I don't know if anyone has mentioned this yet, but if you are able to get an M16 type, my understanding is you can sub out all the many uppers for it legally, since the lower is the registered machine gun. Full auto .50 beowulf, 9mm, .45, .25 WSSM, .223 gas piston upper, younameit! So that makes the question a no-brainer in my book.
hoyt, mighty nice looking "truck gun" you've got there.