I don't know if Daniel implied that it had to be concealed carry in his original post, but if NFA rules went away I would find a late production Thompson "Tommy-gun" with the fixed firing pin and right side cocking handle.
It'd probably spend most of the time in my truck, but I've just always been fascinated by them. I have yet to pick one up let alone shoot one, but I bet it's a real kick.
I've seen them shoot them on the History Channel and they don't look too uncontrollable, that thing throws a crap load of lead in short order. I bet with practice it could stitch a BG without many misses.