<P> The M4 is also capable of mounting the M203 and M320 grenade launchers . The distinctive step in its barrel is for mounting the M203 with the standard hardware . The M4 is 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 fully automatic modes (like the M16A1 and M16A3). </P> <P> 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". "Nevertheless, as a short - range weapon it is quite adequate and thus, (despite) its caliber, (the XM177 "Commando") is classed as a submachine gun ." In 1984, Colt began work on a new carbine design called the XM4 combining the best features of the Colt Commando and later the M16A2 rifles . </P> <P> In 1984, the first model was made, and it was tested in May 1985 . The first model have a Upper A1 Sight, and was given a shorter 11.5 - barrel, but later it was giving a longer 14.5 - inch barrel for the bayonet and the M203 Grenade Launcher . The second model was made in May 1986, and it was tested from May 1986 though May 1987, at the time it have a A2 Upper Sight, and it have the M16A2's 1: 7 inch rifle twist, to use the heavier 62 - grain M855 rounds . The extended barrel improved the XM4's ballistics, reduced muzzle blast and gave the XM4 the ability to mount a bayonet and the M203 grenade launcher . The XM4 was also given the cartridge deflector, as well as other minor refinements . In May 1991, the XM4 was renamed to the M4, and Colt made a manual . In 1994, the U.S. military officially accepted the M4 into service to replace M16A2s in certain roles . The M4 carbine has also replaced most submachine guns and selected handguns in U.S. military service, as it fires more effective rifle ammunition that offers superior stopping power and is better able to penetrate modern body armor . </P> <P> The United States Marine Corps has ordered its officers (up to the rank of Lieutenant Colonel) and staff non-commissioned officers to carry the M4 carbine instead of the M9 handgun . This is in keeping with the Marine Corps doctrine, "Every Marine a rifleman". The Marine Corps, however, chose the full - sized M16A4 over the M4 as its standard infantry rifle . United States Navy corpsmen E5 and below are also issued M4s instead of the M9 . While ordinary riflemen in the Marine Corps were armed with M16A4s, M4s were fielded by troops in positions where a full - length rifle would be too bulky, including vehicle operators and fireteam and squad leaders . As of 2013, the U.S. Marine Corps had 80,000 M4 carbines in their inventory . </P>

When did the army switch to the m4