The M231 Port Firing Weapon was developed in the 1970s as a part of the M2 Bradley Fighting Vehicle Project. A modern relative of the WW2 Krummlauf, the weapon was intended to provide close-in firepower against infantry that might attempt to overrun the M2. It has no sights or buttstock, and fires from an open bolt only as 1100-1200 rounds/minute. It is intended to be used with M196 tracer ammunition to aim. Early versions were issued with rudimentary sights and a wire collapsing stock (akin to that of an M3A1 Grease Gun), but the weapon proved so difficult to control from the shoulder that the stocks were discarded and policy changed to dictate that the guns never be removed form the Bradleys.
The unique fitting on the front of the hand guard is a coarse thread to screw the gun into the Bradley’s firing port sockets. The fire control system is entirely different form a standard M16, with the hammer being removed entirely in favor of a submachine gun like dropping sear. The recoil system was also completely changed, and in the M231 consists of simply three separate recoil springs nested inside one another.