When the American military adopted the M16 rifle, based off the Armalite AR-15, an interest in customization and improvements to the weapon developed. After the AR-15 patents expired, an active marketplace emerged around the rifle's design of using an intermediate cartridge, a semi-automatic system, STANAG magazines and interchangeable upper and lower receivers. Notable variations include replacing the original direct impingement gas-operation with a long/short stroke piston operation, or utilizing new cartridges such as .300 AAC Blackout.
The prototype AR-15 rifle was designed by ArmaLite as a selective fire weapon for military purposes. Armalite sold the design to Colt due to financial difficulties. After some modifications, the rifle eventually became the US Army's M16 rifle, with a civilian version marketed as the Colt AR-15 rifle.
The term "AR-15" signifies "Armalite rifle, design 15". The trademark "AR15" or "AR-15" is registered to Colt, which requires the term to be used only to refer to their products. Colt uses "AR-15" for its semi-automatic civilian rifles and thus many use the term only for Colt AR-15s and clones made by other manufacturers. Other manufacturers make AR-15 clones and variants marketed under separate designations, although these are frequently referred to as AR-15s.