Theoretically, you can put a short 10.5" barrel on a semi auto Thompson, but it is a good idea to "paper" it first, by doing the NFA form 1 application, paying the $200, etc BEFORE you do anything else, and get the BARF approval. One can convert a regular rifle to an "SBR" (short barreled rifle), but you have to proceed legally. Auto-Ord did produce some SBR's, and may still be making them, but it may be cheaper just to make your own.