I just saw that in this mos "Sport Aviation". I foresee a govt contract for air defense when the rest of the USAF is mothballed as planned.
He went thru hell getting this past the govt., as they thought he was an al quaeda operative or some such nonsense. He not only had to get past the EAA officials (most of whom couldn't tell a P-51 from a 747, or an M-2 from a Brown Bess flintlock), then the BATF (the moronic agency that handles "alcohol, tobacco,and firearms - 3 closely related subjects
) and God knows what the locals hassled him with.
He had to buy a $200 NFA tax stamp on each M-2 (like I had to do on my Uzi and RPB MAC-11 decades ago). It took at least as much effort to do that as to restore the 51 probably.
Those guns had a much higher rate of fire than the ground version (and you can always ID them from the straight cooling jacket with holes), and that ammo cost several $ a rd. This is a well heeled guy who knows how to spend his money. I love these guys. They won't let him load the weapons when he's flying though - pity. This has the potential for a great movie plot.
Yrs ago a guy had an RC Me-410 (I believe) with a 50MM gun......it was a .22 semi auto that could fire blanks or live ammo. An early attack drone
.