The AR-15 is a lightweight, intermediate cartridge magazine-fed, air-cooled rifle with a rotating lock bolt, actuated by direct impingement gas operation or long/short stroke piston operation. It has been produced in many different versions, including numerous semi-automatic and selective fire variants. It is manufactured with extensive use of aluminum alloys and synthetic materials. The AR-15 was first built by ArmaLite as a small arms rifle for the United States armed forces. Because of financial problems, ArmaLite sold the AR-15 design to Colt. After modifications, the new redesigned rifle was subsequently adopted 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 and the term has been used to refer to semiautomatic-only versions of the rifle since then. 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.