Colt M231 FPW
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Designed by the Rock Island Armory as a dedicated Firing Port Weapon (or FPW) for the M2/M3 Bradley AFV, the Colt M231 FPW remains in service, although all but the rear two firing ports on the Bradley have been removed.
History
Work started in 1972 on a dedicated Firing Port Weapon to go along with the Mechanized Infantry Combat Vehicle (MICV) program also started at that time. A requirement for these vehicles was to have firing ports for troops riding inside, and so it was decided that a specific weapon also be developed. The Rock Island Arsenal headed up the project working with the M3 SMG, an FPW created by HK based around the HK 33, and a modified AR-15/M16 pattern weapon. The qualities of the last weapon made it most promising, and by 1974 it had been designated XM231. Colt was given the contract and continued to work on the design, and by 1979 the finalized weapon was adopted as the M231.Although most of the Bradley AFV's ports have since been removed, these weapons are maintained, and are used by crews for self-defense, close-quarters situations, and for firing from the rear door firing ports as intended.
Specifics
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The M231 is different in many ways from a standard M16. The original RIA FPW fired from an open bolt, with an extremely high rate of firing (1,050 rpm). The Colt XM231 introduced a special buffer and spring assembly, with three springs nested one within the other. This was done to allow the rate of fire to be lowered to 200 rpm, because the theory at that period was that, using all tracer magazines, if the high ROF was retained troops would exhaust their magazines before the weapon was brought to target. The original RIA FPW had an 11 in (280 mm) barrel, while the Colt prototypes and the production M231 both had 15.6 in (396 mm) barrels. Initially the FPWs had no locking mechanism and had flip up sights, along with a metal wire stock akin to that on the M3 SMG, to allow the weapon to be used outside of the vehicle more effectively. Later this was dropped, and a new handguard introduced with a screw type locking mechanism to fix the weapon into the port. Late XM231s had no rear sights either. By the time the M231 was finalized the wire stock had been done away with as the weapon had a tendency to unfasten itself from the firing port and the stock was deemed to be dangerous in the confines of the vehicle. The firing rate had also returned to a higher rate of 1,100-1,200 round/min.
These weapons are all capable of only fully automatic fire, with the special FPW upper receiver "locking out" any additional movement of the selector switch with a special side plate that extends down to the lower receiver. These weapons retain a 65% commonality with standard M16 rifles.
See also
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