Operation of the HK-53 was modified so that the HK-53 fired from an open bolt instead of the traditional H&K method of firing from a closed bolt, as open-bolt operation allows for better cooling of the barrel and bolt mechanism and made the HK-53 more compatible with the XM-723’s need to vent firing gasses to the outside of the vehicle. Finally, the original HK53 fire control group mechanism was used, and the cyclic rate of the HK-53 was almost doubled as per the firing port weapon competition guidlines requirements. In the end however, the US Army decided to adopt an AR15/M16 patterned weapon instead, being designated the XM231. Colt was given the contract and continued to modify their design finalizing the weapon as the M-231 (NSN: 1005-01-081-4582) which was adopted by the US Army in August, of 1979. Undeterred, Heckler & Koch continued to improve and shop around the HK-53 MICV (particularly to the German Bundeswehr, who was at the time looking for a firing port weapon for their Marder IFV), but no military ever adopted the weapon so the HK-53 MICV eventually became one of those interesting designs that never went into use, and is now a very rare item.
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.
