Hv-400
A theoretical German 400mm anti-tank gun proposed in the final year of World War 2. The Hv-400 was intended to be a super-heavy anti-tank weapon capable of destroying any Allied tank, but it remained purely a theoretical study.
Dm. 400
A Soviet 400mm railway gun employed by the Red Army during World War II. Its massive size and firepower allowed it to bombard enemy positions and fortifications from a distance.