On the letter from the People's Commissar of Tank Production, I report:
- GAU supports the NKTP's request to cease producing the 130 mm gun, indicated in the GOKO degree issued on December 27th, 1943.
GAU does not take part in the arguments between NKTP and TsAKB on the possibility of installing a 130 mm gun into the SPG on the IS chassis, knowing that if the organizations are willing to make concessions, a decision that satisfies both sides can always be reached.
GAU considers it necessary to cancel the 130 mm SPG, as it requires a more reasonable and mass produceable artillery system for tanks and SPGs, which is possible to build in 1944.
- Our current SPGs with 122 mm and 152 mm guns model 1937/43, with ballistics of corps level guns A-19 and ML-20 are powerful weapons against German heavy tanks and SPGs, but it is necessary to further increase the firepower of these weapons.
These requirements are confirmed by developments of the arms race on the battlefield and possible technical progress by our enemy.
Considering this scenario, OKB-172 has designed a high power 122 mm SPG gun according to GAU specifications, indexed OBM-50. The technical project has been reviewed and approved by Artkom on December 18th, 1943. An experimental prototype has been ordered, and should be completed by factory #172 in February. The production of the 122 mm OBM-50 gun is confirmed by GOKO decree #4851ss issued on December 27th, 1943. In order to perform factory and proving grounds trials of the 122 mm high power gun, we need you to order the People's Commissariat of Ammunition to produce new tube nitroglycerine propellant.
GAU considers this gun equivalent in power to the 130 mm gun proposed by comrade Grabin, but the 122 mm high power gun has many advantages over the 130 mm gun.
- The 152 mm OBM-43 gun is a completely different class of weapon than comrade Grabin's 130 mm gun, and can't be compared.
The OBM-43 has been built according to GAU specifications as a towed gun. It is possible to install it in an IS SPG using the oscillating part and mount from the 152 mm model 1937/43 SPG gun (SU-152), which was ordered by comrade Malyshev and carried out by factory #100 and OKB-172 engineers.
This 152 mm gun is not fully developed and is only an intermediate step, and its trials are of little interest. GAU does not oppose the transfer of the 152 mm OBM-43 gun from the NKV to factory #100 for mounting in an IS SPG to carry out trials at the Gorohovets proving grounds with assistance from the GBTU in order to test several design solutions and use cases.
- The installation of a special 152 mm SPG gun with the ballistics of the OBM-43 in an SPG has been foreseen by GAU. The project for such a gun, indexed OBM-53 has been reviewed and approved by Artkom on December 18th, 1943, at the same time as the OBM-50 project.
No prototype is being produced, as the NKV has not issued the appropriate orders to factory #172. I ask you to order the People's Commissar of Armament, comrade Ustinov, to produce an experimental OBM-53 152 mm SPG gun by February 25th.
GAU considers the 152 mm OBM-53 gun to be a very powerful weapon, and, due to design specifics and high power, it cannot be a mass produced tank escort weapon. It must be built in limited numbers to achieve objectives set by the Supreme Command.
- GAU supports the NKTP proposal to test a 100 mm gun designed by factory #9. At the same time, GAU considers it necessary to test the 100 mm gun designed by TsAKB.
- Along with the 100 mm gun, GAU considers it necessary to test an 85 mm gun with the muzzle velocity of 1000-1100 m/s.
This gun has nearly the same firepower and penetration as the 100 mm gun, but the weight and size give it an advantage over the 100 mm gun, both of the gun and the shell.
Additionally, the production of an 85 mm gun and ammunition for it would not pose any difficulty, as the design of the new gun and its shells is an organic progression from the existing items produced at NKV and NKB factories. Savings in metal (approximately 30% for the gun and 40% for its shells) are also a benefit.
- When installing powerful guns with barrels from 5600 mm (85 and 100 mm guns) to 7000-7500 mm (122 and 152 mm guns) in length, the barrel will exceed the front of the SPG by at least 3500 and up to 5000 mm (with the 152 mm gun). Because of this, when moving off-road, it will hit objects and terrain, which will cause damage to the gun and knock it out during battlefield maneuvers.
In January of this year, the commander of artillery of the 3rd Ukrainian Front sent a report to GAU, regarding the SU-85 SPG. In this SPG, the barrel exceeds the length of the SPG hull by about 2300 mm. The report says that when going over obstacles, there were cases of sand getting into the barrel, and strong enough hits against the ground that the normally reliable sliding shaft of the elevation mechanism bends, and the SPG cannot be used. The possibility of this happening with high power SPGs is much greater.
Measures suggested by the NKTP (pushing the barrel back by 800 mm, 10 degrees of gun elevation) help only on the march, but not in combat, where damage is being done to these guns.
Based on the above, GAU considers it necessary to, in parallel with the design of new powerful guns, to design new SPGs with rear fighting compartments (like the SU-76).
This will preserve the high combat maneuverability of SPGs and allow the increase of their front armour.
- The armament of a self propelled gun must not only be examined from the point of view of increasing the firepower of tank units, but from the point of view of increasing the firepower of infantry units, maneuverability and firepower of special tank destroyer units, and creation of special weapons for the Supreme Command Reserve.
GAU considers it necessary to have the following weapons in self propelled artillery:
- Infantry support SPGs
- 76 mm ZiS-3 gun (F-34) on a light SPG chassis made from T-70 components.
- 122 mm D-5 howitzer or 152 mm D-15 howitzer on a medium SPG chassis made from T-34 components.
- Tank escort SPGs
- 85 mm D-5 gun (with AA gun ballistics) on a medium SPG chassis made from T-34 components, already in production.
- 85 mm high power gun or 100 mm gun on a medium SPG chassis made from T-34 components, planned for production in 1944.
- 122 mm D-25 gun or 122 mm A-19S gun or 152 mm mod. 1937/43 gun, already in production, installed on a chassis made from IS tank components.
- High power 122 mm OBM-50 gun on an SPG chassis made from IS tank parts, planned for production in 1944.
- Supreme Command reserve SPGs
- 152 mm OBM-53 gun installed on an SPG chassis made from IS tank parts, planned for production in 1944.
- 203 mm howitzer on a half-covered chassis made from IS tank parts.
- Tank artillery
- Medium tanks:
- 85 mm D-5 gun (AA gun ballistics) or 85 mm high power gun.
- Heavy tanks:
- 122 mm D-25 gun.
- 100 mm gun or 85 mm high power gun.
- GAU considers existing artillery systems in tanks and SPGs incomplete. The correlation between quality and quantity of SPGs still needs refinement. Attempts to correct this balance, as experience shows, appear as individual and sometimes poorly thought out proposals from many design organizations. The demands of the front and capabilities of production are not considered.
When these individual projects are built, antagonistic relationships form between organizations. This can cause unrefined prototypes to be pushed into production prematurely or the opposite, a delay before introducing this weapon into the Red Army.
In order to bring order to the design of tank and SPG armament, GAU deems it necessary to hold a meeting of representatives of the General Staff, GBTU, GAU, NKV, NKTP, NKB, and design organizations in order to refine tank and SPG artillery systems.
Red Army GAU Chief, Colonel-General of Artillery, Yakovlev
January 24th, 1944"