For context, the largest battleships ever built, the Japanese Yamato-class, were armed with massive 460 mm guns. The Gustav dwarfed even these with 10,000-pound 800 mm shells. Construction began in 1937, with the aim of breaking sieges in future battles. The weapon was impressive, but in general its size was more of a detriment than anything else. The overall weapon was 38 feet tall and required over 3,000 personnel to lay out its two dedicated railway tracks and set it up for use. It was a sluggish process that rendered its use rare, with both barely reaching Stalingrad and Leningrad before being pulled back. When the Gustav and its twin, "Dora," did see use, they wrought destruction unprecedented for a single gun (via War History Online).
In 1942, during one of the rare instances of its use in combat, the Gustav fired 47 rounds at Soviet fortifications and ammunition depots in Sevastopol (via All That's Interesting). Beyond this, the gigantic artillery pieces seldom saw any real use, and no more were manufactured. As Germany lost air superiority, such massive guns would have only represented target practice for Allied aircraft. Germany hid them away in favor of the V-1 flying bomb and V-2 rocket programs, which allowed them to fire similarly sized projectiles with greater range and far fewer resources than the Gustav.
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