I. Definition and role.
1. The auxiliary-propelled ARGK guns consist of an open system with the traversing mechanism of a 152 mm gun model 1935 (203 mm howitzer model 1931) mounted on a special chassis incorporating assemblies from KV or IS tanks and having light armor protection only in the front.
2. The ARGK auxiliary-propelled guns are heavy mobile artillery guns attached to large units on the main axis of advance.
3. The guns are intended to accomplish the following fire missions: destruction of enemy concrete fortifications, including by direct fire at ranges of 1500–2000 meters, and the destruction and suppression of enemy artillery.
II. Main specifications
A. The cannon unit.
1. The following basic specifications apply to the cannon unit of the ARGK auxiliary-propelled guns:
- Vertical arc of fire from -3° to +65°
- horizontal arc of fire 60°
- traverse rate: same as for the BR-2 and B4 systems
Rate of fire:
- for the 152 mm gun 2–3 rounds per minute
- For the 203 mm: 1–2 rounds per minute
Basic load transported with the gun: 10–12 rounds
Gun crew (including driver-mechanic): 8
Time for transition to and from travel and firing positions: not more than 2 minutes
2. Weight of gun in firing position: 45–48 tonnes
3. The gun’s front armor protection must consist of a gun shield with lateral extensions about 35 mm thick (for protection against shrapnel).
4. For convenience of crew operation in the travel position, the area of the platform may be extended by means of folding decks that are stored for travel.
5. To support delivery of fire throughout a 60° arc, vehicles may be equipped with side-mounted (anchor type) trails.
6. For long marches, the guns may have provision for a special travel position accomplished either by rotating the traversing mechanism by 180° or by retracting the barrel.
7. For loading the guns, a special device must be developed that is actuated either by means of a motor or manually (requiring no more than 4 people to operate).
The guns may have a set loading angle in accordance with the design of the BR-2 and B-4.
8. The guns must incorporate the sights normally used for the B-4 and BR-2 that support both direct fire and fire from maximum range.
9. Each gun must be equipped for both radio and telephone communications.
B. The chassis.
10. To achieve the specified weight, the chassis must be made of high-hardness steel armor plates 35 mm thick.
11. Special braces may be used inside the hull of the vehicle.
12. The chassis may be lengthened as required to accommodate an additional road wheel.
13. It would be desirable for the gun to retain the speed of movement and mobility of the IS-152 vehicle.
14. Fuel endurance: 100–120 km
III. General requirements.
1. The gun crew must be provided with seats for long marches.
2. The chassis must have areas for storage of a SPT&A for the gun and vehicle, the telephonic communications equipment, a first-aid kit, dry rations, and the crew’s personal gear.
3. The bow and the stern must have tow hitches like those on the IS tank.
IV. Additional specifications.
1. For transporting ammunition in excess of that stored on the vehicle and intended for carrying out a specific fire mission, develop a tracked armored trailer capable of carrying 50 rounds for the B-4 or 100 rounds for the BR-2 that can be towed behind the vehicle.{9}
Ironically, the specifications for vehicle type and weight differed little from those for the heavy SU-14 SP gun developed 10 years previously.
Manufacture of the SP gun, dubbed the S-51, began in January 1944. In theory, mounting PB-4 on the KV-1S chassis required extending the hull and adding an additional pair of road wheels. Also, talk about a special chassis during wartime was tantamount to putting a project on the back burner. Therefore, TsAKB decided not to modify the running gear. The only changes made to the KV-1S in repair status had to do with mounting the new gun on it.
The tipping parts of the B-4 203 mm howitzer were adopted without change. The gun was moved far forward; its base partially overhung the driver’s compartment. The howitzer barrel was retracted in the travel position. The gun was mounted on a special frame that partially obscured the view from the driver-mechanic’s vision block. The system did not have a gun shield at first. Later, a 7 mm thick gun shield was installed to protect the crew against shrapnel. The shield consisted of two parts that folded forward to facilitate movement of the B-4 barrel from the travel position to the firing position. When that was done, the shield rested on special supports on the vehicle.
A platform to hold some crew members when the gun was in firing position was built on shelves over the S-51’s tracks. The platform’s internal spaces served as containers for the SPT&A, and the platform was given a railing to prevent accidents. The main ammunition was stored in regular wooden boxes that were attached to rails and lay on the floor of the platform while the gun was in travel position. Fold-out ladders were added to the SP gun’s stern to facilitate getting on and off the vehicle. Folding guide rails on which a loading tray was placed were located between the ladders. The guide rails were primarily needed for loading the gun from the ground. The S-51’s basic load consisted of just 12 rounds. When firing from cover, therefore, the basic calculation was based on the ammunition carried. It should be noted that when the S-51 was in firing position, part of its 10-man crew was located on the vehicle and the rest on the ground.
The heavy, open-type SP gun program entered its active phase in early February. The S-51 continued to be a low priority and therefore is not included on the list of high-priority programs mentioned above. The SP gun was first mentioned in a letter Ustinov wrote to Yakovlev on February 15, 1944, concerning the finished vehicle:
This is to inform you that the TsAKB of the People’s Commissariat of Arms has manufactured the B-4 203 mm howitzer on a KV-tank chassis (the S-51 system). The S-51 system underwent factory firing tests at Factory No. 88’s test range (at low elevation angles) and at the Sofrinsky Artillery Range of the People’s Commissariat of Munitions (at high elevation angles). During the factory tests, 49 rounds were fired. The S-51 system past both the factory firing tests and the road test done at the same time. During the tests, a rate of fire 30% higher than that obtained on the V-4 system on this normal carriage was measured.
Before the S-51 system is handed over for proving-ground tests, TsAKB is modifying the system in accordance with instructions received from the Artillery Committee Chairman Lieut. Gen. of Artillery V. I. Khokhlov (it is adding an armored shield for protection against small arms fire).
I hereby request your guidance on conducting proving-ground tests of the S-51. It would be advisable to carry out these tests at the Sofrinsky Artillery Range of the People’s Commissariat of Munitions because the Gorokhovets Artillery Proving Ground lacks the hoisting equipment needed to disassemble the system before and after testing.