Named after the head of the Krupp family, the Gustav Gun weighed in at a massive 1344 tons, so heavy that even though it was attached to a rail car, it still had to be disassembled before moving so as to not destroy the twin set of tracks as it passed over. This 4-story (12 meters) behemoth stood 20 feet wide (7 meters) and 140 feet long (47 meters). Its 500 man crew, commanded by a Major General, needed nearly three full days (54 hours, to be exact) to set it up and prep for firing. With a maximum elevation of 48 degrees, the Gustav shell could fire shells weighing seven tons to a range of 47 kilometers (29 miles). The caliber was 80 cm, and Gustav could fire 1 round every 30 to 45 minutes.
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, forcing them to surrender uneventfully and making their destruction unnecessary.
Gustav was later 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.
After Sevastopol, Gustav trundled out of the lime-light. It was taken off to besieged Leningrad, but the Russians had other ideas and had pushed the German army back before the Gustav could be made ready. Its only other recorded appearance was outside Warsaw in 1944 when some 30 shells were fired into the city during the abortive rising. After that Gustav vanished. Numerous reports of its discovery in pieces, its scrapping, its capture or abandonment have been suggested but none of them stand up to very close examination; spare barrels and ammunition were found, but the gun itself was never seen again (Despite some reports that it was found wrecked on its special train by a US army unit in Bavaria at the end of the war). Gustav was destroyed near the end of the war in 1945 to avoid capture by the Red Army.
Gustav had cost 10 million Marks, and the price of the ammunition is unknown, but its only achievement seems to have been the demolition of a few Soviet and Polish defences and one ammunition dump, which was hardly a great achievement for a weapon that had cost so much in effort and money. For propaganda, or for boosting morale, or for frightening an unsophisticated enemy, Gustav, and the other super-guns, may have had their uses, but as a cost effective weapon of war it was nevertheless a non-starter.