The M4 Carbine is an assault rifle currently in usage by the US Armed forces. 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. It is a shorter and lighter version of the M16A2 assault rifle, with 80% parts commonality. The M4 has selective fire options including semi-automatic and three-round burst (like the M16A2), while the M4A1 has a "full auto" option in place of the three-round burst.
The Colt company developed various carbine versions of the AR-15/M16 rifle since the 1970s. These carbines were intended for all markets - military, law enforcement, and civilian. The US Military (and some other armies, most notably - The Israeli Self-Defense Forces) ha adopted the Colt CAR-15 Commando and XM-117 carbines during the 1970s and 1980s. But in the early 1990s, the old idea of replacing the pistols in the hands of troops with some more effective, shoulder fired weapon, rose again in the heads of the US Military. This idea can be dated back to the US M1 Carbine of 1941. So, in 1994, the US Army adopted the Colt Model 720 selective-fire carbine (basically, a shortened M16A2 rifle), as the US M4 Carbine. This weapon was intended to replace some aged M3A1 submachine guns and some M16A2 rifles. The M4 was much more handy and comfortable to carry than the long M16A2 rifle, so the US Special Operations Command (SOCOM) put its eye on the M4 as a possible universal weapon for all Special Operations. For this purpose, the M4 was later modified with the M16A3-style flat-top reciever with integral Picatinny-type accessory rail instead of the M16A2-type integral carrying handle. This modification retained the M4 index. The only difference between the M4A1 and M4 is that the M4A1 fires fully automatic instead of the 3-round burst used by the M4. The US Naval Surface Warfare Center developed a SOPMOD M4 kit, that consisted of the M4A1 equipped with Rail Interface System (RIS) instead of the standard handguards. The kit also includes a variety of accessories, such as various sights (ACOG 4x telescopic, ACOG Reflex red-dot, detachable back-up open sights), laser pointers (visible and infra-red), deatachable sound supressor (silencer), modified M203 40mm grenade launcher (with shortened barrel and improved sights). The kit also included a detachable front grip and tactical light.
The Individual Carbine is the planned successor of the M4A1 Carbine for the U.S Army. Testing has commenced and results are not expected until 2013, and the weapon will not be fielded until 2016-2018.