"Work on SPGs, especially heavy ones, stopped in the USSR after the start of the Great Patriotic War. This was largely caused by the fact that the factories were busy with other orders. In addition, many factories were evacuated eastward. Only light SPGs were put into production at the start of the war, and these were largely improvised.
Meanwhile, due to the number of factories that switched from making artillery tractors to tanks, the artillery branch was forced to revisit SPGs towards the end of 1941. Experience gained in battle and a number of other factors meant that the new generation of SPGs that was built in 1942 was radically different from pre-war designs. This is especially true for medium SPGs, which became assault guns instead of tank destroyers. The SG-122 was one such SPG, built on the chassis of a captured German SPG.
First, there was Atrshturm
In early December of 1941, the GAU prepared a plan for work on SPGs. The plan included an 85 mm tank destroyer on the T-34 chassis, which was to be assigned to factory #8. This project, known as U-20, became a victim of its own requirements. According to them, the factory could not make changes to the T-34 chassis. The designers managed to build such an SPG, but it did not match other requirements…"
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