The M3 is an American .45 caliber submachine gun that uses the .45 ACP (11.43x23mm ACP) pistol cartridge. It entered US Army service December 12 1942 as the United States Submachine Gun, Cal. .45, M3 and began to replace the .45 caliber Thompson series submachine guns: the M1928A1, M1 and M1A1 that were slowly being withdrawn from use. The weapon’s designer was G. Hyde, while F. Sampson – GMC’s Inland Division chief engineer – was responsible for preparing and organizing production. Even at the development stage, the weapon’s design was focused on simplified production, ease of use and the ability to convert the weapon to the 9x19mm Parabellum pistol cartridge. The weapon is commonly nicknamed the "grease gun," owing to its similarity in appearance to the common mechanic's tool.
Initial prototypes carried the experimental designation T15, and after removing the fire safety selector - T20. In 1942 the submachine guns were subjected to a series of military evaluations, after which they received slightly extended stocks. This variant was then approved for production at GMC’s Guide Lamp Division facility. Close to 600,000 weapons were produced by the end of World War II, including approx. 25,000 models chambered for the 9x19mm Parabellum cartridge (which was achieved through swapping out the barrel, bolt and applying an adapter for use with the British 9 mm Sten submachine gun magazines), which were delivered to the OSS in 1944. The OSS also requested approx. 1000 .45-caliber submachine guns with an integral sound suppressor (designed by Bell Laboratories).
The M3 is an automatic only, blowback-operated firearm (the weapon’s bolt is assisted by two parallel wire-guide action springs) that fires from an open bolt. A spring extractor is located inside the bolt, while the fixed ejector – in the cocking handle assembly. The weapon is striker fired with the firing pin contained inside the bolt, the return springs act as the striker spring. The weapon is secured from accidental firing by closing the spring-loaded ejection port cover, which has a lug that engages notches in the bolt assembly’s surface, locking it in both its forward and rear positions. The firearm is fed from a double-column 30-round detachable box magazine.
The M3 employs metal stamping and pressing, spot welding and welding. The bolt rides on two rods that are simultaneously the return spring guide rods. The weapon has a removable, rifled barrel. A conical flash hider (developed later) could be attached. The fixed iron sights consist of a rear aperture set for firing at 100 yards (approx. 91 m) and a blade foresight. The M3 is equipped with a folding (telescoping) metal wire stock, which features threads at both ends used to attach a bore brush (creates a cleaning rod).
In December 1944 a modernized version of the M3 was introduced into service known as the M3A1, which has several new features, among them the crank-type cocking mechanism was removed and a recess was machined into the bolt allowing the bolt to be cocked with the user’s finger, the ejection port and ejection port cover were enlarged, a magazine release button cover was added and a magazine loading device was welded to the wire stock. Additionally the bolt latch was changed, the barrel bushing received cuts that assist in barrel removal with the use of the stock, a larger oiler was used and a spare lubricant mount was removed (located at the left side of the cocking lever assembly), replaced with an oiler installed inside the pistol grip. The modifications resulted in a reduced weight and improved reliability; these changes also improved maintenance and field stripping (i.e. to remove the bolt and recoil assembly the user simply has to unscrew the barrel, in the M3 however it was necessary to first remove the trigger guard, then detach the cocking crank assembly from the receiver housing). In 1945 the Guide Lamp factory manufactured close to 15,500 M3A1 submachine guns, additionally during the Korean War – another 33,000 units were built at the Ithaca Gun Co.
The M3 and M3A1 were officially retired from US service in 1957; however they continued to be used until the early 1990s by armored vehicle crews and truck drivers. For example, during the Gulf War of 1991, drivers of the 19th Engineer Battalion, attached to the 1st Armored Division, deployed with and could utilize the M3A1.
In 1955 production was initiated at the Argentinean FMAP plant in Rosario of the PAM1 submachine gun (an M3 chambered in the 9x19mm Parabellum pistol caliber), and later the PAM2 (PAM1 modernized with the addition of a grip safety).