"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.
SG-122(A), first prototype.
SG-122(A), first prototype, cutaway, showing maximum gun elevation.
SG-122(A), first prototype, top view, cutaway.
After trials in September, the following deficiencies in the design were discovered:
- Insufficient off-road performance on soft terrain, and high pressure on front road wheels.
- High pressure on the commander, due to also performing the duties of a gunner.
- Insufficient operational range.
- Difficulty in using gun ports due the position of the ammo rack.
- Insufficient robustness of casemate armour plates.
- Rapid buildup of gases in the fighting compartment.
Two new prototypes were built on the PzIII chassis to resolve these issues. The vehicle was lightened by using 35 mm armour plates instead of 45 mm, and was equipped with wider winter tracks. The commander and gunner switched places, with the commander now performing horizontal aiming. Front hatches were removed to increase protection. The commander was also given an improver periscope and panoramic sight. The gun ports were replaced with those of superior design from the SU-76I. Their size allowed not only the use of the Nagant revolver, but any small arms used by the Red Army. The rate of fire was increased (according to Svirin, to a very rapid 7-8 RPM) by adding a loading tray. Large and awkward gun shields were replaced with an improved mantlet, more capable of protecting the weapon from bullets and shrapnel. An exhaust fan directly above the gun helped vent gases after firing.
SG-122, improved prototype, front view, cutaway.
They were tested at the Gorohovets proving grounds shortly after the SU-122 decree, on December 5th. The SU-35 (SU-122) participated in these trials as well. Based on trial results, orders for the SG-122 were cancelled. Only 21 of these vehicles were produced. It is unknown how many used PzIII hulls and how many used StuG hulls, but M. Svirin writes that 8 StuG hulls were used for conversion into SU-76Is and SG-122s.
Actual combat use was complicated by a lack of spare parts and the use of a captured radio, which operated on a different range of frequencies than Soviet ones. In February of 1943, all existing vehicles were transferred to training units.
One experimental vehicle was produced using the PzIV chassis, indexed SU-122/T-4.
Information and images retrieved from Battlefield.ru, Solyankin et al, Domestic Armoured Vehicles of the 20th Century, Volume 2, Exprint, 2005 and M. Svirin, SturmGeshuetz III Assault Gun, Zeughaus, 2007