The M4 is a shorter and lighter variant of the M16A2 assault rifle. The M4 is a 5.56x45mm NATO, air-cooled, gas-operated, magazine-fed assault rifle. It has a 14.5 in (370 mm) barrel and a telescoping stock. The M4 is extensively used by the United States Armed Forces and is largely replacing the M16 rifle in the U.S Army and United States Marine Corps combat units as the primary infantry weapon and service rifle. The M4 is also capable of mounting the M203 and M320 grenade launchers.
Built in 1984, the first models had an upper receiver with an A1 sight and were given a longer 14.5-inch barrel for the bayonet and the M203 grenade launcher. Gas-operated, magazine-fed, selective fire firearms with either a multi-position telescoping stock or a fixed A2 or LE tactical stock the M4 has 80% parts commonality with the M16A2 rifle. The M4 is handy and more convenient to carry than a full-length rifle. It is more compact in size, carries a shortened barrel of 14.5 in, and a telescoping buttstock. Known to be a very reliable firearm, the M4 has become widely used by the U.S military.