A very common mistake made, especially in the media, is the the AR in AR-15, AR-10, etc. stands for Assault Rifle. The truth is that is a designation AR represents ArmaLite, which was the designation given to it’s original producer back in the 1950′s. An Assault Rifle is a selective fire, either fully automatic or burst-capable, rifle, which the civilian version is not capable of, that is used for assaulting or repealing an advancing force in the military.
The AR-15 (ArmaLite Rifle-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 toColt. 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.