The M231 FPW ("Firing Port Weapon") was a highly-modified, specialized variant of the Colt M16A1 assault rifle series. Also chambered for the 5.56x45mm NATO cartridge and operating on the same gas system, the M231 was specifically-designed for the crew and passengers of the American Bradley Infantry Fighting Vehicle (IFV), engaging enemy infantry through the available firing ports of the Bradley. Design work on the product began in 1972 by Rock Island Arsenal to coincide with the arrival of a new mechanized infantry combat vehicle - this eventually becoming the Bradley vehicle. Introduced in 1980, the weapon saw use in the 1991 Gulf War as well as the 2003 Iraq war and remains in circulation today (June 2014).
Much of the general form and function of the M16A1 was retained though an open bolt design was accepted. A threaded section along the forend was used for passing the barrel through the available firing ports of the Bradley vehicle. The weapon could be fixed in this manner or removed and operated as a stand-alone carbine weapon in-the-field. Since the gunner would be engaging through the vehicle's vision blocks and periscopes, there were no iron sights fitted though the carrying handle could be used as a rather basic aiming reference. Feeding was through the same 30-round STANAG curved detachable box magazines featured in the M16A1. The M231 FPW showcased a rate of fire of about 1,225 rounds-per-minute though short bursts were generally the call of the day. Effective range was out to 330 yards which offered the needed protection against infantry.
For its intended role, the dimensions of the M231 were purposely compact - an overall length of 28.25 inches with a barrel measuring 15.6 inches.
Today, the Bradley line has seen most of her firing ports removed save for the rear-facing positions which, in turn, has limited the tactical value of such a specialized weapon as the M231 FPW.