The US began looking for a cost-effective replacement for the Thompson submachine gun in 1942, and the “Grease Gun” was the result. Designed by George Hyde (a noted firearms designer at the time) and Frederick Sampson (GM/Inland chief engineer), it was a very simple and almost entirely stamped firearm. Chambered for the .45ACP cartridge, it is notable for its very low rate of fire – 350-400 rpm, which made it quite controllable and easy to shoot for relatively inexperienced troops.
The M3 was a quite reliable gun (and what problems it did have were mostly due to its single-feed magazine and not the gun itself), but a revision program was begun in April 1944. This would produce the M3A1, which further simplified the design by removing the charging handle (which had been the one mechanical trouble point of the M3 anyway) and replacing it simply with a notch in the bolt to cock the gun with a finger.
While the M3 and M3A1 were replaced in front-line service in 1957, they would remain in military inventory as armament for tank crews and truck drivers until 1992 – quite the legacy for such a crude looking weapon!