Schwerer Gustav (English: Heavy Gustav) was a German railway gun. It was developed in the late 1930s by Krupp in 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 , and could fire shells weighing to a range of . The gun was designed in preparation for the Battle of France, but was not ready for action when that battle began, and in any case the Wehrmacht's Blitzkrieg offensive through Belgium rapidly outflanked and isolated the Maginot Line's static defences, which were then besieged with more conventional heavy guns until French capitulation. 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 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 uprising 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 Soviet 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 was surpassed in calibre only by the unused British Mallet's Mortar and the American Little David bomb-testing mortar—both at —but was the only one of the three to be used in combat. Development In 1934, the German Army High Command (Oberkommando des Heeres (OKH)) commissioned Krupp of Essen to design a gun to destroy the forts of the French Maginot Line that were nearing completion. The gun's shells had to punch through seven metres of reinforced concrete or one full metre of steel armour plate, from beyond the range of French artillery. Krupp engineer Erich Müller calculated that the task would require a weapon with a calibre of around , firing a projectile weighing from a barrel long. The weapon would have a weight of over . The size and weight meant that to be at all movable it would need to be supported on twin sets of railway tracks. In common with smaller railway guns, the only barrel movement on the