The M4 Carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16; all based on the original AR-15 made by ArmaLite. The M4 was developed and produced for the US government by Colt Firearms, which has an exclusive contract to produce the M4 family of weapons through 2009. Due to the success and acclaim of the M4, a number of other manufacturers offer comparable M4-like firearms. These firearms cartridge the 5.56x45mm NATO round and are gas-operated, air-cooled, magazine-fed, selective fire firearms with a 4-position telescoping stock.
Currently, the M4 Carbine and its variants are among the primary weapons used by the US Army infantry; replacing the M16A2. In addition to serving the US/Iraqi forces in Iraq and Afghanistan, the M4 family is used by military and law enforcement agencies in over 30 countries.
M4s are easy to accessorize due to their picatinney rails. Some examples are forward pistol grips, various optical sights, carry handles, and large capacity magazines.