The finding on the SU-203 project was approved on May 21, 1943. Unfortunately, it was not in the project’s favor. There were no complaints about the SP gun’s design as such; the problem lay elsewhere entirely. The main reason for the M-4’s failure was the following:
The shell of the 203 mm self-propelled mortar submitted for review offers no firepower advantage over that of the 152 mm self-propelled gun-howitzer currently in the Red Army’s inventory because the 152 mm projectile with its velocity of 600 m/s at impact penetrates a layer of concrete 1.4 m thick, and the 203 mm with its velocity of 350 m/s penetrates only 0.8 m; in addition, the rate of fire (theoretical—according to calculations) of the 203 mm mortar is 1.5 rounds per minute, whereas the practical rate of fire of the 152 mm SP gun-howitzer is 2.8 rounds per minute.
This will fully satisfy the requirement of the Field Service Regulations that prohibit the use of a large-caliber gun if a combat mission can be achieved with a smaller caliber gun.
Thus, the 203 mm self-propelled mortar should be considered a gun of the fourth type—a heavy, auxiliary-propelled gun.
For that, however, the following requirements must be met:
a) Remove the strong, elaborate mantlet;
b) Increase the elevation angle to 65°–70°.
Based on its review of the project, the Artillery Committee of the Main Artillery Directorate of the Red Army has come to the following conclusions:
1. The 203 mm self-propelled gun must be considered a heavy, auxiliary-propelled gun.
2. The M-4 203 mm mortar is a low-power gun with low muzzle velocity and insufficient range.
3. The Artillery Committee believes it necessary to propose that OKB-172 develop the 203 mm howitzer model 1931 (B-4).
4. The OKB-172 of the People’s Commissariat of Arms is to revise the design in order to eliminate the elaborate, strong mantlet and must limit itself to a light gun shield. Increase the elevation angle to 60–70°. The total weight of the system must not exceed the weight of the SU-152 152 mm self-propelled gun-howitzer, i.e., 45 tonnes.{4}
Meanwhile, OKB-172 had two heavy corps-level 203 mm howitzers that used the same munitions as the B-4. One was the BL-39, which had been developed between 1938 and 1939 by the NKVD’s Separate Technical Bureau (OTB) (which later became OKB-172). The NKVD’s OTB was located in the infamous Kresty Prison, and the gun’s designator stood for nothing other than “Beria Lavrenty.” This OTB was one of the infamous “sharashkas,” R&D laboratories in the Soviet Gulag labor camp system where designers worked who had been sentenced to prison under various articles (primarily for subversive activities and as enemies of the people).
Between 1941 and 1942, the NKVD’s OTB was evacuated to the city of Molotov (now named Perm), where it was renamed OKB-172. Work on the BL-39 continued, but now it was in competition with the M-40 corpslevel heavy howitzer, which had been developed between 1938 and 1939 by Factory No. 172’s design bureau. The BL-39 lost out in joint testing to the U-3, which, however, never went into production either.
The M-40 was also in an unenviable position. The howitzer could not pass proving-ground tests in 1940 because its wheels dug into the ground when it was fired. In 1942, therefore, the M-40 was considered, if not hopeless, at least a system that was being marginalized. The howitzer continued to be mentioned in correspondence dating from 1942, but on October 23, GAU chief Col. Gen. Yakovlev received a letter from the GAU’s Artillery Committee that contained the following proposals:
1. Stop all work on the M-40 203 mm corps-level howitzer.
2. Compensation for actual costs based on the factory’s accounting calculation as certified by GAU’s regional engineer is within the contractual amount.{5}
Nevertheless, in the spring of 1943, the M-40 was the gun that Factory No. 172’s design bureau used as the armament for its SP gun. The SP gun assigned the factory designation M-17 was an ambitious project for the factory that had been approved by People’s Commissar of Arms Ustinov. The design was submitted for review on June 5, 1943, i.e., two weeks after OKB-172’s failure with the SU-203. Unfortunately, only a verbal description and engineering analysis of the gun remain.
The concept for the M-17 resembled the SU-203, and Factory No. 172 design bureau chief V. A. Ilyin headed up the effort to develop it. Minimal design changes to the KV-1S chassis were required, and the design made maximum use of parts from the SU-152’s superstructure. The fixed mantlet, shield, and frame were taken unchanged from the production SP gun.
Unlike the SU-203, the M-17 was the same height as the SU-152: 2450 mm. The crew makeup remained the same, as did the radio equipment, superstructure hatches, and vision devices. To achieve a 3° angle of depression, the fighting compartment roof was designed to slant forward at the same angle. Also, to achieve the needed fighting compartment size, the sides were extended to the full width of the vehicle in the rear. The fuel tank was increased in size, and the second tank, which was located along the left wall of the fighting compartment, was made smaller. In order to reduce the M-17’s combat weight, the thickness of the mantlet was reduced to 65 mm, the front plate of the superstructure to 60 mm, and the superstructure’s sides and rear to 45 mm. That was supposed to make it weigh 45.8 tonnes. Another version under consideration retained the plate thickness of the SU-152’s superstructure, giving it a combat weight of 46.8 tonnes.
The M-40 howitzer was chosen as the gun because its barrel was more suitable for use in the M-17. The design called for the M-40 barrel to be mounted on the ML-20S carriage, which underwent minimal modification. In addition, the M-40 barrel received a dual-chamber muzzle brake that was more than a meter long. The T-5 sight was used for direct laying.
According to the design, the basic load was to consist of 16 rounds. Shells were located in two magazines on the left side of the superstructure—nine in the rear and seven in the front. Charges were to be located in two iron boxes, one of which was located under the system, and the other in the right corner of the fighting compartment. The basic load also included 21 drums for the PPSh submachine gun and 25 F-1 hand grenades.