The MAC mle 1931 machine gun (official French designation Mitrailleuse modèle 1931 - machine gun, model of 1931), was a machine gun used in French tanks of the World War II era, as well as in fortifications such as the Maginot line. It is also sometimes known as the JM Reibel, from Jumelage de mitrailleuses, or Reibel twin-mounted guns and really refers to the specialized twin-mounting frame used in JM cloche cupolas on the Maginot Line fortifications, while MAC mle 1931 refers specifically to the gun. The JM twin-mounts were the standard emplacement for the mle 1931 in fixed fortifications, while tanks and other AFVs received single guns. Overview The Reibel machine gun is a gas operated weapon chambered in the 7.5 mm MAS cartridge and was loaded with vertical, side-mounted, 150-round pan magazines. The variant used in fortifications was modified with a different rifling to accommodate the balle D heavy ball type of ammunition. Some other guns in French service during the late 1940s were converted to ground role, with adoption of the side-mounted 35-round box magazines and adapters for US M2 tripods. The modele 1931 machine gun is a gas operated weapon which fires from open bolt and in full automatic only. It is derived from the French FM 24/29 Fusil-Mitrailleur (light machine gun) also designed by Lt Colonel Reibel, and based on the Browning Automatic Rifle. The FM 24/29 was developed into a heavier machine gun capable of relatively sustained fire, by giving it an extremely thick and massive barrel, to act as a heat sink. This was necessary, since the FM 24/29 lacked a quick-change barrel or water-cooling and its normal light barrel would rapidly overheat and wear out, if fired in more than short bursts, with cooling rests between. The long stroke gas piston is located below the barrel and operates the vertically tilting bolt group. Ammunition is fed from side-mounted multi-layer 150-round pan magazines (with bullets pointing to the center of the round magazine). The gun can be modified to take magazines on either the left or right side, to ease magazine changes while mounted in the standard side-by-side JM twin-mount. Ejection is straight down, through the short chute attached to the base of the receiver, which in fortifications, usually led into a longer tube or chute that directed the spent cases into the ditches outside. The gun was fitted with a forward-curved pistol