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The success of the SU-152 forced Soviet tank designers to develop another vehicles based on new JS tank, which production begun in Cheluabinsk. Works on "Object 241" performed at the same time as the works on JS-1 (JS-85) heavy tank. New assault gun looked like its predecessor - SU-152, though the new one had a higher compartment. Internal capacity of both vehicles was equal, so they had an equal amount of ammunition - 20 shots. Both vehicles had the same armament - 152 mm ML-20S gun-howitzer. In summer 1943 the new "object 241" was showed to the GKO and after that it was accepted to the Red Army service under "JSU-152" designation.
The main problem during mass production of the new assault gun was lack of ML-20S cannons. Soviet artillery plants weren't able to produce them in proper quantity. Although there were plenty of 122 mm A-19 cannons and huge number of 122 mm ammunition. As a result, in summer 1943 the SKB-2 (design bureau in Chelyabinsk) has been ordered to try to mount an A-19 cannon instead ML-20S.
It was easy because both cannons had the same gun-carriage. At the end 1943, the very first prototype - "Object 242" - has been finished. Then it was tested and accepted for service under the JSU-122 designation. By the end of 1943, the first party of the new JSU-152 and JSU-122 were ready, simultaneously with the first JS-1 heavy tanks.
During the war there were several attempts to increase the firepower of the JSU-122. The A-19 cannon was modernised for usage with JS-2 tanks. For this purpose cannon has been equipped with a new semi-automatic block-breech and renamed as D-25S. After modernisation, the practice rate of fire (don't mix with maximum ROF!) increased from 1.5 to 3 shots per minute.
Soon, the idea to mount D-25S on the JSU-122 was suggested. The new prototype with the D-25S was named "Object 249". This vehicle had a new gun mantlet, also crew's layout was improved. "Object 249" was accepted for service and renamed as JSU-122S (or JSU-122-2) designation.

The JSU-152, JSU-122 and JSU-122S heavy assault guns were based on JS-2's chassis. Most of their mechanisms and devices were unifiewd with the JS-2's ones. Although those assault guns were equipped with different fuel tanks.

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