ArmaLite AR-15

The AR-15 is a lightweight, 5.56 mm, magazine-fed, semi-automatic rifle, with a rotating-lock bolt, actuated by direct impingement gas operation or long/short stroke piston operation. It is manufactured with the extensive use of aluminum alloys and synthetic materials.

The AR-15 was first built by ArmaLite as an assault rifle for the United States armed forces. Because of financial problems, ArmaLite sold the AR-15 design to Colt. The select-fire version of the AR-15 entered the U.S. military system as the M16 rifle. Colt then started selling the semi-automatic version of the M16 rifle as the Colt AR-15 for civilian sales in 1963. Although the name "AR-15" remains a Colt registered trademark, variants of the firearm are independently made, modified and sold under various names by multiple manufacturers.

Semi-automatic AR-15s for sale to civilians are internally different from the full automatic M16, although nearly identical in external 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 to satisfy United States Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) requirements that civilian weapons may not be easily convertible to full-automatic. In the late 1970s and early '80s, items such as the "Drop In Auto Sear" or "lightning-link," conversion to full automatic was very straightforward (sometimes requiring machining of the lower receiver with use of a lathe and M16 Bolt Carrier Group). Such modifications, unless using registered and transferable parts made prior to May 19, 1986, are illegal. (The Firearm Owners Protection Act in 1986 has redefined a machine gun to include individual components where a semi-automatic firearm can be converted to full-automatic based on a 1981 ATF ruling on machine gun parts.) Since 1993, The Bolt Carrier Groups used in AR-15 type rifles for civilians have employed additional measures to prevent modification to full auto. Colt AR-15's use a metal alloy wall separating the Fire Control group from the Sear, preventing use of such items.

Automatic variants have a three-position rotating selective fire switch, allowing the operator to select between three modes: safe, semi-automatic, and either automatic or three-round burst, depending on model. Civilian Colt AR-15 models do not have three-round burst or automatic settings on the fire selector, though some other makers may mark that way for collectors and re-enactors, even though the guns will not fire in that mode. In semi-automatic only variants, the selector only rotates between safe and semi-automatic. Due to this, weapons modified to full automatic using a lightning-link are capable of full automatic fire only—unless a special full automatic fire select mechanism and modified selector-switch is substituted.