Why U.S. Army Loved the 'Tommy' Gun

In 1917, when America entered the First World War, the United States Army tasked Brigadier General John T. Thompson, Chief of the Small Arms Section of its Ordnance Department, with designing a short-range, rapid-firing, large capacity infantry trench weapon. The general turned the job over to his own firearms firm, the Auto-Ordnance Corporation (AOC).

Thompson and his team determined the .45-inch Automatic Colt Pistol cartridge used in the Colt M1911 semi-automatic pistol provided the most suitable rounds for the automatic weapon they envisioned. Due to the ammunition slated for the new weapon, the term “submachine gun” came into general use, referring to the gun's sub-rifle caliber cartridge and its capability for fully automatic fire.