As expected as there's no federal law against owning a fully-operable tank or fighter jet. Well the tank howitzer may need to be registered as a destructive device with $200 tax stamp in accordance with the National Firearms Act due to the bore width.
It's also legal to equip them with machine guns, as long as the gun's transferable under the Hughes Amendment. An M2 Browning would set one back $20K or so.[1] But a guy like Paul Allen could splurge and get a GE M134 chain gun for $500K.[2]
One could also equip them with explosive ordnance as long as each piece of ordnance is registered as a destructive device (along with $200 tax stamp).
These are minor obstacles to the wealthy. Gun control is for the poor.
[1]: https://www.morphyauctions.com/jamesdjulia/item/lot-1009-m2-...