|Region of Origin:||United States|
|Range:||Sights fixed to 100 yards (91 m)|
|Rate of Fire:||500 rounds/min|
|Action:||Recoil-operated, roller locked|
|Years Active:||1942 - 1945|
The M3 was an American .45-caliber submachine gun adopted for U.S. Army service on 12 December 1942, as the United States Submachine Gun, Cal. .45, M3. The M3 was a superior alternative to the Thompson submachine gun as it was cheaper to produce, lighter, more accurate, and was also chambered in .45 ACP. The M3 was commonly referred to as the "Grease Gun" or simply "the Greaser", owing to its visual similarity to the mechanic's tool.
Intended as a replacement for the .45-caliber Thompson series of submachine guns, the M3 and its improved successor, the M3A1 began to replace the Thompson in first-line service in late 1944 and early 1945. Due to delays caused by production issues and approved specification changes, the M3/M3A1 saw relatively little combat use in World War II
In 1941 the U.S. Army Ordnance Board observed the effectiveness of submachine guns employed in Western Europe, particularly the German 9mm MP 40 and British Sten guns, and initiated a study to develop its own Sten-type submachine gun in October 1942. The Ordnance Department requested the Army to submit a list of requirements for the new weapon, and Ordnance in turn received a separate list of requirements from both the Infantry and Cavalry branches for a shoulder-fired weapon with full- or semi-automatic fire capability in caliber .45 ACP or .30 Carbine. The two lists of requirements received by Ordnance were then reviewed and amended by officials at Aberdeen Proving Ground. The amended requirement called for an all-metal weapon of sheet metal construction in .45 ACP caliber, designed for fast and inexpensive production with a minimum of machining, and featuring a dual auto and semi-automatic fire capability, a heavy bolt to keep the cylic rate under 500rpm, and the ability to place 90 percent of all shots fired from a standing position in full-automatic mode on a 6x6 foot target at a range of 50 yards. The benchmark for testing the M3's performance would be the M1928A1 Thompson.
George Hyde of General Motors's Inland Division was given the task of designing the new weapon, while Frederick Sampson, Inland Division's chief engineer, was responsible for preparing and organizing tooling for production. The original T15 specifications of 8 October 1942 were altered to remove a semi-automatic fire function, as well as to permit installation of a kit to convert the weapon's original .45 caliber to that of 9mm Parabellum. The new designation for the 9mm/.45 full-automatic-only weapon was the T20. Five prototype models of the .45 T20 and five 9mm conversion kits were built by General Motors for testing. At the initial military trials, the T20 successfully completed its accuracy trials with a score of 97 out of 100. In the endurance test, the test weapon fired more than 5,000 rounds of brass-case ammunition, with only two failures to feed. Four Army test boards composed of multiple Army service branches independently tested and reviewed the T-20 prototype weapons including the Airborne Command, the Amphibious Warfare Board, the Infantry Board, and the Armored Forces Board. All four branches reported malfunctions caused by the M3 magazine, mostly attributed to defective or jammed magazine followers.
The T-20 was formally approved by U.S. Army Ordnance for production at GM's Guide Lamp Division in Anderson, Indiana in December 1942 as the U.S. Submachine Gun, Caliber .45, M3. Guide Lamp produced 606,694 of the M3 variant submachine gun between 1943 and 1945. Although reports of malfunctions caused by the single-feed magazine design appeared during the initial firing trials, no changes were made to the M3 magazine.