The M16A3 rifle is an M16A2 rifle with an M16A1’s fire-mode control (semi-automatic fire, automatic fire) used only by the U.S. Navy.
The M16A4 rifle was standard issue for the United States Marine Corps in Operation Iraqi Freedom; it replaced the M16A2 in front line units. In the U.S. Army the M16A2 rifle is being supplemented with two rifle models, the M16A4 and the M4 carbine as the standard issue assault rifle. The M16A4 has a flat-top receiver developed for the M4 carbine, a handguard with four Picatinny rails for mounting a sight, laser, night vision device, forward handgrip, removable handle, or a flashlight.
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Semi-automatic AR-15s for sale to civilians are internally quite different from fully automatic versions for sale to law enforcement and military customers, though nearly identical in appearance. The hammer and trigger mechanisms are of a different design. The bolt carrier and internal lower receiver of semi-automatic versions are milled differently, so that the firing mechanisms are not interchangeable. This was done specifically to satisfy BATF requirements that civilian weapons may not be easily convertible to fully-automatic. Despite this, through use of a “Drop In Auto Sear” or “lightning-link,” conversion to full automatic is very straightforward (sometimes requiring slight modification to the bolt carrier).[13][14] Such modifications, unless using registered and transferrable parts made prior to May 19, 1986, are illegal. (The Firearm Owners Protection Act in 1986 has redefined a machinegun to include individual components where a semiautomatic firearm can be converted to full-automatic based on a 1981 BATF ruling on machinegun parts.) An illegally converted fully-automatic AR-15 was used in the North Hollywood shootout.