The KAC Master-Key is basically a Colt M4 carbine with a Remington 870 pump-action 12-guage shotgun slung under the barrel. The concept isn’t new, but what KAC brought to the weapon was the use of the Rail Interface System which allows the user to quickly detach the shotgun if necessary. The shotgun is slightly modified in that a Picatinny rail is mounted on the dorsal side, along with a shortened barrel. It holds five shells in a tubular magazine.
