The M231 is different in many ways from a standard M16. The original RIA (Rock Island Arsenal) 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, to prevent the risk for soldiers opening fire from the moving vehicle of exhausting their magazine before the weapon was brought to target. The original RIA FPW had an 11-inch (280 mm) barrel, while the Colt prototypes and the production M231 both had 15.6 inch (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 submachine gun, to allow the weapon to be used outside of the vehicle more effectively. Later this was dropped, and a new hand-guard 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 (or, alternately, Army officials omitted the feature to discourage troops from employing the M231 in lieu of the issue M16 rifles). Likewise the intended mechanism to bring down the firing rate had been removed from the design again thus bringing the rate of fire back up to a rate of 1,100-1,200 round/min.