The M3 "Grease Gun" was an economical submachine gun weapon designed during World War 2 (1939-1945) intended to arm the large quantity of incoming American troops with an equally-large quantity of small arms. The M3 appeared in number during 1943 in time to succeed the excellent-but-expensive (and complicated) M1 Thompson ("Tommy Gun") series of submachine guns. Design work on the new submachine gun began in 1942 and production ran from 1943 until 1945 to which some 700,000 examples were ultimately manufactured.
The nickname of "Grease Gun" was eventually given to the weapon American soldiers who likened its general appearance to that of a mechanic's grease gun.
Indeed the general appearance of the weapon was tubular and cast as large components to minimize parts needed. The pistol grip sat at the rear of the receiver in the usual way and a wire stock could be slid out from the sides of the receiver to provide a make-shift shoulder support. The trigger unit (solid trigger assembly) sat under the receiver and ahead of the pistol grip in the usual way. Very little in the way of ergonomics was afforded the shooter - the M3 was a utilitarian weapon to be sure. The magazine well sat ahead the receiver and accepted straight, detachable box magazines and also served as the forward grip (as in the German MP38/40 submachine gun series). The barrel was a simple cylindrical assembly set ahead of the receiver. Sighting devices were fitted over the weapon for some semblance of accurized fire but the submachine gun was always a short-to-medium-ranged weapon at its core.
A variant of the M3 series became the "M3A1" and this was a further evolution of the design . It followed the original into service during December of 1944 and was essentially developed to further simplify the production process and practices in order to streamline the end-product and ship the weapon out in the quantities required in wartime. Additionally, this period of revision allowed for a second look into correcting some inherent deficiencies in the original. As such the M3A1 was given a redesigned bolt-retracting mechanism which forced the prominent hinged cover to be enlarged some, allowing the user to fit his finger into the recess and pull the bolt back as needed. In another effort to make the weapon more versatile and - in effect more "battlefield friendly" - a reservoir of oil for in-the-field lubrication of parts was also incorporated into the pistol grip and various parts of the operating system were re-engineered to double as tools when the weapon was stripped down to its bare components.
By all accounts, the M3A1 was not that much of an improvement over the original M3 to which neither system ever achieved any level of acceptability or likeness with soldiers - they instead preferred their trusty M1 Carbines and M1 Thompsons for their sheer reliability and man-stopping power. Nevertheless, the M3 series went on to see extensive combat service through to the end of World War 2 and the series was in widespread circulation by the time of the Korean War (1950-1953).
Beyond that, production also occurred outside of the United States by foreign forces eager to take on a cheap, proven weapon that was nonetheless robust and fairly easy to operate.
The M3 model was produced by the Guide Lamp Division of General Motors in Detroit, Michigan, as well as the Ithaca Gun Company of Ithaca, New York. Some M3 models incorporated unusual-looking additions like muzzle-mounted flash hiders. Argentina designated their M3s as "PAM1" and "PAM2". The PAM1 was notable for its chambering of the 9x19 Parabellum German pistol cartridge while the PAM2 featured a grip-mounted safety. A suppressed model (detailed elsewhere on this site) was also noted which added an oversized assembly over the barrel to help reduce the telltale "crack" of the outgoing bullet for clandestine operations.