Author: Yuri Pasholok
On 15.4.1942, on a general meeting of all the members of the Artillery directorate of the Red Army (Артком ГАУ КА), several self-propelled gun projects were accepted. One of them was a heavy bunker destroyer, built on KV chassis, equipped with the 152mm BR-2 gun.
Since the authors of the project couldn't get their hands on such a vehicle, it was instead decided to use the KV-7 platform and the 152mm ML-20 howitzer. As a parellel project, in spring 1942, works began on a similiar vehicle in Factory No.8 under the leadership of F.F.Petrov. The chief designer of the SPG project - now designated as ZIK-20 - became T.A.Sandler (he already was the general chief designer of the Factory No.8). The project documentation was ready in September, 1942.
At that point, the designers became somewhat confused as to what exactly a "KV-7" is. In their mind, any KV model with fixed casemate was designated KV-7 and they basically managed to scrape 20 such hulls to work with. The ML-20 howitzer was also changed significantly and the re-worked gun itself recieved the ZIK-20 designation. The biggest change was the absent muzzle brake.
Eventually, after a discussion within the technological committee of the artillery directorate, several changes were made and a permission was given to manufacture a full-sized mockup. However, when the mock-up was ready, the project became kinda obsolete, as was the original KV-1 hull. KV-1S became the new platform for the project. Also, the vehicle was supposed to weight 53 tons and there were serious doubts about the reliability of the drivetrain (it would apparently be overloaded by the weight). The whole project was transferred to Chelyabinsk and eventually dropped.
There were other projects undertaken in the Factory No.8 under F.F.Petrov. 7 projects were made in total, of 2 a prototype chassis (SS: or metallic mockup, unclear) was built and one of them had a gun installed, the 152mm D-1. Amongst them, a project was concieved - when F.F.Petrov was working on the ZIK-20, a thought came to him that the chassis could carry even a bigger gun - and so he made a project where he replaced the ZIK-20 gun with the BR-2.
He wrote rather optimistically about it. The ZIK-20 gun was to be replaced by the 152mm BR-2 gun on the same carriage in a fixed casemate. He noted that the BR-2 gun would fit the ML-20 mount. An armor steel muzzle brake was to be fitted also. The cast steel breech however would have to be re-worked, the recoil brake was more or less compatible (one part had to be adjusted). The vehicle would have following characteristics:
Caliber - 152mm
Shell weight - 49kg
Muzzle velocity - 880 m/s
Type of loading - two-piece shells, the shell and the artillery cartridge
On KV-1 platform - 55 tons
On KV-1S platform - 48 tons
Amount of shells carried: 40-47 pieces
Frontal - KV platform - 75mm, KV-1S platform - 60mm
Side - KV platform - 75mm, KV-1S platform - 60mm
Superstructure - KV platform - 75mm
Vertical gun range: +/-5 degrees
Elevation, depression: +15, -3 degrees
Barrel overlap length beyond the shape of the tank: 3000mm
Line of fire height: 2206mm
According to the constructor, the vehicle with the BR-2 is different compared to the ML-20 mount, and the installation is not without problems, but the gun is twice as powerful as the ML-20. The length of the barrel (including the muzzle brake) was 8000mm (L/52) - it was a massive beast and the designer does mention potential difficulties when aiming such a monster. Furthermore, F.F.Petrov stated that since the 152mm BR-2 shares the same mount and the recoil compensator with the 203mm howitzer, it's possible to potentially refit the vehicle with the 203mm U-3 howitzer.
This project (just as the original ZIK-20) never moved forward, but according to Y.Pasholok, it started a series of efforts to refit the SU-152 with something bigger, an idea that would stay around for quite a while.