The Thompson submachine gun is a true classic in small arms. It is an icon of gangsters and GIs alike. The standard Thompson went through a series of modfications during its service life. The Tommy Gun, as standard, was chambered in .45 ACP (outside of some British multi-caliber efforts like 9mm and .30 Mauser,) except for an attempt by Thompson engineers to create a light support weapon using the submachine gun as its base.
The Model 1923 was that experiment.
Seeking a light weight alternative to the Browning Automatic Rifle, Thompson engineers attempted to squeeze more out of the compact envelope of the submachine gun. To do that, instead of completely re-chambering in a rifle caliber, weapons designers put energy into expanding the .45 itself.
The Model 1923 was built around the .45 Remington-Thompson cartridge. The bullet and case were heavier and longer than the standard .45 ACP round of the Thompson submachine gun. The M1923's .45 Remington-Thompson possessed a case length one-eight of an inch longer than the standard .45 ACP ( 22.8mm.) The .45 caliber round seated in the lengthened case was 250 grain FMJ. The standard .45 ACP bullet of the period was 230 grains.
The heavier round and longer case resulted in the .45 Remington-Thompson achieving a muzzle velocity of 1,450 ft/sec; about 500 ft/sec above the standard 230 grain .45 ACP.
In order to exploit this larger, faster round, the Thompson M1923 was built out with an extended barrel of 14 inches. The longer barrel allowed the .45 Remington-Thompson to achieve the 1,400 ft/sec + muzzle velocities.
To complete the support weapon envelope, Thompson engineers added a heavier barrel in one version. However, the standard M1923 was fitted with a bi-pod and option for a bayonet.
The M1923 Thompson while a good submachine gun, based on a reliable weapons platform, was not dramatically more effective than the BAR. It wouldn't be adopted by the United States military and it would fade into relative obscurity.