Quoted: Quoted: IIRC, the original Thompson's fire from an open bolt which is a big no-no with the ATF even if semi-auto. Due to that issue, I don't think they've been popular to build up from kits All but the M1A1 fire from closed bolt, AFAIK.

The Thompson SMG's are all open bolts (read 1921 colt produced and then the same rifles just over stamped to something else).  Later when the government adapt the SMG, a few of the parts where changed, but they still where all open bolt SMG's.

Only the semi autos rifles produced by Auto Ordnances (the later companies are closed bolt semi automatic rifles (read 16" barrels).

Back to the original question, since the SMG kit parts are open bolt designed, very few parts are usable as is for a semi auto build, and the parts would need to be converted over to a closed bolt system. Simply, you can buy a semi completely built for the same price as just the kit, and the real use for the kits is replacement parts for a registered SMG.