KAC Masterkey

The Masterkey is a door breaching system manufactured by Knight's Armament Company.

The Masterkey project was initiated during the 1980s to provide assault rifles with a potent built-in breaching tool. Individual soldiers were often forced to carry a breaching shotgun in addition to their standard issue rifle, but the Mastekey removes this need. The system consists of a Remington 870 12 gauge pump-action shotgun mounted on a M16/M4 rifle in an underbarrel configuration, much like the M203 grenade launcher. The shotgun cannot be operated independently because it has no grip behind the trigger – when firing the Masterkey, the M16's magazine must be used as a makeshift pistol grip. Independent stocks and grips for the 870 are available for use, but they are incompatible with the M16 barrel mount.

The Masterkey has been largely superseded by the XM-26 Lightweight Shotgun System (LSS).