US Marine during the Battle for Hue, 1968. The M3 is an extremely simple blowback weapon. There is no safety, fitted and the weapon only fires fully automatic. However, since the cyclic rate of fire is relatively low at 450-rounds per minute, this allowed the firer to control the movement of the gun when firing bursts and even to squeeze off single shots.. It fires the standard .45 ACP pistol cartridge (as used in the M1911A1) from a straight 30-round, single-column box magazine. There was provision in the original design for conversion to 9-mm Parabellum. This involved changing the barrel, breech block and magazine, all of which could be done without tools. A small number of weapons so converted appeared in Europe, but the vast majority of the 700,000 US-made M3s were in 45 calibre.