Colt M4 The M4 carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M - 16 , 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 with the M16A2 . The M4 has selective fire options including semi - auutomatic and three - round burst ( like the M16A2 ) , while the M4A1 has a " full auto " option in place of the three - round burst . Description . The Colt company developed various carbine versions of the basic AR - 15 / M16 rifle since 1970s . These carbines were intended for all markets - military , law enforcement , civilian . US Military ( and some other armies , most notably - Israeli Self - Defense Forces ) had adopted the Colt CAR - 15 Commando and XM - 177 carbines during the 1970s and 1980s . But early in 1990s the old idea of replacing the pistols in the hands of the troops with some more effective , shoulder fired weapon , rise again in the heads of the US Military . In fact , this idea can be dated back to the US M1 Carbine of 1941 , but good ideas never die . So , in the 1994 , U . S 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 in service some Beretta M9 pistols , as well as some aged M3A1 submachine guns and some M16A2 rifles . New weapon 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 community . For this purpose M4 was latter modified with the M16A3 - style flat - top receiver with integral Picatinny - type accessory rail instead of the M16A2 / M4 - type integral carrying handle . This modificatin retained the M4 index . The only difference between the M4A1 and M4 is that its trigger unit of M4A1 is modified to fire full - auto instead of the three shots bursts in M4 . Specially for the SOCOM M4A1s US Naval Surface Warfare Center developed a M4 SOPMOD kit , that consisted of the M4A1 carbine equipped with Rail Interface System ( RIS ) instead of the standard handguards . The kit also includes a variety of the add - on goodies , such as various sights ( ACOG 4X telescopic , ACOG Reflex red - dot , detachable back - up open sights ) , laser pointers ( visible and infra - red ) , detachable sound suppressor ( silencer ) , modified M203 40mm grenade launcher ( with shortened barrel and improved sights ) . The kit also included a detachable front grip and tactical light . At the present time , the M4 carbine is used as a front - line weapon by US Army , Marine Corps and SOCOM operators in Iraq and Afghanistan . Combat experience with thos weapon resulted in update program , which will , as of now ( mid - 2010 ) following steps . First , Army wanted to install heavier barrel to allow more sustained firepower , combined with full - automatic mode of fire instead of 3 - round burst , and ambidextrous safety / selector switch . Second stage will see improved rail adapter and a new , improved bolt carrier , and a possible third stage will include change of operating system ( most probably , from direct gas to the gas piston ) . At the present time , the Colt company still is the prime M4 carbine manufacturer for U . S Armed forces , but many other companies build similar " milspec " weapons for other US and foreign customers , such as government and private security organizations , law enforcement etc . To name just few , M4 - type carbines are manufactured by Bushmaster Firearms , Olympic Arms , THOR Global Defense Group and others Accessories . Like all variations of M16 , The M4A1 can be equipped with many accessories such as night vision devices , suppressors , laser pointers , telescopic sights , bipods , grenade launcher or M203 and M320 , the M26 rifle Mass , and all compatible with the Picatinny rail . Other common accessories include AN / PEQ - 2 , Advanced Combat Optical Gunsight , and M68 CCO . EOTech holographic weapon sights are part of the SOPMOD II package . Visible and IR ( infrared ) lights of various manufacturers are also commonly associated with various mounting methods . As with all versions of M16 the M4A1 accepts an accessory blank - firing attachment ( BFA ) for training purposes . Issues . Despite being more successful than its predecessors , the M4A1 still carries some problems M16 As the frequency with which accidents occur when the mechanism is subjected to dirt common in the jungle environment , and the complexity of this mechanism , which makes it very laborious process of disassembly and maintenance . Nevertheless , it is relatively inexpensive , so it is used by various organizations in many countries .