http://en.wikipedia.org/wiki/M16_rifleThe
M16A3 is a fully-automatic variant of the
M16A2, issued primarily within the United States Navy. The
M16A2, in turn, is currently being supplemented by the
M16A4, which incorporates the flattop receiver unit developed for the M4 Carbine, and Picatinny rail System.
M16A3The M16A3 was a fully-automatic variant of the M16A2 adopted in small numbers around the time of the introduction of the M16A2, primarily by the U.S. Navy for use by SEAL, SeaBee, and Security units.[22] It features the M16A1 trigger group providing "safe", "semi-automatic", and "fully-automatic" modes.
The M16A3 is often incorrectly described as the fully-automatic version of the M16A4 or an M16A2 with a Picatinny rail. This misunderstanding likely stems from the use of the "A3" designation by Colt and other manufacturers to describe commercial AR-15 type rifles before the official adoption of the M16A3 or M16A4. Colt used the "A3" designation in the hopes of winning military contracts as they also did with the terms, "M4" and "M5".
M16A4The M16A4, now standard issue for front-line U.S. Marine Corps and almost all U.S. Army units, replaces the combination fixed carry handle/rear iron sight with a MIL-STD-1913 Picatinny rail, allowing for the rifle to be equipped with a carry handle and/or most military and consumer scopes or sighting systems (usually either an ACOG or M68 CCO scope).[22] Most of the U.S. Marine Corps' M16A4s are equipped with a Knight's Armament Company M5 RAS handguard, allowing vertical grips, lasers, tactical lights, and other accessories to be attached. In U.S. Army Field Manuals, M16A4s fitted with the RAS are sometimes referred to as M16A4 MWS or Modular Weapon System. This model retains the 3-round burst mode of the M16A2.[22]
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