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The M4 Carbine is a 5.56×45mm NATO, air-cooled, gas-operated, direct impingement, magazine-fed, select fire carbine. It has a 14.5 in (370 mm) barrel and a telescoping stock. It is essentially a lighter and shorter variant of the M16A2 assault rifle.
The M4 is extensively used by the United States Armed Forces and is largely replacing the M16 rifle in United States Army and United States Marine Corps (USMC) combat units as the primary infantry weapon and service rifle. The M203 and M320 grenade launchers can be mounted on the lower hand guard of the carbine (see where bipod is attached on the photo). The distinctive step in its barrel is for mounting the M203 with the standard hardware. The M4 has semi-automatic and three-round burst firing modes (like the M16A2 and M16A4), while the M4A1 has semi-automatic and fully automatic firing modes (like the M16A1 and M16A3).

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