The Thompson submachine gun is an American submachine gun that became infamous uring 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", "Chopper", "Chicago Typewriter","The Trench Broom" and "Chicago Piano". The Thompson was favored by soldiers and civilians alike for its compactness, large .45 ACP bullet, and high volume of automatic fire.