Masterkey是由KAC出产的枪管下挂式霰弹枪，发射12号口径霰弹。

Masterkey项目在80年代出现，主要目的是在步枪或卡宾枪上下挂一把用于破门、重量较轻的霰弹枪，而不需另外独立携带霰弹枪。Masterkey由泵动的雷明登870截短改成，以挂架安装在M16突击步枪及M4卡宾枪系列的枪管下，安装方法类似M203榴弹发射器。发射发射12号口径霰弹，内置管式弹仓，可装填3发子弹，若加上枪膛中待射的1发，共4发子弹。

由于发射时需要以弹匣充当握把，Masterkey拆下时无法作独立发射。

从马克·鲍顿的军事小说Black Hawk Down: A Story of Modern War中提到，美军三角洲特种部队队员（Delta SFC. Paul Howe）的CAR-15卡宾枪上也安装了Masterkey霰弹枪。

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 a 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.