If you can buy it on a 4473 with no nfa paperwork. Than you can build it yourself. This is an other, not in the sense of an aow but similar to a lower receiver, in that it doesn't fit into any definitions. This is why it's legal. Because in the absence of a state definition the federal definition takes hold. And this gun has no matching description on either state or federal level. There for it is just an other that you can own just as if it was just a lower receiver.
This guy gets it.