The Colt M4 is a carbine rifle originally produced by Colt's Manufacturing Company. The design is now owned by the United States military and can be made by other manufacturers. It takes the 5.56x45 mm NATO round, which is .223 caliber.
The M4 was developed as a lighter and smaller alternative to the M16A2 assault rifle. It follows in the lineage of carbine rifles that began with the CAR-15 and M16. Features of the rifle include gas-operated reloading, a telescoping stock and selective fire. The rifle was designed with close-quarters combat in mind, with a shorter barrel than previous carbines and a standard bayonet mount.
Prior to the U.S. Army's acquisition of the Colt M rights in 2009, the rifle was manufactured only by Colt and was first manufactured in 1994. The Army has made the design available to other contractors, and the M4s of the Army are set to be produced by FN Herstal until 2018.
Ownership of select-fire and automatic Colt M4s is mostly restricted in the United States to military and law enforcement agencies. Only prototypes made prior to 1986 are legal for civilian ownership. Civilian-legal variants are semi-automatic and have a longer barrel than the military M4.