IIRC on the defunct Discovery show "Sons of Guns", Will Hayden (currently in prison) converted a M2 Browning parts kit to full automatic (for a permitted LEO buyer, I think a Sheriff).
Also IIRC, the machine work looked like it took only a few minutes.
(This is HIGHLY illegal for the hoi polloi, but so is murder and mayhem.)
Just to go a bit further -- a semi-automatic M2 would be perfectly legal. People buy "parts kits" of machine guns with the fully automatic receiver cut into pieces, and also purchase a semi-auto receiver to build a semi-automatic and therefore legal firearm around, using all of the other parts in the kit.
This is probably more commonly done with AK47s but apparently can also be done with M1919s and M2s.