I'm not so sure about the "All Spitfires are considered mechineguns". The "problem" with them according to the ATF was their open bolt method of operation, as it was relatively easy to convert to full auto.
IIRC, In the 80's or so the ATF declared these semi-auto open bolt designs "readily convertible into a machinegun" and banned new manufacture, but any in circulation were grandfathered in. Open bolt semi-auto MAC-10's are still for sale on the used market, but typically command a collectors premium. I don't see how the Spitfire would be any different.
Yes, if you built a new one today the ATF would declare it a MG, but not the ones built before that ruling. Can anyone confirm this?
I don't carry a gun to go looking for trouble, I carry a gun in case trouble finds me.