Schwerer Gustav was the name of a German 800mm railway gun. It was the largest-calibre rifled weapon ever used in combat, the heaviest mobile artillery piece ever built in terms of overall weight, and fired the heaviest shells of any artillery piece. 

Gustav was employed in the Soviet Union at the siege of Sevastopol during Operation Barbarossa, where among other things, it destroyed a munitions depot buried in the bedrock under a bay. The siege of Sevastopol was the gun's first combat test.