In the summer of 1942, S.J. Kotin and other designers, were given the task to develop a medium self-propelled gun initially called the SU-35.2
The Central Artillery Directorate (GAU) in April 1942 gave instructions to several design teams to work on a self propelled vehicle that could carry the 122 mm gun.1
The five man crew were able to work inside the roomy (compaired to tanks with turrets) superstructure.1
The gun was held in a cast mantlet with fabricated armor cover for the recoil system. Excelled at attacking strong points but was weak on antitank performance. HEAT ammunition was developed but is was only effective at short ranges and still wasn't capable of piercing much armor.
Late models had similar ball mantlet as the SU-85.2
The Uralsky Heavy Machine Tool Factory (UZTM) designated its design as the U-35.1 They used a T-34 chassis to build a box like superstructure onto.1,2 The superstructure had a 122 mm M-30 gun installed.1 The frontal armor was 1.77" / 45 mm thick.1
Zavod Nr 592 Team
The Zavod team designated its design as the SG-122.1 This used a German PzKpfw III chassis with a new hull.1 It posed great difficulties with mainenance of a foreign made chassis without spare parts being available.1
First issued in January 1943. In late 1943 production was phased out in favor of SU-152.
Production: December 1942 - 19441
1942: 252, 261
SU-122P: Prototype.1,2 Self propelled gun with a 122 mm 1931/37 corps cannon.2 Was to be a tank destroyer.1
Used in platoons of 3 vehicles each.
In the spring of 1943 self propelled gun regiments were formed with 17 SU-76s and 8 SU-122s.2 In April they had 4 or 5 batteries of 5 guns each of the same type.2 They were to support the infantry and defend against tank attacks.2