"Grease Gun" refers to the M3A1, a submachine gun introduced in World War II to replace the Thompson M1A1 submachine gun. It has a high rate of fire and can be converted to use the ammunition and magazines of several other weapons, such as the British Sten gun. These characteristics made it a perfect weapon in Vietnam. SMGs were used widely by all groups in Vietnam; many were weapons originally used in WWII. Although the M3A1 became outdated, it was employed by several other nations and even by U.S. helicopter pilots.