Despite less-than-satisfactory accuracy and a number of flaws with the carriage, the test commission issued a positive finding on the gun on March 25, 1943:
When the defects noted above and indicated in this report are corrected, the M-4 203 mm mortar can be recommended for service with the Red Army.
The M-4 203 mm mortar is deserving of attention because it fires a powerful shell that can be effective against enemy fortifications, it is highly maneuverable, and it can be put into production quickly and rapidly manufactured in appropriate numbers for forces in the field.{2}
On June 12, 1943, Stalin signed State Defense Committee Decree No. 3564ss “On Preparation for Production of the M-4 203 mm Mortar.” Factory No. 172 was to manufacture a batch consisting of four mortars for troop trials by September 1, 1943. Factory No. 172 manufactured the four mortars specified in the decree during September and October. One underwent extensive checkout testing between October 30 and November 10. A commission issued the following finding based on those tests:
1. The M-4 203 mm mortar No. 5/7557 successfully underwent extensive checkout testing during which it fired 207 rounds and traveled 275 km.
2. Based on the tests, the Commission believes that all four of the M-4 mortars that were manufactured by Factory No. 172 and underwent factory testing can be released for troop trials.{3}
On November 25, the GAU Artillery Committee’s chief decided that two M-4’s would remain at the Gorokhovets Test Range and the other two systems would be transferred to the Gorokhovets Artillery Training Camp. Nevertheless, work on the mortar ground to a halt in early 1944. The M-4 suffered the unenviable fate of a prototype that did not make it into mass production.
Since things went well with development of the mortar, it was decided to use it as the gun for a self-propelled artillery system. This time, the work was not an initiative project. According to the records, the originator of the SP gun mounting the M-4 mortar was Maj. Gen. A. A. Tolochkov of the Engineer Artillery Service, who at the time was chief of the experimental design sector of the Engineering Council of the People’s Commissariat of Arms. OKB-172 coordinated with him on the preliminary specifications for the design. Exactly when the program to develop a heavy SP gun mounting the M-4 mortar got started is unclear. Judging by the dates on the project’s technical documentation, the work was already underway in April 1943.
Satel, chairman of the Technical Committee of the People’s Commissariat of Arms, and Lieut. Gen. Khokhlov, chairman of the GAU’s Artillery Committee, were sent conceptual designs for two SP guns on May 12, 1943. The first was the SU-2-122, a twin-barreled M-30 122 mm howitzer mounted on a T-34 medium tank chassis. This huge double-barreled shotgun, which was supposed to be capable of firing salvos, required the T-34 chassis to be lengthened by one road wheel. Of greater interest to us, however, is the second design, which was assigned the factory designation SU-203. According to its description, the following specifications guided the SU-203’s development:
1. Design it along the lines of the KV-14;
2. Maximize the number of rounds it can carry;
3. Make it convenient for the crew to operate;
4. Simplify feeding and loading without requiring extensive effort and give it a rate of fire of approximately one round every 1–1.5 minutes;
5. Give the system together with its ammunition a weight of about 46 tonnes;
6. Make the standard telescopic sight mounted on the M-4 howitzer its main sight. For observation, use the PTK tank commander’s panoramic periscope;
7. Armor thickness: glacis—70 mm, side—60 mm, top and sides—20–25 mm;
8. Secondary armament: antiaircraft machine gun. In addition, the armor must have small-arms ports with plugs.
The expression “along the lines of the KV-14” was highly appropriate for the SU-203, because A. F. Smirnov’s team developed it (drawings for the SU-203 and the SU-2-122 show Smirnov as the project manager). The project retained the mantlet and fixed armor from the original SP gun project. Everything else was an entirely new SP gun based on the KV-1S chassis. Because the tasker for the project did not specify maximum use of the SU-152 superstructure design, the OKB-172’s designers had freedom to maneuver. The result was an SP gun design that showed a great deal of thought had been given to the crew members’ jobs.
Because the M-4 mortar’s laying mechanism left too little room for the driver-mechanic, his area was shifted to the right. To improve working conditions for the driver-mechanic, it was suggested that the gun mount be moved slightly to the left. However, it was considered possible that the driver-mechanic could be returned to his regular position, in which case it was proposed to modify the breech mechanism drive to make it more compact.
A simple solution was found for the problem of the commander’s position, which was located just to the right of the gun in the SU-152—his function was combined with that of the gunner. A radio set was not included in the SU-203 on the principle that this made the idea of combining the functions of commander and gunner quite logical, especially since that did not reduce the crew size—it was given two loaders. The loader located on the left side of the gun enjoyed truly luxurious accommodations. He had a commander’s cupola with five vision blocks taken from the KV-1S tank. The right-hand loader’s spot was no less luxurious: he had a hatch with an integrated mount for the DT antiaircraft machine gun. The breechblock operator was given a PTK panoramic periscope. The commander had only an ST-10 telescopic sight coupled to the gun for use in firing from cover. Thus, the gunner was commander in name only.
The main group of 14 rounds was located in the sides (seven on each side), so the tanks on the right side had to be removed. A special mechanism was developed for removing the shells from the rack; their weight would have made them extremely difficult to remove without it. The outermost shell was taken during firing, and the next round rolled into the spot where it had been. The charges were located in individual canisters inserted in the ammunition racks. An additional two shells were located in recesses that extended outside the superstructure. Another two shells were placed in the reload trays as needed. These additional shells constituted secondary storage and needed to be transferred to the main storage rack as shells were removed from it. The dense placement of the ammunition meant the height of the SU-203’s fighting compartment was just 10 cm higher than that of the SU-152. In addition to the ammunition, the increased height was required for the crane equipment (without which handling the 100 kg shells would have been extremely problematic). Smirnov’s design team must be given its due; unlike the truly monstrous size and weight of the U-19, the SU-203 was as compact as possible and still had room to accommodate the crew in comfort.