Schwerer Gustav (English: Heavy Gustaf) was a German 80 cm (31.5 in.) railway gun.
It was developed in the late 1930s by Krupp in Darłowo (then Rügenwalde) as siege artillery for the explicit purpose of destroying the main forts of the French Maginot Line, the strongest fortifications in existence at the time. The fully assembled gun weighed nearly 1,350 tonnes, and could fire shells weighing seven tonnes to a range of 47 kilometres (29 mi). The gun was designed in preparation for the Battle of France, but was not ready for action when the battle began, and in any case the Wehrmacht's Blitzkrieg offensive through Belgium rapidly outflanked and isolated the Maginot Line's static defenses, eventually forcing the French to surrender and making their destruction unnecessary. Gustav was later deployed in the Soviet Union during the Battle of Sevastopol, part of Operation Barbarossa, where among other things, it destroyed a munitions depot located roughly 30 meters below ground level. The gun was moved to Leningrad, and may have been intended to be used in the Warsaw Uprising like other German heavy siege pieces, but the rebellion was crushed before it could be prepared to fire. Gustav was destroyed by the Germans near the end of the war in 1945 to avoid capture by the Red Army.
Schwerer Gustav was the largest-calibre rifled weapon ever used in combat and, in terms of overall weight, the heaviest mobile artillery piece ever built. It fired the heaviest shells of any artillery piece. It is surpassed in calibre only by the unused British Mallet's Mortar and the American Little David bomb-testing mortar (both 36 inch; 914 mm).