A similar letter went that same day to A. I. Nekhovsky, director of Factory No. 172, and Col. N. A. Ivanov, chief of OKB-172. It called for the joint development of a gun with similar characteristics by Factory No. 172’s design bureau and OKB-172; the conceptual design was expected by October 20. The operational requirement for signing the gun was drafted three days later and approved on October 4 by the chief of the GAU’s Artillery Committee:
I. Role and mission
1. The 122 mm tank and heavy self-propelled guns are intended for arming heavy tanks (like the IS) and self-propelled guns manufactured using assemblies from heavy tank chassis.
2. The chief fire mission of the 122 mm heavy gun is the destruction of heavy enemy tanks and self-propelled guns (with armor up to 200 mm) at ranges of 1000–1500 meters and the destruction of the armored covers of concrete bunkers and armored embrasures.
The gun is also intended for destroying enemy personnel and artillery.
II. Ballistic data.
3. The 122 mm tank and self-propelled guns must have the following ballistics:
a. Caliber: 122 mm
b. Projectile weight: 25 kg
c. Muzzle velocity: 1000 m/s (for an armor-piercing shell)
d. Charge density: no more than 0.75
e. Powder grade: 22/1 or 24/1
f. Maximum pressure: approximately 3200 kg per cm²
In addition, the basic load must include the high-explosive fragmentation shell of the A-19 gun. The muzzle velocity of the HE/fragmentation shell must be less than 1000 m/s in order to increase the service life of the barrel and obtain larger angles of descent for the projectile.
III. Tactical specifications.
4. The gun must have the following characteristics:
a. Rate of fire: 6–7 rounds per minute
b. Accuracy at a range of 1000 m: 0.15 mu (axes of zone of dispersion)
c. Effort on laying mechanism flywheels:
To start: no more than 4–5 kg
While turning: no more than 2–3 kg
d. Laying mechanism flywheel play: 1/4 turn
e. Effort on mechanical trigger mechanism: no more than 10 kg
f. Vertical arc of fire: -5° to +20°
Flywheel aiming rate: -0.5 per turn
5. The gun must have separate loading.
6. The gun’s main sight is a collapsing telescopic sight. The tank has a conventional telescopic sight for redundancy.
7. The tank gun must have a clinometer for firing from cover. The self-propelled gun has a ZIS-3-type field sight.
8. During firing, the gun is served by a crew of three: the gunner and two loaders.
9. The gun must have a coaxial machine gun.
IV. General design specifications.
10. The tipping parts of the 122 mm gun must be designed such that it can be mounted in a rotating tank turret and in a self-propelled artillery vehicle’s hull.
11. The overall layout of the gun must provide for the lowest line-of-fire height and the most favorable loading level.
12. The weight of the gun without armor must be no more than 3500 kg, and the weight of the recoil mechanism must be less than 2700 kg.
13. The dimensions of the gun when mounted in a turret with an 1800 mm support ring must support convenient working conditions for the crew.
14. It would be desirable to have a natural balancing action; however, an equilibrator may be used.
15. A barrier must be provided for the recoiling parts.
16. The strength margins of the gun parts must provide the opportunity to further improve the ballistics and mount a 152 mm barrel with the ballistics of the BR-2.
17. Domestic factories must be able to employ the materials stipulated for manufacture of the gun. High-alloy steels and nonferrous metals must be kept to a minimum.
18. The gun must be simple to manufacture and rely on existing technological processes. Commonality of fasteners, threads (in terms of dimensions, classes, precision, and types), holes, and slots must be extensive.
19. The overall design of the gun must allow for the lack of adjustment mechanisms in field units.{11}
The TsAKB did not handle the project in a conventional manner. According to correspondence, the bureau received it and started work on September 25. The project for a long-barreled 122 mm gun was assigned the factory designation S-26-1. However, Grabin decided to go his own way. Simultaneously with the S-26-1, TsAKB was able to push the idea of a 130 mm and SP gun, which it developed on the basis of the ballistics of the B-13 naval gun. Development of the project, which received the designation S-26, was delayed. It was approved in early 1944, when the SU-152 was no longer relevant. For that reason, the ISU-152 SP gun began to emerge as the base chassis for the S-26 and S-26-1.
In this instance, it was not assigned to work jointly with the Perm design bureau. Instead, Factory No. 172 and OKB-172 began developing competing projects. The first with this important task was OKB-172, which succeeded in drafting the documentation in record time—less than two weeks. M. Yu. Tsirulnikov, OKB-172’s chief designer for ground artillery, oversaw the design bureau’s efforts. Like other sharashka workers, he was a prisoner, but he received an early release (by State Defense Committee Decree No. 3612, dated June 18, 1943) when the M42 45 mm antitank gun project got underway.
By October 5, 1943, a conceptual design for the gun lay on the desk of the chief of the GAU’s Artillery Committee. To be more precise, it was a conceptual design for more than one gun, because in addition to the 122 mm high-power gun, OKB-172 had developed a project to mount the OBM-43 152 mm system with the ballistics of the BR-2 in the SU-152:
By special telegram No. 5014, dated 24 September of this year, in accordance with a decision by the People’s Commissariat of Arms and the chief of the Main Artillery Directorate, OKB-172 and Molotov Factory No. 172 have been assigned to develop a 122 mm self-propelled gun having a muzzle velocity of 1000 m/s with a projectile weight of 25 kg.
OKB-172 of the People’s Commissariat of Arms places a great deal of importance on the project and has completed the conceptual design ahead of schedule. It is hereby submitted for your review.
Preparation of the conceptual design resulted in the following determinations:
1. The task of developing a 122 mm self-propelled gun with a muzzle velocity of 100 m/s can be completed by placing a new barrel on the KV-14 vehicle in place of the model 1937 gun howitzer and using a single-action hydro-pneumatic balancing mechanism.
2. Since the power of self-propelled guns is constantly growing, our proposed installation of the OBM-43 barrel, which has a muzzle velocity of 880 m/s with a shell weighing 43.5 kg, on a KV-14 vehicle instead of the assigned 122 mm barrel with a muzzle velocity of 1000 m/s cannot be neglected, because doing so would increase the power of the system by 33.5% and enhance the concrete-penetrating and explosive effect of the shell.
This approach both employs an extremely powerful gun for the vehicle and takes advantage of sharing components with the field gun, which would significantly facilitate production and use of these guns.