"Report on the development of the 152 mm ML-20SM SPG
- The Journal of the Artillery Committee #0254 1943 indicates that, as a result of trials and review of blueprints of the 152 mm model 1937/43 SPG, work should be done to improve the following areas:
- Gun/breech: get rid of the muzzle brake, introduce a sliding quarter or semi automatic breech.
- Cradle and recoil elements: simplify the recoil brake, transition to constant resistance to recoil and constant recoil length.
- Turning mechanism: improve the design to reduce drifting of the aim and reduce effort of turning the aiming flywheel.
- Frame, loading tray, recoil guard: review the design in order to standardize the parts and reduce their number.
- By January of 1944, OKB-172 performed a modernization of the ML-20S, which consisted of the following:
- Removal of the muzzle brake.
- Introduction of a simpler recoil brake with a constant resistance and constant recoil length.
- Shorter cradle and recoil elements.
- Factory #172 began to work on producing a prototype of the ML-20SM. OKB-172 was told that it was expected by February 10th, 1944.
In practice, work on the experimental prototype lasted until September of 1944.
- After multiple requests made by the GAU to the NKTP to install the ML-20SM into a mass production ISU-152 SPG at the Kirov factory, this order was only given on October 2nd, 1944.
- The experimental ML-20SM gun was delivered to the Kirov factory in the first half of October of 1944. As it was established later (December 12th, 1944), the gun was incomplete (lacked a turning mechanism, lacked a prepared location to install it, factory #172 did not respect the established connection between the gun and mantlet)
- The GAU raised the issue of completing the ML-20SM to factory #172. Factory #172 representatives arrived at the Kirov factory to perform the work there in mid-February of 1945.
- Factory #172 communicated the following to the GAU in early May of 1945:
- The ML-20SM is being installed at the Kirov factory not in the mass production ISU-152, but in the experimental "Kirovets-2". Because of this, it is necessary to shorten the cradle and recoil elements. An agreement was reached to make changes to mass production samples, and for now the experimental ML-20SM was installed with a special mantlet (to compensate for the longer recoil elements) and presented to proving grounds trials in this manner.
- The GAU informed comrades Ustinov, Malyshev, and Fedorenko by letter that it will only accept a fully completed ML-20SM for trials, with all changes required to install it in the new SPG. At the same time, the question of how future-proof the Kirovets-2 is was raised (May 16th, 1945).
- The GBTU and NKTP replied that they were in agreement with the GAU, and urged the NKV to complete modifications of the ML-20SM, after which it would be sent to trials.The NKV insisted that the trials are to be carried out at the GAU main proving grounds without changes to the gun, claiming that it is necessary to discover additional requirements for its improvement.
- The GAU replied that, if the trials are considered factory trials, the GAU has no issue with trials aimed at determining requirements being carried out at the GAU main proving grounds, and will aid the NKV with necessary service and supplies.
- The NKV presented a program of trials at the start of August, and the GAU main proving grounds were ready for the trials in mid-August.
- Factory trials of the ML-20SM at the Main proving grounds began on September 24th, 1945, and concluded on November 13th, 1945. The start and end of the trials were delayed due to a lack of NKV representatives. The GAU informed the NKV that they must be sent urgently on two occasions (September 9th and October 15th).
- The report on factory trials arrived at the GAU on December 6th, 1945. The main goal of the trials was to:
Determine the quality of the weapon.
Determine the robustness of the system as a whole and its individual components.
- In total, 65 rounds with normal charge and 244 with increased charge were fired. Overall, including the rounds fired before arriving at the proving grounds, the gun made 576 shots, 440 of them with increased charge.
- The SPG will be sent to the NIBT proving grounds for GBTU trials on December 21st-25th.
- The People's Commissariat of Armament gave the order to factory #172 to produce a new MK-20SM including the required improvements described as a result of trials at the GAU proving grounds and NIBT proving grounds.
Senior Engineer of the 2nd Department of the Artillery Committee, Engineer-Major Gorbachev
December 19th, 1945"