Beginnings of the D-25

After the IS-85 was selected from the original 4 IS prototypes (1-4, the 85 mm one being #3), its armament proved insufficient for a modern heavy tank, especially considering that the 85 mm gun was already available on the T-34-85 and SU-85. An upgrade was needed, and the A-19 122 mm corps gun model 1931/37 was chosen as a candidate.

"The experimental IS tank with a 122 mm gun (IS-3) is built on the base of the IS tank and differs from it in armament. Instead of the 85 mm D-5 gun, the 122 mm D-25 gun is used, the most powerful of all currently known foreign or domestic tank guns.

The D-25 has the ballistics of the model 1931/37 A-19 corps gun.

Characteristics of the D-25 gun

• Muzzle velocity: 800 m/s
• Shell mass: 25 kg
• Muzzle energy: 833 ton-meters*
• Caliber: 121.92 mm
• Barrel length: 47 calibers
• Maximum recoil length: 550 mm

The D-25 tank gun uses the following shells:

1. Armour piercing
2. High explosive fragmentation
3. Concrete piercing

The D-25 is installed on the gun mount of the 85 mm D-5 gun in a slightly altered IS turret. The pinions are moved by 110 mm, leading to a slight change in the immobile armouring of the turret. The gun slit is enlarged. The periscope is moved forward, the ammunition rack is altered. The radio is moved to the turret bay in order to ensure proper operation of the screw breech. The rear machinegun is absent. 

*The muzzle energy of the 85 mm D-5 gun is 305 ton-meters, or 2.7 times less"

CAMD RF 38-11355-1702

So there you have it, both the IS-1 and IS-2 were an IS-3.