Several shotgun designs have been modified to be mounted under the barrel of combat rifles or submachineguns. The shotgun is mounted in a similar fashion to the M-203 grade launcher under the M-16. Masterkey shotguns have been mounted under the M-16, M-4, and MP-5 weapons. The Knight masterkey shotgun is one such under rifle mounted shotgun and is primarily designed as a breaching took. That is a weapon designed to blow doors open. As such, the weapon is most commonly used by law enforcement. The shotgun is part of the Knight Corporation's Modular Weapon Concept.

The Knight Masterkey has a short 10 inch barrel and is fairly lightweight at under 6 lbs. Still, a combat rifle or submachinegun can be a fairly heavy combination. The shotgun is punp action and has a three round internal magazine. One more round can be carried in the chamber when needed. Like the M-203 grenade launcher, the recoil is destributed to the host weapon's barrel nut and barrel. It also uses the rifles magazine as the handgrip to fire the shotgun. The shotgun uses an aluminum adapter to simulate the receiver dimensions of the M-203 grenade launcher. The shotgun is designed to be quickly detached and is designed with a special folding butt stock to be used independently.

Weight:	5.5 lbs (2.5 kg) free weight, 9.1 lbs (4.12 kg) assembled to Stand Alone assembly.
Caliber:	12 Gauge.
Barrel Length:	10 inches (25.4 cm).
Overall Length:	16.7 inches (43.2 cm) free length, 29.5 inches (75.0 cm) assembled to Stand Alone assembly with butt-stock extended, & 20.7 inches (52.7 cm) with stock closed.
Action:	Pump Action.
Mode of Fire:	Single shot only.
Range:	300 ft (90 m).
Magazine:	3 round internal.
Cost:	1900
Made in:	USA.
Special:	Lightweight, can be mounted under the barrel of combat rifles and submachines.

[Knight, Masterkey, M-16, M-4, and MP-5 are copyright of their respective owners]

By Mischa