The timeline for development of these self-propelled systems, both the conceptual designs and the follow-on development stages, have not been established; they are working at their own initiative.
In view of the great importance of and urgency associated the development of self-propelled howitzers as a means of suppressing and destroying earth-and-timber emplacements and concrete bunkers, Factory No. 8 must be given strict deadlines, specifically for completion of the conceptual designs by June 15, 1942, and their submission to Moscow for consideration by the Red Army’s GAU Artillery Committee in conjunction with the People’s Commissariat of Arms and the People’s Commissariat of the Tank Industry.
A decision will be made and a general plan for implementation of these self-propelled systems will be drawn up after due consideration.
I hereby request that the director of Factory No. 8 be issued the appropriate instructions.{8}
In other words, Factory No. 8’s design bureau had been involved in the heavy bunker buster project since the spring of 1942. As was the case with the U-18, the factory’s design bureau had been working on mounting the ML-20. Work on the chassis was assigned to the Chelyabinsk Kirov Factory. In contrast to the U-18, on which correspondence is almost entirely lacking, the Factory No. 8 project designated the ZIK-20 was much discussed in both the GAU and the GABTU. Work on the heavy SP gun was led by T. A. Sandler, Factory No. 8’s chief designer (prior to the evacuation he had headed up the quality control department at Factory No. 8). The absence of any reference to the designation ZIK-20 in correspondence caused confusion later. According to GAU Artillery Committee documents dating from the second half of 1943, the projects of the Ural Heavy Machinery Plant and Factory No. 8 came to be perceived as a single entity, and that perception was reflected in subsequent correspondence and the Ural Heavy Machinery Plant’s summary report. As a result, many researchers saw the ZIK-20 as the U-18, and much confusion arose because no graphic materials existed for the two SP guns.
The GAU also attempted to assign development of the much-desired bunker buster fitted with the BR-2 to Factory No. 8. This is particularly evident from a letter that A. A. Goreglyad, Deputy People’s Commissar of the Tank Industry, wrote to GAU chief Col. Gen. N. D. Yakovlev on June 23, 1942:
In response to your letter No. 538884 of April 23, 1942, I hereby inform you of our agreement to accept for implementation part of the experimental work on self-propelled artillery that you have proposed.
The self-propelled gun projects you mention have undergone a preliminary workup at factories supervised by our Commissariat, allowing us to clarify their possible implementation as follows:
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3) Kirov Factory. The manufacture of a prototype self-propelled 152 mm gun-howitzer model 1937 (ML-20) using a type KV-7 hull can be accepted.
As regards your proposal to design a 152 mm self-propelled gun mounting the BR-2 gun, we consider even design work on this project to be inadvisable inasmuch as manufacture of this type of self-propelled gun would be an unrealistic undertaking for the near future.
The People’s Commissariat of Arms must be instructed to manufacture two models of the 152 mm gun model 1937 with muzzle velocity increased to 750–780 meters per second by lengthening the barrel and introducing a muzzle brake.
This version has been developed by the artillery design bureau of Factory No. 8 (Chief, Comrade F. F. Petrov).
To speed up the work and improve communications with the artillery design bureau, we consider it necessary to request that the People’s Commissariat of Arms assign the design work associated with the artillery systems and their modification for self-propelled artillery to Factory No. 8’s design bureau (Chief, F. F. Petrov), with factories of the People’s Commissariat of Arms to manufacture the artillery systems for the self-propelled guns at its discretion.
Upon receipt of your approval of these developmental self-propelled artillery projects, we will instruct the factories to implement them.{9}
Goreglyad’s proposal played a cruel joke on Factory No. 8’s design bureau. In addition to the ZIK-20, Petrov’s design bureau was working on a number of SP gun projects of all types at the same time. They included the ZIK-5 25 mm SP air defense gun on a chassis incorporating assemblies from the T-60 (a copy of Factory No. 37’s SU-32), the ZIK-7 76 mm SP gun on a chassis incorporating assemblies from the T-70 (a copy of Factory No. 37’s SU-31), and the ZIK-10 and ZIK-11 122 mm SP guns on the T-34 chassis (similar to the U-35 under development at the Ural Heavy Machinery Plant’s design bureau). And that list leaves out its impressive number of gun projects, from antitank guns to corps-level artillery! With a workload like that, it is not surprising that ZIK-20 development was significantly delayed. In fact, the 152 mm bunker busters completely disappeared from the development plans of the GAU and the GABTU after mid-summer. However, some progress was being made by the end of the summer. On August 14, 1942, Khokhlov sent a letter to Yakovlev:
According to the GAU Artillery Committee’s development plan, installation of the 152 mm gun-howitzer model 1937 on a chassis incorporating assemblies from the KV-7 tank should be completed in 1942.
The People’s Commissariat of Arms and the People’s Commissariat of the Tank Industry accepted these projects and assigned them to Factory No. 8 (Sverdlovsk) and the Kirov Factory (Chelyabinsk).
The design is now complete.
For manufacture of the prototype of the 152 mm self-propelled howitzer, I hereby request that you instruct the Chief of the GAU’s Artillery Equipment Supply Office, Maj. Gen. of Artillery Sokolov, to send Factory No. 100 (Chelyabinsk) one 152 mm gun-howitzer model 1937 without a limber and with the gun and battery kit of spare tools and accessories by September 1 of this year.{10}
However, it was clearly too early to issue that instruction. First of all, the first ZIK-20 drawings would not be ready until early September and had not yet been reviewed by the GAU’s Artillery Committee. Second, the bunker buster as it was taking shape differed greatly from the U-18, in terms of both the gun and the fighting compartment. That is evident from the system description:
The following issues received particular attention during development:
1. Keep the design and location of the KV tank units and mechanisms and the artillery system parts and units unchanged as much as possible.
2. Make the operations of the artillery crew with the gun as convenient as possible within the existing dimensions of the tank and thereby increase its rate of fire.
3. Increase the number of rounds and make their location as convenient as possible from the standpoint of increasing the rate of fire.
4. Increase the artillery system’s traverse and elevation angles as much as possible.
5. Minimize any weight increases to the artillery system and thus the total weight of the tank.
6. Develop the best armor protection for both the artillery system and the gun crew, thus enabling the tank to be used for close-quarters direct fire on the enemy’s fortified lines and strongpoints.