122 mm gun M1931/37

122 mm corps gun M1931/37 was a Soviet field gun developed in late 1930s by combining the barrel of the 122 mm gun M1931 and the carriage of the 152 mm howitzer-gun M1937. The gun was in production from 1939 until 1946. It saw action in World War II and remained in service for a long time after the end of the war. Vehicle-mounted variants of the gun were fitted to the IS-2 and IS-3 tanks of the Iosif Stalin series of tanks and the ISU-122 self-propelled gun.

Source: Wikipedia