203 mm howitzer M1931

203 mm howitzer M1931 was a 203 mm Soviet high-power heavy howitzer. During the Second World War, it was under the command of the Stavka's strategic reserve. It was nicknamed "Stalin's sledgehammer" by German soldiers. These guns were used with success against Finnish pillboxes at the Mannerheim Line, heavy German fortifications and in urban combat for destroying protected buildings and bunkers.

Source: Wikipedia