The SU-152 and Related Vehicles

CHAPTER 8. Thicker, Longer, More Powerful!

The adoption of the KV-14 did not mean that the military had finally come to a decision about the gun system needed for a bunker buster. The ML-20S was one quarter weaker than the BR-2 that the artillerymen continued to dream about. However, several things had occurred by the winter of 1943 that finally killed the BR-2 SP gun idea. People’s Commissariat of Arms Factory No. 221, the developer and producer of the BR-2 and other heavy guns, was heavily bombed in August 1942, and then it became the site of one of the bloody battles for Stalingrad. Efforts to rebuild the factory got underway in the spring of 1943, but the Soviet Union could forget about heavy guns for a long time to come.

Nevertheless, one project to mount a BR-2 on the KV-14 still took place. This project, for which only a sketch remains, was developed by a team at Factory No. 9’s design bureau under the direction of F. F. Petrov. No textual data on the project has been preserved, but the project by and large was a continuation of the idea of installing a BR-2 in the ZIK-20 SP gun that dated back to October 1942. As with the ZIK-20, they took the barrel from a BR-2 that had been modified to fit on an ML-20 cradle. A dual-chamber muzzle brake was added to the barrel to reduce recoil. Judging by a similar project to mount the BR-2 on the ZIK-20, the combat weight of the KV-14 with a heavier gun would have been 2 tonnes greater, which would have been acceptable. The later projects for the ISU-152-1 and the ISU-152-2 with the BL-8 and BL-10 guns show that Petrov’s concept of rearming the SU-152 with a BR-2 was quite feasible. However, the project did not get beyond the conceptual design stage.

As mentioned above, mounting a BR-2 on the ZIK-20 chassis was not the only concept Petrov had come up with. He also floated the idea of putting the barrel of the U-3 203 mm corps-level howitzer on an ML-20’s elevating mechanism. Matters did not proceed beyond the talking stage with the ZIK-20, but in 1943 Factory No. 9’s design bureau decided to work up a conceptual design for mounting the U-3 on the KV-14 chassis. Some scholars have taken the sketches of this project to be for the U-19 SP gun, even though there is an interval of more than six months between the two vehicles. The barrel was the only difference between the U-3 and the installation of the BR-2 on the KV-14. To reduce recoil, the barrel was given a massive, dual-chamber muzzle brake. Unlike the KV-14 with the BR-2, this brainchild of Petrov raises several questions concerning implementation. The projectile for the U-3, which was identical to that of the B-4 203 mm heavy howitzer, weighed 100 kg. It is not entirely clear how the ammunition would fit in the KV-14’s extremely narrow and low superstructure or, most importantly, how the loader would be able to load rounds into the chamber by hand. Given the low height of the KV-14’s superstructure, it would hardly be possible to fit a crane like the one on the U-19 inside it. In addition, in 1943 the heavy U-3 203 mm howitzer’s star had decidedly waned. The gun did not go into production, and that meant that the project’s future had become even more hazy. The idea of putting a U-3 barrel on an ML-20 carriage also went unrealized, although they did receive instructions to manufacture a prototype in February 1943. An upgraded version of the U-3 howitzer that received the designation U-3 BM went unrealized. It was a U-3 with a barrel lengthened to give it the ballistics of the B-4.

For one reason or another, both projects sank into oblivion during the spring of 1943. Factory No. 9’s design bureau never revisited the problem of installing heavy gun systems on a KV tank chassis. It concentrated on tank guns and systems for medium SP guns.

Petrov’s abandonment of the idea of installing a BR-2 and U-3 in the KV-14 did not mean that the issue was dead. As the saying goes, nature abhors a vacuum, and Factory No. 172 took Factory No. 9’s place with a vengeance. This turn of events was logical: the Perm enterprise had developed and was the only producer of the ML-20. The factory’s design bureau also explored numerous heavy artillery projects using that gun-howitzer’s tipping parts.

In March 1942, Factory No. 172’s design bureau began developing a 203 mm corps-level mortar. S. P. Gurenko spearheaded work on the system, which received the factory designation M-4. Red Army artillery had no system like that, making it an enterprising project. In developing the M-4, Gurenko’s designers were guided by the following:

In other words, Factory No. 172’s design bureau decided to take a shortened B-4 howitzer barrel and mount it on the ML-20 gun-howitzer carriage. Unlike Factory No. 8’s design bureau, which in October 1942 submitted only a conceptual design for a U-3 howitzer barrel on an ML-20 carriage, its competitors went much further. On May 14, 1942, the GAU’s Artillery Committee issued a finding on the M-4 project that basically endorsed the idea. The first tests on the mortar took place from May 18 through 20, 1942, and involved an experimental carriage. A ballistic barrel made from the prototype M-40 203 mm howitzer’s tube and the B-30 152 mm gun’s breech mechanism was used for the test.

A small number of modifications were made to the ML-20 design for mounting the heavier and more powerful barrel. The top carriage was given a special fitting for attaching a purchase block and tackle for use in retracting the barrel for travel. The bottom carriage was given a limiter associated with the purchase block and tackle, also a small modification; and a trail was added to the design. The equilibrator was given a stronger spring, and the makeup of the SPT&A kit was altered somewhat.

A monobloc barrel was manufactured for the mortar in August 1942. The breech mechanism was taken from the B-4 howitzer with minor modifications. The tube was manufactured from scratch. The barrel was equipped with a massive slot-type muzzle brake to reduce recoil. The length of the barrel to the muzzle face was 3693 mm (18.2 calibers); with the muzzle brake added it was 4328 mm. For ammunition, the mortar used the normal shells for the B-4 203 mm howitzer. The shorter barrel reduced the muzzle velocity to 353 m/s.

According to factory records, the M-4 mortar was developed to destroy field fortifications. The carriage enabled it to fire at angles from -2° to 65° and at ranges of 3 to 9.8 km. The M-4 weighed 8300 kg in travel position and 7500 kg when configured for combat, which exceeded the same figures for the 152 mm gun-howitzer by less than half a tonne.

The M-4 began factory testing with a monobloc barrel on August 25, 1942, and 71 rounds had been fired by September 18. It fired a total of about 100 rounds during testing. In addition, it underwent towing tests between September 18 and October 21. In all, the M-4 traveled 400 km. No warping was detected during the tests. One defect did show up, but it was not due to design flaws. While climbing a slope, the prime mover towing the howitzer moved backwards and struck the left trail, bending the ammunition tray.

On November 5, 1940, the M-4 system was shipped to GAU’s Gorokhovets Artillery Scientific and Technical Experimental Test Range (GANIOP) (Ilyino Gorkovskaya Railroad Station) by order of GAU chief Col. Gen. Yakovlev. The technical documentation for the system accompanied it. The tests lasted from January 21 to March 13, 1943. During that time, the mortar fired 456 rounds and traveled 458 km over various types of snow-covered roads. The road tests were not without incident: while avoiding an oncoming vehicle on February 27, the Voroshilovets prime mover that was towing the mortar performed a maneuver that caused the M-4 to fall in a ditch, resulting in some damage.

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:

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:

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:

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.

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:

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:

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.

Following the design review, Eng. Col. Alymov, chief of the Self-Propelled Artillery Office of the Red Army’s Main Armor Directorate (USA GBTU KA), and Eng. Lt. Col. Kovalev, chief of the 3rd Department of the USA GBTU KA, prepared a report that was dated June 25, 1943:

These findings appeared to greenlight the project, and an SP assault gun with a 203 mm howitzer would finally take shape in metal. Or so they thought in the Self-Propelled Artillery Office—the artillerymen held an entirely different opinion. The main role of the M-17 was destruction of enemy fortifications, but some nuances emerged. Calculations for penetration by the M-40 howitzer’s shell revealed that at a range of 2000 meters it was 4% better than the ML-20. The two rounds were equal at 1000 meters, and the M-40 was 6% better at 200 meters! And those were the ranges from which concrete bunkers would be fired on. Thus, the rationale behind the M-17 was called into question, as reflected in a letter from the GAU’s Artillery Committee on July 26, 1943:

E. A. Satel, chairman of the Technical Council of the People’s Commissariat of Arms, expressed the same opinion in his letter of July 28, 1943:

The M-17 was the next-to-last project that involved mounting the 203 mm howitzer in an enclosed superstructure on either the SU-152 chassis or the later ISU-152. The Central Artillery Design Bureau (TsAKB, in Kalingrad, now Korolev) was the last organization to work on a similar project. In October 1943, it developed an enclosed SP gun that would mount either a 152 mm gun with the ballistics of the BR-2 or a 203 mm howitzer with the ballistics of the B-4. The work took place under the S-51 project, but it went no further than the conceptual stage.

After that, the design effort proceeded along two lines. According to the requirements of the Technical Council of the People’s Commissariat of Arms, work began on designs for an open auxiliary-powered system mounting the B-4 203 mm heavy howitzer (this system will be discussed in the next chapter). Attempts to mount a more powerful system on the SU-152 without changing its superstructure continued.

It should be noted that work to install a system other than the ML-20 had begun in the spring of 1943. As mentioned above, the experimental projects involving the SU-152 included mention of installation of an A-19 122 mm corps-level gun-howitzer in an SP gun. The GAU’s Artillery Committee authorized the work, and the Chelyabinsk Kirov Factory and Factory No. 172 were to carry it out. By April 29, 1943, the Chelyabinsk Kirov Factory had received one A-19 system, and work was underway to install it and develop the ammunition rack. A prototype SU-152 mounting the A-19 gun was expected by May 10, but work stalled for various reasons.

Meanwhile, in the summer of 1943 the Red Army came face-to-face with an opponent more formidable than the Tiger. Among the new armored vehicles employed by the Germans on the Kursk Salient, the 8.8 cm PaK 43/2 Sfl L/71 Panzerjäger Tiger (P) SP gun, better known as the “Ferdinand,” stood out. The glacis and superstructure of the heavy German tank destroyer boasted armor 200 mm thick, which meant it could survive fire even from corps-level artillery.

To say that the Soviet military took note of the Ferdinand greatly understates their reaction. The appearance on the front of this vehicle, which had had a production run of 90 units, sparked a number of projects. Suffice it to say that the “Object 701” heavy tank (the future IS-4), for which specifications were developed in November 1943, was created specifically to counter the Ferdinand. Work on the D-25 122 mm tank gun was also related to Ferdinand Porsche’s creation, although, in all fairness, development of that gun had begun earlier.

The M75 107 mm antitank gun reappeared among a list of tank, antitank, and self-propelled artillery projects dated September 15, 1943. One reason for the revival of that project, which had been abandoned in 1942, was that the gun was based on the ML-20 gun-howitzer carriage. It was theoretically possible that the idea of putting the M75 on a self-propelled vehicle, which had been buried in the summer of 1941, might have a chance at revival two years later, especially since targets worthy of it had appeared. However, the return to the 107 mm antitank gun project was very short-lived. In the same letter that mentioned the M75, GAU Artillery Committee chief Maj. Gen. Khokhlov suggested dropping a number of projects and replacing them with more promising ones:

Thus, instead of reviving the idea of using a 107 mm antitank gun, it was decided to develop a new gun based on the A-19 122 mm gun. That was approved at a meeting of the Technical Council of the People’s Commissariat of Arms and the GAU’s Artillery Committee held that same day—September 15, 1943.

On September 24, the chairman of the Technical Council of the People’s Commissariat of Arms sent a letter to TsAKB chief V. G. Grabin:

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:

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:

Moreover, three versions of the OBM-50 were submitted, all with the same muzzle velocity—1000 m/s. The first version of the gun (referred to as “Version A” in documentation), which was discussed in a memorandum, had a barrel 8566 mm (70.2 calibers) long and a maximum pressure of 2670 kg/cm². According to its design, this version resembled the A-19 120 mm gun very closely. Two additional versions of the OBM-50 were submitted on October 28. They were referred to as “Versions I and II.” Version I’s barrel was shortened to 8390 mm (68.7 calibers) and its chamber lengthened to 990 mm. Version II featured many more differences. The length of its barrel was reduced to 7430 mm (60.9 calibers); it also had a 990 mm chamber. Its maximum pressure was increased to 3000 kg/cm². The increased pressure required a new design for the breech end, which was given a sliding wedge breechblock.

The GAU’s Artillery Committee issued its finding on the OBM-50 conceptual design on November 12, 1943. Interestingly, the Artillery Committee’s 1st Department proposed its own ballistic solution for the gun in addition to these three versions. It had a 7440 mm barrel with a maximum pressure reaching 3200 kg/cm². Thus, there were four projects to choose from:

Thus, Version II’s design was the most suitable for the OBM-50 project. The breech end with a sliding wedge breechblock stipulated in the specifications for the gun was favored. However, a requirement to increase the spacing between the barrel gibs or modify them was expressed, because the existing design could result in excessive pitching or vibration. The designers were also required to simplify the breechblock because it required a great deal of machining, and reduce its width to 420–450 mm. They were also required to simplify the breech mechanism, which needed to be equipped with a former-type semiautomatic system.

Modifying the gun was not OKB-172’s only task. The last SU-152’s that the gun was designed for were rolling off the assembly line as the finding on the OBM-50 at the Chelyabinsk Kirov Factory was being signed. Therefore, the SP gun needed to be changed.

The engineering drawings for the modified system were reviewed on December 4, and OKB-172 submitted the design documentation for mounting the OBM-50 on the ISU-152 by the 14th. In addition to the heavy 122 mm gun, the project also included mounting the OBM-43 152 mm system on the ISU-152. The self-propelled version of that gun was assigned the designation OBM-53.

And what was Factory No. 172’s design bureau doing at the time? According to documentation, the conceptual design of the heavy 122 mm gun designated the M21 was finished by October 9, 1943. But something strange happened: according to the same documentation, the chairman of GAU’s Artillery Committee received the conceptual design from the Technical Council of the People’s Commissariat of Arms only on November 12; that is, on the same day the decision on the OBM-50 was reached. Why the documentation sat, going nowhere, for over a month is unclear.

Work on the M21 was led by Factory No. 172’s artillery design bureau chief V. A. Ilyin (who was also serving as acting chief designer at the same time). The design took the elevation and traversing mechanisms, cradle armor, and sights from the ML-20S. It also incorporated the frame and cradle, with some modifications. The barrel was 7747 mm (63.5 calibers) in length. According to calculations, the M21 could penetrate 208.4 mm of armor from a range of 1000 m, and it could penetrate 169.8 mm at a 30° angle of incidence.

As required by the gun’s specifications, the barrel was equipped with a single-chamber muzzle brake. According to the calculations, it absorbed up to 54% of the energy from a shot. Also in accordance with the specifications, the M21 used a sliding wedge breechblock design copied from the M75 107 mm antitank gun. Minor modifications to some parts were the only differences between the new gun’s breech mechanism and that of the M75. The firing mechanism design was similar to that of the ZIS-3. The cradle differed only in that the recoil mechanism included a counter-rod that provided a shorter recoil length, and the trigger mechanism was modified for use of the sliding wedge breechblock. No changes were planned to the design of the SU-152’s superstructure; only the storage racks needed modifying for the new rounds.

There was no further correspondence after the GAU’s Artillery Committee received the M21 documentation. The delay in reviewing the project in the Technical Council of the People’s Commissariat of Arms took its toll: by mid-November, there was no longer a need for another heavy gun, especially since the design would have to be modified for mounting on the ISU-152.