The M3 sub-machinegun, also known as Grease gun, was developed as a cheaper war-time alternative to famous Thompson M1A1 and M1928 sub-machineguns. M3 and M3A1 were developed and manufactured by General Motors Corp. M3 was introduced in 1942, and simplified M3A1 was introduced in 1944, and remained in service in the USA until 1960 or so, and also was exported outside the USA.
M3A1's were able to be converted to use 9mm Luger rounds by replacing the barrel, bolt, and installing the magazine adaptor to use british Sten magazines. Both M3 and M3A1 fired from the open bolt (round is chambered after the trigger is pulled). The retractable stock, made from steel wire, could be used as cleaning rod (when detached), it also featured a magazine loading tool. Some M3s and M3A1a were also fitted with flash hiders. The sights were fixed and located at the top of the receiver.
 
M3A1 "Grease Gun"
       
  M3 is full-auto only, blowback operated firearm. The receiver is made from steel stampings. M3 featured spring-loaded ejection port cover (also acted as safety) and crank-type bolt retracting (cocking) handle at the right side of the receiver. In M3A1 the designers removed cocking handle assembly (it was prone to malfunctions) and replaced it with simple finger hole in the bolt body, accessible through enlarged ejection window.

-(Shortened and/or modified version of the article at world.guns.ru)

 
M3A1s stock retracted and flash suppressor removed
       
 
M3A1 unloaded
  GAME DATA: M3A1

The M3 is a relic of the WW2. A cheap war time SMG for the troops. It's made from stamped scrap metal and the only reason for it's long time of service (US tank crew used it untill the early 90s) is the shear number of them. But an automatic (400-450 rounds per minute) .45 ACP sub-machinegun is better than nothing in a tight spot.
Used by Hong Kong Triads and Colombian Rebels. Also by some Soviet troops in Prague.

   
Name: M3A1  
Slot: 4, Sub-machineguns  
Caliber: .45 ACP  
Capacity: 30 rounds  
Firemodes: Full- auto  
Accuracy: Fair  
   
   
   

.