Where I live it is perfectly legal.
I think you'd better check again...and the local range guys better read the BATF regs...
Post this thread in the NFA section...
be legal- with the proper paperwork and the $200 tax stamp registering it as a short-barreled rifle- which it becomes as soon as you put the stock on it.
Proceed carefully, there's a reservation at Club Fed for those that do this improperly.