Hv-300
A concept for a German 300mm anti-tank gun that was proposed in the final year of World War 2. The Hv-300 was intended to be a super-heavy anti-tank weapon capable of destroying any Allied tank, but it remained purely a theoretical study.
Dm. 300
A Soviet 300mm railway gun deployed by the Red Army during World War II. Its massive size and firepower allowed it to bombard enemy positions and fortifications from a distance.