"To comrade L.P. Beria, on the installation of a 100 mm D-10 gun produced by factory #9 into an IS tank instead of a 122 mm D-25 gun of the same factory
In March of this year, I reported to you that factory #100, jointly with factory #9, designed an experimental prototype of a heavy IS tank with a 100 mm D-10 gun designed at factory #9. From March 30th to April 6th, the tank was tested at the GAU proving grounds. Trials of the IS tank armed with a 100 mm gun demonstrated its advantages over the IS tank armed with a 122 mm gun. The advantages are as follows.
- Combat rate of fire is up to 8 RPM (on average 6 RPM), 2-3 times higher than the 122 mm D-25. This high rate of fire improves combat performance of the IS tank.
- A lack of a muzzle brake on the D-10 improves the conditions of the crew compared to the 122 mm D-25 gun, which does have a muzzle brake.
- The D-10 gun weighs 300-400 kg less than the D-25 gun.
- The part of the D-10 inside the tank is slightly smaller than the D-25, improving crew conditions.
- The one piece round of the D-10 gun is safer in case of fire or explosion than the two piece D-25 round.
- The penetration of the D-10 gun is (if insignificantly) higher than that of the D-25 at 1000 meters.
Considering that factory #9 managed to create a 100 mm gun based on an existing family of guns (85 mm D-5 and 122 mm D-25), not needing many alterations to the design of the IS tank, I consider that production of a 100 mm gun in place of a 122 mm gun and the installation of these guns into IS tanks at Kirov factory will not be difficult and will not require a lot of time.
Considering the above, it is imperative to begin production of IS tanks with 100 mm guns instead of 122 mm guns within 2-3 months. In case of your agreement, the NKTP and NKV can draft a GOKO decree within five days.
Malyshev, April 25th"
Malyshev himself approves! Surely the fate of the D-25 is sealed! However...
"To the commander of the Armoured and Motorized Forces of the Red Army, Marshall of the Armoured Forces, comrade Fedorenko.
Having examined the results of the comparative trials of the 122 mm D-25 and 100 mm D-10 tank guns, I have come to the following conclusions:
- The rate of fire of the D-10 gun is thrice that of the D-25, which is a significant advantage of the D-10. The 100 mm D-10 gun currently has an insufficiently robust armour piercing shell, and thus cannot reliably penetrate the front of a Panther tank at over 1200 meters. Before replacing the D-25 with the D-10, an improvements in the AP shell and semi-automatic mechanism must be made. Considering that the D-10 gun and its ammunition, even in its present state, is superior to the D-5 gun, SU-85 SPGs should be re-armed with 100 mm guns. A trial batch of SU-85 SPGs armed with 100 mm guns should be ordered, in order to correct defects of the SPG and gun determined in trials.
- The 122 mm D-25 gun reliably penetrates the Panther at a range of more than 2000 meters. The AP shell has sufficient robustness, and the HE grenade has a powerful explosive and fragmentation effect. Until the perfection of D-10 mechanisms and ammunition, it is necessary to retain the D-25 gun as the main armament of the IS tank. Trials to investigate a one piece 122 mm shell are reasonable, but consider that the chamber, shell, and casing will have to be changed. The size and weight of a one-piece shell would complicate the work of the loader, which will not allow increasing the rate of fire of the D-25 to the rate of the D-10.
Regardless of the comparative results of the D-25 and D-10, continue the installation and trials of the S-34 100 mm gun on tanks and SPGs.
Chief Marshall of Artillery, Voronov
Marshall of Artillery, Yakovlev"