9mm x 19 Parabellum cartridge. This gun is reknowm for it's incredible cyclic rate of fire- 1145 rpm. The gun fires from an open bolt (causing minimal accuracy) and it is mainly made of pressed steel. MAC strands for Marietta Armament Corperation, the company that made most of these guns. Another version, the Mac 11, fires the shorter .380 ACP round. Both guns were made to fit a Sionics Company suppressor/silencer.
I cannot stress enough that a MAC 10 is NOT an Uzi. They are two completely different weapons. Get it through your heads.
The MAC-11 is a sweet gun for CQB. But I'd pick an M733 or an MP5 over it anyday.