The M4 Carbine is a shorter and lighter variant of the M16A2 assault rifle. The M4 is a 5.56x45mm NATO, air-cooled, direct impingement gas-operated, magazine-fed select-fire carbine. It has a 14.5 in (370 mm) barrel and a six-position telescoping stock.
Reese used this as his personal rifle in Rambo. Reese's M4, however, was equipped with an M203 40mm grenade launcher, a scope on top of the detachable carrying handle and a tan desert camouflage paint scheme.
Following the adoption of the M16 rifle, carbine variants were also adopted for close quarters operations. The CAR-15 family of weapons served through the Vietnam War. However, these carbines had design issues, as "the barrel length was halved" to 10 inches which "upset the ballistics", reducing its range and accuracy and "led to considerable muzzle flash and blast, so that a large flash suppressor had to be fitted". In 1988, Colt began work on a new carbine design called the XM4 combining the best features of the Colt Commando and M16A2 rifles. The rifle they came up with ended up being the M4, featuring a strengthened six-position telescoping stock, contoured barrel for mounting the M203 grenade launcher, detachable carrying handle with adjustable A2 sights and fat handguards capable of housing two heat shields. The original M4 had a lower capable of firing in semi-automatic and three-round burst modes (like the M16A2 and M16A4), while the M4A1 is capable of firing in semi-auto and full automatic modes (like the M16A1 and M16A3). The M4 carbine is heavily used by the United States Armed Forces and is replacing the M16 rifle in most United States Army and United States Marine Corps combat units as the primary infantry weapon.