The original open bolt models (preferably full auto if it is legal in your jurisdiction) if you can find one at a reasonable price. The original M11 was in 380 and not much bigger than a 1911. I used to know someone with a registered m11 and suppressor. This was truely a very cool, room clearing setup, especially with a 20 round mag in the gun and 2 32 round mags in the shoulder holster.
Next would be an open bolt 45 suppressed m10.
Sure the tax stamps are expensive but it costs money to have a good time.