In late 1942, SU-122s and SU-76es entered production. However, as it often happens, not enough vehicles were produced to meet the army's needs. The need for a 76 mm SPG was solved with the SU-76I. In case of a medium SPG armed with a 122 mm howitzer, the military was luckier. Work on a StuG III with a 122 mm howitzer began earlier that year.
"To the chief of the repairs directorate of ABTU KA, brigade engineer Sosenkov
Copy to factory #592 director Pankratov
In accordance with the decision of the Deputy People's Commissar of Defense Lieutenant-General of the Tank Forces comrade Fedorenko on the re-armament of captured StuGs with model 1938 122 mm howitzers at factory #592, I ask you to arrange for four repaired StuGs to be delivered to factory #592. In order for work to proceed without delay, the first StuG should be received by April 25th.
April 13th, 1942
Chairman of the technical council, E. Satel"
The vehicle was indexed Self-Propelled Howitzer SG-122 (StuG) (Samohodnaya Gaubitsa SG-122 (Artshturm)), more commonly called just SG-122 and sometimes SU-122/T-3. The crew consisted of a driver, a commander, two loaders, and a gunner. As with many pre-war Soviet indirect fire guns, the M-30 had vertical and horizontal traverse controls on opposite sides of the gun, forcing the commander to aim horizontally, and the gunner to aim vertically. One of the loaders also performed the job of a radio operator.
The vehicle was armed with a 122 mm model 1938 howitzer (M-30). Much like the SU-76I, the SG-122 was protected with up to 45 mm of armour. The majority of the vehicle's insides remained the same. The radio was moved from the left side to the right, into a special addition to the vehicle's hull. External gas tanks were added to increase the vehicle's operational range.