During World War II, various companies and weapons designers vied for the coveted contract that ultimately became the M1 Carbine in .30 Carbine. One of those companies was Auto Ordnance, which did submit a “light” rifle design, but as a “just in case” also developed a .30 Carbine version of their Thompson SMG.
Originally chambered for the .45 ACP pistol cartridge, it took a bit of engineering to bring the Tommy Gun up to .30 Carbine specification. Specifically, a new magazine and receiver was required to handle the longer and more powerful load. From there, various internal mechanical bits were reinforced, but the primary operating action was the same.
The weapon was truly a “just in case” submission to the Army, which was summarily rejected. The .30 Carbine Thompson SMF was nearly double the weight requirement for the light carbine, coming in past 12 pounds, unloaded. Auto Ordnance hoped to win the contract as minimal retooling was required to get the weapon into production.
All the common quirks of the ubiquitous Tommy Gun remained. The magazine release was activated upward, the charging handle was on the top, which reciprocated as the gun fired, and the safety and selectors were in their usual unusual locations.
To see all the details of the .30 Caliber Thompson, check out the video from Forgotten Weapons: