When people bring up this I don't think they realize that there are already very few limits on what you can own in the US... if you have the money. There's nothing inherently illegal about owning fighters or tanks, for example [0] [1]. If you browse through [2] you'll find plenty of examples of fully functional civilian owned large caliber cannon and artillery. So if you want your jet or tank to be fully functional that's totally doable. Cannons larger than .50 caliber and any kind of explosive (exploding rounds, bombs, missiles) are perfectly legal, but they're classified as destructive devices that require ATF approval and $200 tax to purchase (that's $200 per bomb or projectile). Of course if you're in a position like Mr. Private Air Force in the first link, you can get to buy these things pretty freely without the tax or oversight.

As for needing more than 5 rounds for self defense, well, it's a bit tongue in cheek but I go along with this logic: https://www.reddit.com/r/liberalgunowners/comments/am4epf/en...

0: https://www.thedrive.com/the-war-zone/32869/this-man-owns-th...

1: https://www.hotcars.com/here-are-the-worlds-largest-private-...

2: https://www.forgottenweapons.com/category/artillery/