The Thompson submachine gun, a.k.a. the Tommy gun, was an early submachine gun design. It was popular among soldiers, police, and civilians alike. It was invented by Brigadier General John T. Thompson at the end of World War I as a "trench-boom" weapon, or a weapon that could clear out trenches very quickly. The war ended before the submachine gun could be used in battle. In 1919, Brigadier General Thompson had the company he founded, Auto-Ordnance Company, modify the gun for civilian use. The gun was classified as a submachine gun, meaning a small, fully automatic, hand-held gun that fires pistol ammunition. It remains an iconic firearm that is very popular with collectors. It fires .45 ACP caliber bullets, an effective and accurate combat bullet against human targets.