The Masterkey is a door breaching shotgun system manufactured by Knight's Armament Company. The Masterkey project was initiated during the 1980s to provide assault rifles with a potent built-in door breaching tool. Individual soldiers were often forced to carry a breaching shotgun in addition to their standard issue rifle, but the Masterkey removes this need. The system consists of a Remington 870 12 gauge pump-action shotgun mounted on an M16 or M4 assault rifle in an underbarrel configuration, much like the M203 grenade launcher. It has a 3-round internal magazine and can carry a fourth round in the chamber.