"April 6th, 1942
On the draft project of the 122 mm SPG called "122-SG"
1. Contents of the project
The GAU Artillery Committee received a draft project of a 122 mm howitzer on the chassis of a captured StuG from the OKB-16 independent design group. The proposal was met with a question (letter #537868, March 17th, 1942) regarding the utility of replacing the German 75 mm gun with a domestic 122 mm M-30 mod. 1938 howitzer.
The NKV Technical Council had a positive reaction towards this proposal and offered the OKB-16 independent design group, led by comrade E.V. Sinilshikov, to execute the project.
The draft project was finished in a short time and presented to the NKV Technical Council and the GAU Artillery Committee.
The NKV Technical Council approved the project.
The material received by the GAU Artillery Committee consists of overall blueprints of the SPG and its technical characteristics.
The main characteristics of the 122 mm self propelled howitzer are as follows:
- The 122 mm SPH is designed to destroy dugouts, field fortifications, combat against artillery and motomechanized forces, as well as enemy personnel.
Direct and indirect fire is possible.
- Muzzle velocity: 515 m/s
- Shell mass: 21.76 kg
- Maximum gun elevation: 42 degrees
- Minimum gun depression: [illegible]
- Horizontal gun range: +/- 6 degrees
- Bore axis height: 1670 mm
- Ammunition capacity: 60 rounds
- Top speed: 70 kph
- Normal speed: 40 kph
- Thickness of front armour: 70 mm
- Height: 2050 mm
- Length (with barrel): 5850 mm
- Clearance: 400 mm
- Crew: 5
- Horizontal aimer/commander
- Vertical aimer/gunner
- Breech operator
The design of the re-armament project had the aim of using the 122 mm mod. 1938 divisional M-30 howitzer without changes of the oscillating part.
The SPG could be changed only to allow for the installation of the new armament, namely the front and top of the casemate.
The OKB-16 independent design group solved this problem completely. The oscillating part of the M-30 is installed without changes. The lower mount, made from cast steel, is new. Due to the larger dimensions of the M-30 compared to the 75 mm German gun, the height of the SPH increased by 100 mm.
The front of the StuG is protected by 20 mm of additional armour, which is a reasonable modernization.
The installation of a 122 mm howitzer increased the mass of the StuG by about 2.5 tons, which comes from the following:
- The howitzer: 1000 kg
- The ammunition: 900 kg
- 20 mm applique armour: 400 kg
Installation of the howitzer with the stock sight created poor conditions for observation of the battlefield and target. Improvement of conditions required elevating the sight by adding additional plugs to the sight mount.
Due to the addition of the M-30 howitzer recoil system armour, it is doubtful that the gun will still be balanced. It is necessary to recalculate the ratio of the balancing springs and Belleville washers for this new system.
It is necessary to allow the horizontal aimer to fire the gun. At the same time, add a device that prevents firing if the horizontal aimer is lagging behind.
Add a special railing to protect the loader that prevents him from coming up behind the gun breech.
- Approve the proposed project of a 122 mm SPH on the captured StuG chassis and begin development, taking into account the changes requested by the 2nd Department of the GAU Artkom.
- Ask the NKV to issue a production base for equipping captured StuG SPGs with 122 mm mod. 1938 M-30 howitzers.
- Ask the GABTU Chief to promptly restore two StuG vehicles and send them to the NKV in order to produce experimental SG-122 SPGs.
Acting Chief of the GAU Artillery Committee 2nd Department, Military Engineer 2nd Class, Sorkin
Assistant Chief of the GAU Artillery Committee 2nd Department, Military Engineer 2nd Class, Getmanov"