KAC Masterkey shotgun mounted under an M4-type rifle.
Source: DrBaker - © Public domain
The Masterkey is an underbarrel shotgun of US origin. It was developed in the 1980's by KAC for special forces use. The Masterkey is intended to be mounted under the barrel of the CAR-15 or M4 carbine. This allows for a fast transition to the shotgun, which is useful in close quarter combat.
The Masterkey is based on an existing M870 shotgun. The barrel and tubular magazine are shortened. The stock and rifle style grip are removed. The mounting bracket is fixed to the top of the receiver. The mounting bracket is similar to the one used with the M203 grenade launcher. The magazine of the rifle acts as a sort of pistol grip for the shotgun.
The Masterkey fires 12ga shells with a case length of 2.75 or 3 inch. The tubular magazine holds three rounds, and one round can be carried in the chamber. Although intended for breaching, the muzzle does not feature a muzzle device as often seen on breaching shotguns since the 2000's.
The Masterkey was adopted by US special mission units, including the US Army Special Forces. It is also in foreign use, for instance with Australian SASR.
The CAR-15 was the first type of rifle to be fitted with the Masterkey. Upon introduction of the M4 that became the main platform.
The M4 carbine is the primary platform on which the Masterkey is currently used. The Masterkey is mounted in a similar manner as the M203 underbarrel grenade launcher.
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