Schwerer Gustav (English: Heavy Gustav) and Dora were the names under which the German 80 cm K (E) railway guns were known. They were developed in the late 1930s by Krupp in order to destroy large, heavily fortified targets. They weighed nearly 1,344 tons, and could fire a shell that weighed more than 7 tons at distances up to 37 km (23 miles). Designed in preparation for World War II, they were intended to be used against the Maginot Line. But instead of a frontal assault, the Wehrmacht outflanked the line during the Battle of France. One of the guns was used in the Soviet Union at the siege of Sevastopol during Operation Barbarossa. It was destroyed near the end of the war to avoid capture.

It is the largest calibre rifled weapon in the history of artillery and fired the heaviest shells of any artillery piece. It is only surpassed in calibre by the American 36-inch Little David mortar and a handful of earlier siege mortars which all fired smaller shells.[1]

80 cm Gustav gun shell, note the size relative to the tank

 

Heres a video of the gun in action. This gives you a good idea as to how massive this thing was. Note: Gustav gun comes in at 1:21.