"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.
- 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.