I don't think there are laws against owning tanks (even ones that work).
Well, they are not street legal and after paying fees and such you could tool around your private property, but fully operational and firing the main gun I think are more than problematic...like getting an FFL I think the hurdles are higher to get the proper permits and such and I have no idea how hard it would be to get ordnance. And like FFL's any such persons would be among the first people government would go after if it had nefarious ideas toward we citizens.
I guess my original point is - we should not be so encumbered by the Fedcoats.