The M3 Grease Gun was, much like the British Sten, designed as a simpler, cheaper replacement for the Thompson sub machinegun. It used the same .45 caliber round as the Thompson, but it could easily be converted to use 9mm rounds, as used in the Sten and MP40. Despite it being cheap to manufacture, it was a generally reliable weapon and thanks to its low rate of fire it was easy to control. The M3 has a rate of fire of 400 rounds per minute, while other sub machineguns had much higher rates. The sten and MP40 have 500 RPM and the M1 Thompson 650 RPM. At the end of World War 2 600,000 Grease Guns (both M3 and M3A1) were made.
Platoon leader and close combat soldiers are mainly equipped with it. Tanker kits also have it as a random choice. It is easy to hit a target as Grease Gun has hardly open sight. But definitely be aware that when you are re-holding the Grease Gun, and do a motion such as lifting it up, the view is narrowed immediately after re-holding.