The Ciener Ultimate Over/Under system is a modification of the Remington 870 shotgun by Johnathan Arthur Ciener. It is identical in purpose to the Knight's Armament Company Masterkey or the accessory weapon configuration of the Remington 870 MPS.

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. The method of attachment differs in that the Ciener system attaches to the host weapon using a yoke adaptor to join the receivers of both weapons and an adaptor to join the shotgun's barrel to the rifle's bayonet lug.

The shotgun cannot be operated independently because of the yoke adaptor. The M16's magazine well must be used as a makeshift pistol grip. Independent stocks and grips for the 870 are available for use, but they are incompatible with the M16 barrel mount.

The Ciener Ultimate has allegedly been used by Delta Force, and the design's unique mounting system is mentioned in Mark Bowden's Black Hawk Down: A Story of Modern War, Delta SFC. Paul Howe was using a "CAR-15, a black futuristic-looking weapon with a pump-action shotgun attached to the bayonet lug in front." The Ciener design was the only underbarreled shotgun to use the host weapon's bayonet lug as an attachment point at that time.

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