Takes alot more than just welding a few parts together as far as macs are concerned.The open bolt smg mac is a different animal altogether than the semiauto version that uses a hammer and a spring loaded short firing pin.The internal parts are totally different as also the bolt is different.
Without assembling an open bolt smg such as the mac series, correctly, could create a situation that might cause premature detonation of the primer,aka instant handgrenade, plus the trip to the bighouse and fines.
To get around the SBR ,you would have to start with a virgin receiver that started off as a pistol and registerd originally as a pistol receiver.Ak pistols are legal and dont require an special 200 dollar nfa tax, which is why you see them without the stock.Adding the stock is what changes its status.