Wikipedia
"Karl-Gerät" (040/041) ( German literally "Karl-device"), also known as Mörser Karl, was a World War II German self-propelled siege mortar (Mörser) designed and built by Rheinmetall. Its heaviest munition was a diameter, shell, and the range for its lightest shell of was just over . Each gun had to be accompanied by a crane, a heavy transport trailer, and several modified tanks to carry shells.
Seven guns were built, six of which saw combat between 1941 and 1945. It was used in attacking the Soviet fortresses of Brest-Litovsk and Sevastopol, bombarded Polish resistance fighters in Warsaw, participated in the Battle of the Bulge, and was used to try to destroy the Ludendorff Bridge during the Battle of Remagen. One Karl-Gerät has survived and the remainder were scrapped after the war.