A machine gun, representatively a 30 mm machine gun, is mounted on an outer end portion of a support plank structure projecting out of the cabin area of a helicopter using a roller cradle assembly secured to the outer plank end. A variety of structural improvements are incorporated into the machine gun to (1) reduce its mechanical complexity, (2) reduce its firing recoil to make the gun more suitable for light aircraft mounting, and to improve the gun's firing controllability and accuracy, and (3) to make the gun easily and quickly field strippable, for cleaning, inspection and repair purposes, without requiring the services of a highly skilled armament mechanic or taking the gun to a specialized repair facility.