Object 249 was immediately launched in mass production as the ISU-122S (ИСУ-122С) self-propelled gun. However, the original ISU-122 remained in production (along with the ISU-152) due to a large stock of A-19 guns (the ML-20 and D-25 came directly from artillery factories). Mass production of the ISU-122 and ISU-122S ceased at the end of 1945. ChKZ produced 1,735 ISU-122 and 675 ISU-122S variants in total. After World War II, many surviving ISU-122s were rebuilt as rocket launchers, very large calibre gun chassis or supply vehicles. The small number of ISU-122s that kept their original armament were modernized in 1958. This modernization was not as complete as that of the ISU-152. Most ISU-122s did not receive a new engine, only upgraded gun sights and radio sets. In the beginning of 1960, the ISU-122 was withdrawn from Soviet Army service (the ISU-152 served much longer). Some disarmed ISU-122s were transferred to civil organizations, to be used as emergency vehicles on Soviet railways or as tracked transport in Arctic areas of the Soviet Union. However, most ISU-122s (both variants) remained in service longer in Warsaw Pact states and, with unconfirmed reports, in service in North Korea and China, having both variants. Variants Some vehicles were fitted with a 12.7 mm DShK anti-aircraft machine gun. ISU-122S - In later models, the A-19 gun was modified to have a semi-automatic breech block. This gun was designated D-25S, and the vehicle mounting it ISU-122S. The modification increased rate of fire from 1.5 rounds/minute to 3 rounds/minute. The ISU-122S variant is recognizable by its ball-shaped gun mantlet and double-baffle muzzle brake. The ISU-122S model was coded 'type 249' by the German's in their reporting. Construction and design Construction of the ISU-122 and ISU-152 is the same except for the gun mounting, sights and ammunition stowage. The A-19S or D-25S cannons of the ISU-122 had 18 degrees elevation angle and 30 rounds of ammunition (the ISU-152 had 20 degrees and 20 rounds respectively). The A-19S cannon was equipped with an ST-18 (СТ-18) telescopic sight and the D-25S cannon was equipped with a TSh-17 (ТШ-17) sight. Both types of sights had maximal exact targeting distance of 1.5 km (the ISU-152 could manage only 900 m). Maximum direct fire range of the A-19S or D-25S cannons was 5 km, much further than these sights' targeting abilities. For direct or indirect firing on distances over 1.5 km the gunner used the