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 shortened Remington 870 12 gauge pump-action intended to be mounted under a firearm's barrel, similarly to the M203 grenade launcher.

[1] The weapon is named "Masterkey" in reference to its ability to "unlock" virtually any standard lock.