An open bolt SMG design does cool better, but that is not its main advantage. By using an open bolt, the gun is set up so that it can use advanced primer ignition (API), touching off the primer just BEFORE the bolt fully closes. That means the backward pressure is fighting the momentum of the forward-moving bolt as well as its mass. And that means the bolt can be much lighter and use much lighter springs than if the gun is made to fire from a closed bolt.
Anyone who has fired both the TSMG and the current semi-auto version will know exactly what I am talking about.
There was actually no experimentation with other calibers during the design of the TSMG, the earlier .30-'06 auto rifle having been abandoned. The .45 ACP was chosen simply because it was the U.S. service cartridge and the market was intended to be the U.S. military. Early advertisments say the gun was available in ".22, .30., 38., and .45", but only the .22 (a conversion unit) was produced. I have seen no indication which .30 or .38 cartridges were meant.
Later, in 1926, guns were made in the UK by BSA in 9mm Parabellum, but those, while internally like the standard Thompson, had a different profile.
(Those bulged cases seem to be the result of an ovesize chamber, not the gun opening too soon.)
Last edited by James K; October 30, 2010 at 01:05 PM.