The Thompson submachine gun is an American submachine gun that became infamous during the Prohibition era. It was a common sight of the time, being used by both law enforcement officers and criminals. The Thompson was also known as: the "Tommy Gun", the "Trench Broom", the "Chicago Piano", the "Chicago Typewriter", and the "Chopper". The Thompson was favored by soldiers and civilians alike for its compactness, large .45 ACP cartridge, and high volume of automatic fire.