Schwerer Gustav (English: Heavy Gustaf, or Great Gustaf) is the name of one of two massive World War 2 German railway siege guns. In 1934 the German Army High Command (OKH) commissioned Krupp of Essen, Germany to design a gun to destroy the forts of the French Maginot Line which were then nearing completion. The gun's shells had to punch through seven meters of reinforced concrete or one full meter of steel armor plate, from beyond the range of French artillery. The cannons had barrels 30 meters long, weighed nearly 1,350 tons each, and could fire shells weighing seven tons to a range of 23 miles. The size and weight meant that to be at all movable it would need to be supported on twin sets of railway tracks. The train carrying the gun was of 25 cars, a total length of 1.5 kilometers. It was the largest caliber rifled weapon in the history of artillery to see actual combat, and fired the heaviest shells of any artillery piece.
Though the cannon was not ready for action when the Wehrmacht outflanked the line during the Battle of France, Gustav was used in the Soviet Union at the siege of Sevastopol during Operation Barbarossa. It was moved to Leningrad, and may have been intended for Warsaw. Gustav was captured by US troops and cut up.
Here’s a video of Gustav’s sister cannon, Dora, firing: