Commonly called in real life as M-16, it is made in the United States of America and used in the Vietnam war in the 60's and 70's, it was made as a prototype in 1948 and it uses 5.56 cartridges. Its shorter, automatic fire mode twin known as the M4 (MOO4) exists. Can accept silencers, and a underbarrel M203 grenade launcher. Other variants, like the AR-15 (LOL-15), obviously can exist. Manufactured by Colt in reality.
Can fire in burst fire and single firing modes. It is difficult to repair, and it is poor at short range.
A predicessor to M-16 is the M-14, made in 1957.
Microsoft Sam (Owns a modified MOO-16 with automatic fire and an M203 grenade launcher
Microsoft Mike (Owns a MOO-16 with an underbarrel shotgun)
Adam Demeter (Owns a MOO-16 with a silencer and scope)