The effects of the new tank gun were excellent: coupled with a new pointed AP shell, it could penetrate the front of a Panther at 1400 meters, and the front of a Tiger at 1800 meters. The tank was accepted under the index IS-122. Production IS-122s differed from the trial version by a D-25T gun with a horizontal semi-automatic sliding breech, which increased the rate of fire. Modifications to increase the rate of fire were carried out throughout the tank's life. "I had a habit of following tanks to the front lines," remembered Kotin in a 1977 interview with Voyenniy Vestnik. "I don't ride them into battle, but I examine the aftermath. ... I remember we tested an IS. The shell was heavy, the loaders complained that loading takes too long. I climbed into a tank, tried to lift a shell, and confirmed that it was very heavy. I needed to do something. The [Kirov] factory quickly developed a special mechanism for loading shells into the breech." Kotin's device seems to have been effective. In a letter to the Commissar of Armament Ustinov, concerning re-armament of the IS tank with a 100 mm gun, the engineers wrote "We consider it imperative that the D-25T remain in the IS-2. Latest trials at the ANIOP resulted in a rate of fire of 4-6 rounds per minute".
Shashmurin was busy with a brand new IS tank. Really, the only things it shared with the IS-2 were the gun caliber and its breakthrough role. It even looked vastly different.