The standard breaching shotgun is the Master Key, a tiny Remington 870 shotgun modified by Knight’s Armament Company to fit on the front end of an AR-15 or M-16 rifle. It works, but it’s heavy and racking the action requires you to take your hand off the shotgun. Crye Precision think they have a solution that would let the door kicker fire multiple rounds into a door while still having a finger on the trigger of the rifle . . .
The shotgun has a rotating 6-round magazine that is detachable, so you can reload it on the fly and very quickly. The gun has a solid single stage trigger, and is semi-automatic to make the operator’s life easier. There’s also a convenient blast shield on the front of the magazine to keep the blast from screwing up their hand.
Oh, and they have a standalone version. Which looks BAD ASS!