M231 Firing Port Weapon

The M231 Firing Port Weapon is an adapted version of the M16 rifle built for the United States Army so troops could fire from the firing ports of the M2 Bradley infantry fighting vehicle, since standard M16 rifles were too long for that use. Development began in 1972 and the weapon, chambered in 5.56x45mm NATO, entered service in 1980 with roughly 65 percent parts commonality with standard M16 rifles. It fires only in fully automatic mode at a cyclic rate of about 1,100 to 1,200 rounds per minute. Although most Bradley firing ports have since been removed or welded shut, the weapons remain in inventory for crew self defense and rear port use.