GatDaily article section on this gun, written by Travis Pike:
The rarest of the .30 Cal Thompsons is a .30-06 rifle variant that looks like an actual Thompson. It looks like the creators stretched a standard Thompson SMG to fit a .30-06 cartridge and magazine. The gun did use BAR mags with an added oil pad. The extreme pressure of the operation required an oil pad and lubricated ammo.
Only one of these experimental Thompsons exists, and it was made in 1943. It appears to be the only one and is, or at least was, privately owned. An old VHS uploaded to YouTube shows the most we know about the gun. The presenter explains that it is a blowback-operated firearm and used a delayed blowback, but not the Blish lock.
From what I can understand from the video, it uses some form of plunger-delayed blowback system combined with a strongly tensioned recoil spring. The collector wisely will not fire it due to the fact no one’s sure what will happen if you do. Will it explode? Will someone be eating a recoil spring? It’s too valuable to risk.
Cursed stretch FAL