My employer has bought various class III weapons including a couple M16's --for the tune of $28,000 a piece, so I'm sure it's "legal".
Unfortunately I have to limit my budget to around $600 monthly to buy / build toys with...so a fully automatic attachment -- let alone a "real" tank -- isn't doable. Likely just going to be another small track loader with a modified frame, powertrain, and sheet metal body.