M16
This was the first M16 variant adopted operationally, originally by the U.S. Air Force. It was equipped with triangular handguards, butt stocks without a compartment for the storage of a cleaning kit, a three-pronged flash suppressor, full auto, and no forward assist. Bolt carriers were originally chrome plated and slick-sided, lacking forward assist notches. Later, the chrome plated carriers were dropped in favor of Army issued notched and parkerized carriers though the interior portion of the bolt carrier is still chrome-lined. The Air Force continued to operate these weapons until around 2001, at which time the Air Force converted all of its M16s to the M16A2 configuration.
The M16 was also adopted by the British SAS, who used it during the Falklands War.
XM16E1 and M16A1 (Colt Model 603)
The U.S. Army XM16E1 was essentially the same weapon as the M16 with the addition of a forward assist and corresponding notches in the bolt carrier. The M16A1 was the finalized production model in 1967 and was produced until 1982.
To address issues raised by the XM16E1's testing cycle, a closed, bird-cage flash suppressor replaced the XM16E1's three-pronged flash suppressor which caught on twigs and leaves. Various other changes were made after numerous problems in the field. Cleaning kits were developed and issued while barrels with chrome-plated chambers and later fully lined bores were introduced.
M16A2
The development of the M16A2 rifle was originally requested by the United States Marine Corps as a result of combat experience in Vietnam with the XM16E1 and M16A1. It was officially adopted by the Department of Defense as the "US Rifle, 5.56mm, M16A2" in 1982. The Marines were the first branch of the U.S. Armed Forces to adopt it, in the early/mid-1980s, with the United States Army following suit in the late 1980s.
Modifications to the M16A2 were extensive. In addition to the new rifling, the barrel was made with a greater thickness in front of the front sight post, to resist bending in the field and to allow a longer period of sustained fire without overheating. The rest of the barrel was maintained at the original thickness to enable the M203 grenade launcher to be attached. A new adjustable rear sight was added, allowing the rear sight to be dialed in for specific range settings between 300 and 800 meters to take full advantage of the ballistic characteristics of the new SS109 rounds and to allow windage adjustments without the need of a tool or cartridge. The weapon's reliability allowed it to be widely used around the Marine Corps' special operations divisions as well. The flash suppressor was again modified, this time to be closed on the bottom so it would not kick up dirt or snow when being fired from the prone position, and acting as a recoil compensator.
M16A3
The M16A3 is a modified version of the M16A2 adopted in small numbers by the U.S. Navy SEAL, Seabee, and Security units. It features the M16A1 trigger group providing "safe", "semi-automatic" and "fully automatic" modes instead of the A2's "safe", "semi-automatic", and "burst" modes. Otherwise it is externally identical to the M16A2.
The M16A4 is the fourth generation of the M16 series. It is equipped with a removable carrying handle and a full length quad Picatinny rail for mounting optics and other ancillary devices. The FN M16A4, using safe/semi/burst selective fire, became standard issue for the U.S. Marine Corps. Military issue rifles are also equipped with a Knight's Armament Company M5 RAS hand guard, allowing vertical grips, lasers, tactical lights, and other accessories to be attached, coining the designation M16A4 MWS (or Modular Weapon System) in U.S. Army field manuals.
The M16A4 is the fourth generation of the M16 series. It is equipped with a removable carrying handle and a full length quad Picatinny rail for mounting optics and other ancillary devices. The FN M16A4, using safe/semi/burst selective fire, became standard issue for the U.S. Marine Corps.
Military issue rifles are also equipped with a Knight's Armament Company M5 RAS hand guard, allowing vertical grips, lasers, tactical lights, and other accessories to be attached, coining the designation M16A4 MWS (or Modular Weapon System) in U.S. Army field manuals.
Colt also produces M16A4 models for international purchases:
R0901 / NSN 1005-01-383-2872 (Safe/Semi/Auto)
R0905 (Safe/Semi/Burst)