The SU-122 was the first major variant of the successful T-34 chassis. After the first German Sturmgeschütz assault guns were seen in operations, an order was issued in April 1942 to several design bureaus to devise plans for a prototype SPG. Many heavy guns were tested in the process. It was seen the assault gun formula amassed the advantages of a cheaper, easier produced vehicle with the use of heavier guns, albeit with the disadvantage of having to turn the entire vehicle to aim in combat. In the end, both 122 mm (4.8 in) and 152 mm (5.98 in) field howitzers were retained. Only the 122 mm (4.8 in), developed by F. F. Pietrow's design bureau as the M-30S, was found suitable for a medium tank chassis.