The M4 carbine was developed from various outgrowths of these designs, including a number of -barreled A1 style carbines. The XM4 (Colt Model 720) started its trials in 1984, with a barrel of . The weapon became the M4 in 1991. Officially adopted as a replacement for the M3 "Grease Gun" (and the Beretta M9 and M16A2 for select troops) in 1994, it was used with great success in the Balkans and in more recent conflicts, including the Afghanistan and Iraq theaters. The M4 carbine has a three-round burst firing mode, while the M4A1 carbine has a fully automatic firing mode. Both have a Picatinny rail on the upper receiver, allowing the carry handle/rear sight assembly to be replaced with other sighting devices.