It is, in fact, legal (but very expensive) to own a tank ( https://www.drivetanks.com/ , yes, that's a company, but a rich enough motivated person could fill out the same paperwork). Apparently each exploding shell is a NFA taxed destructive device ( https://youtu.be/GW2U0qORdLE ).

Who do you think makes the tanks? It's typically corporations, but could just as easily be a person... It isn't a socialist/communist company doing the work.

It's pretty much already the case that people can have access to these weapons, I'm just being explicit.