45 ACP...for a carbine

[Toprudder]

The original Thompsons had a Blish delayed blowback system. In the early 1940's, Savage Arms engineers worked to reduced the cost of manufacturer of the Thompson, they changed it to a straight blowback among other cost reductions.

Interesting, I thought they made the change much earlier. I know they had to slow the cyclic rate to be much slower for the military.