KV-1S s 85 mm kanónom S-31 (Objekt 238)

5.5.1943 Stalin signed GKO Decree No. 3289ss "On strengthening the artillery armament of tanks and self-propelled guns". According to it, two samples each of IS and KV-1 heavy tanks with 85 mm guns were to be received by 1.7.1943. It was not a sudden idea to use the 85 mm gun on the KV platform. It had been preceded by several designs for new guns and turrets since 1942. In particular, there was the project of the 85 mm ZIS-25 gun, which was to be lafeted in the newly proposed turret. It was an under-performing gun (muzzle velocity 600 m/s, penetration 90 mm at 500 meters). The shortcomings in performance were discovered in time after the capture of two German heavy tanks Tiger, when the thickness of the frontal armour (100 mm) and the maximum thickness of the armour of the tank's gun shield (120 mm) were detected. Work on this project was cancelled, but the intention to develop a new 85 mm gun was soon dusted off.
On 12.4.1943 preliminary tactical and technical requirements for the 85 mm self-propelled gun - tank fighter were developed.
On 5.5.1943 the official order was created and the design department of Plant No. 9 (D-5 gun) and CAKB (S-31 gun) started work on the new tank gun.
The work on the S-31 gun was led by the head of the 17th department of the CAKB P.F. Muraviev (П. Ф. Муравьев) and the head of the 3rd department of the CAKB E.V. Sinilshchikov (E.V. Sinilshchikov). The design inspiration from the ZIS-25 gun was noticeable on the new project. CAKB developed a new universal turret usable for both KV-1s and IS-1 (KV-13, Object 233). However, this created a problem because no one consulted with the factories involved in the development and production of these tanks to develop this turret. The ČKZ factory in charge did not rush into the production of the turret, the fitting of which into the designed and produced versions of the heavy tanks KV-1s and IS-1(KV-13, Object 233) was problematic. It was eventually produced, but testing did not take place until late 10/1943. At that time, however, the design of both the S-31 gun and the KV-1s platform itself was past its zenith. Hence the history of previous designs.

https://dzen.ru/a/ZDmHkrlCrwJNlAh_

Doubts of the GAU KA (GAУ КА) artillery committee about the suitability of the original design of mounting an 85 mm gun in the KV-1s forced the designer Grabin to look for an alternative solution. He began cooperation with the development department of the No. 100 factory in Chelyabinsk. The project was launched as early as 6/1943. Sometimes the designation KV-85G was used for this project, but this index did not exist. It was some anomaly invented by an unknown source. In fact, this project bore the designation Object 238. The project was finally completed on 17 June 1943, although the first drafts were dated 25 May 1943. The rearmament work KV-1s was directed by G.N. Moskvin (Г.Н Москвин) and Ž.Ja. The modifications to the KV-1s were minimal, as the S-31 gun was based on the original 76.2 mm ZIS-5 gun. The movable shield of the gun was changed. However, in 7/1943 a pair of vehicles were made that were rebuilt from KV-1s produced the same month. The first trials began on 7/31/1943 and continued until 8/15/1943. Object 238 covered only 103 kilometres. During the trials, however, there was a fundamental lack of space for handling rounds and there was no room for the tank commander. At that time, tests were also underway on Object 237 (one with the S-31 gun, the other with the D-5T-85 gun). The S-31 gun had a greater recoil length than the D-5T-85 gun, but was not evaluated as critical. The rate of fire of 3-6 rounds per minute was less than half that of similar systems. Compared to the battlespace of Object 237 (IS-1) and Object 239 (KV-85), the battlespace of Object 238 was clearly inferior. After reviewing all the findings, the KV-85 (Object 239) was approved for series production on 8 August 1943.
One of the prototypes was probably fitted with the original turret, which the authorized firm was reluctant to produce. One of the pair of prototypes with the original turret is still preserved in the military museum in Kubinka. Unfortunately, during its placement in the museum, the tank underwent several tuning modifications (original engine replaced by a new type, wheels from KV-1 tracks from IS-2M/IS-3M, etc.).

https://dzen.ru/a/ZDmHkrlCrwJNlAh_