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 in 1959 by ArmaLite as a small arms rifle for the United States armed forces. Because of financial problems, ArmaLite sold the design to Colt. After modifications, the redesigned rifle was adopted as the M16 rifle. In 1963, Colt started selling the semi-automatic version of the rifle for civilians as the Colt AR-15 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 made, modified, and sold under various names by multiple manufacturers.