The M3 (also known as Grease Gun or Cake Decorator) was a submachine gun that was intended to replace the complex and expensive Model 1928 Thompson SMG as the standard U.S. Military submachine gun. The Grease Gun fired .45 ACP rounds, but could easily be converted to fire captured German 9 x 19 mm Parabellum rounds, making it ideal for partisans and resistance fighters, as well as airmen who might need to bail out over German lines.
Pictured is the M3. To load, a magazine is inserted, the ejection port cover is lifted and the cocking handle, or "crank", located on the right side, is rotated back, drawing the bolt to cocked position. The primary problem was that the crank was always catching on something.
NEVER, under ANY circumstances, insert the magazine with the bolt cocked. Pushing the mag in so the catch engages or "slapping" the mag to seat it, COULD cause the sear to release the bolt, allowing the piece to fire.
The M3 was surpassed by the M3A1 in 1944. The main difference is that the A1 did away with the crank on the right side, made the ejection cover longer, and bored a hole in the top of the bolt. To load a M3A1, insert a magazine, open the ejection cover, insert your finger in the hole in the bolt and pull it back to the cocked position. Doing away with the crank eliminated several parts, making the piece simpler and cheaper to manufacture.
Although it was meant to replace the Thompson, 1.5 million Thompsons had been created and the Thompsons outnumbered the M3's by three to one, so the M3 never replaced the Thompson.
The Grease Gun was invented by George J. Hyde in December 1942 as a light and simple SMG for the U.S. Army. The gun itself was modeled after the German MP-40 SMG and the British Sten sub-machine gun. The M3 was durable, accurate and resistant to dirt jamming it. However one of the main drawbacks was it could only fire in automatic causing an occasional jam from the ammo feed. Another drawback was the firing rate, which was 450 rpm compared to its predecessor, the M1928 Thompson's 675 rpm. It still came out in front of the Thompson because the M3 used state-of-the-art sheet metal pressing, making it cheap and robust. The Grease Gun was used mainly in the Pacific or by the French Resistance because of its small size and reliability.
After 1945 the U.S. sold most M3s manufactured to other countries. The gun was used in the US armed forces until 1992 when it was replaced by more modern sub-machine guns and assault rifles. Over 655,000 M3s and other versions have been produced in many countries from Argentina to Taiwan since 1942, and the reliable sub-machine gun was used by many foreign militaries going even up to the 1990s. As of 2004, the Philippines have revived and revamped mothballed stock of M3 Grease Guns, coated with corrosion-resistant camouflage finish, silencers and picatinny rails.
- Julio S. Guzmán, Las Armas Modernas de Infantería