Behold the KV-500, a horrid mix of oscillating turret, superheavy landship, and casemate tank destroyer. Complete with a 500mm cannon!
Built in the post-apocalyptic 70s by a Soviet successor state, the KV-500 is nothing short of insane. Somehow, despite resource shortages and half of Europe being an irradiated wasteland since the late 40s, this...thing was created. Or at least one prototype was. The entire thing was built around the gun, with all focus on firepower. Despite paper-thin armor, the thing still weighed almost 160 tons.
It never saw battle, and what records remain of its testing describe it failing catastrophically one day, with the turret becoming dislocated and "falling through the hull", hitting the ammunition stowage and detonating said ammunition. The vehicle's thin armor did little to hold back the detonation of all eight shells.
And that's the story of the post-Soviet post-apocalyptic 500mm Bob Semple tank. Nonetheless, it did atomize an old Panther II that was on the firing grounds.