The success of the German StuG's did raise a big interest within the Soviet High Command. Assault guns were cheaper to produce, because of the absence of a turret and some other expensive elements. The larger compartment offered more room, so bigger and more powerful weapons could be installed in it. Because of the absence of a heavy turret, the vehicle's frontal armor could be increased as well.
In April 1942, the GAU ordered several design bureaus to develop an assault guns with 122 mm armament or even greater. All work was coordinated by the Ministry of Tank Industry (Russian abbreviation - NKTP). Each bureau did offer it's own project. Two of these projects were manufactured "in metal". The first of them (U-35 designation) was developed by the Uralmash (Uralsky Machine-Building factory). The second was the SG-122(A) Self-Propelleg Gun, which was developed by the Factory #592.
The U-35 was a fully armored assault gun, armed with the 122 mm M-30 howitzer, mounted in non-traversing compartments. The vehicle was based on the chassis of the T-34 medium tank. The SG-122 was armed with the same howitzer but was based on the chassis of an ex-German StuG III. In July 1942, the SG-122 was accepted for service, but was considered unsuccessful and hard to maintenance, so it was quickly removed from service.
The U-35 was protected with 45 mm of frontal armor. The elevation angles of the M-30 were -3° +26°, the horizontal (traversing) angle ±10°. Engine and transmission were left unchanged to increase the production capability and keep the cost price low. The maximum range of the vehicle was increased by adding some external fuel tanks.
The crew of five men was placed as follows: driver's station at the left front hull, gunner's station at the left behind the driver, commander's station was at the right front and the two loaders (the M-30 loaded separately) were behind the howitzer's breech.
In December 1942, after passing a full set of tests, the GKO ordered to manufacture a small installation party of SU-35 assault guns. Later on the vehicle was renamed SU-122. Till the end of 1942,a total of 25 assault guns were manufactured.
The same month, the first mixed regiment was formed, partially equipped with these SP-guns. The regiment consisted of four batteries of SU-76s SP-guns (16+1 vehicles in total) and two batteries of SU-122s (eight vehicles total). At the end of January 1943, a pair of SU-122 regiments were put into action near Leningrad. In March 1943, another two SU-122 regiments were formed and sent to the front.
The impression of the new regiments was different. Although the appearance of a new assault guns was met with delight, the use of SU-76's together with SU-122's caused many problems because of certain technical troubles with the SU-76.
After they were improved, all SU-76's were re-organized in separate regiments of light self-propelled artillery, while SU-122's were used to form medium self-propelled artillery regiments. These Medium SP regiments consisted of 16 SU-122's (organized in four batteries) and one commander's T-34 tank. Soon, the new SU-122 became very popular due to it's quite powerful armament and good armor protection. In May 1943, the new BP-460A HEAT round was accepted. That ammunition allowed the SU-122 to deal with enemy AFV's at long distances successfully.
Although, even without any hollow-charge ammunition, the SU-122 could deal with heavy German tanks, including Tigers.
And with a lens..
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