The IS-122 prototype replaced the IS-85, and began mass production as the IS-2. The 85 mm guns could be reserved for the new T-34-85 medium tank, and some of the IS-1s built were rearmed before leaving the factory, and issued as IS-2s.
The main production model was the IS-2, with the powerful A-19. It was slightly lighter and faster than the heaviest KV model 1942 tank, with thicker front armour and a much-improved turret design. The tank could carry thicker armour than the KV series, while remaining lighter, due to the better layout of the armour envelope. The KV's armour was less well-shaped and featured heavy armour even on the rear, while the IS series concentrated its armour at the front. The IS-2 weighed about the same as a German Panther and was lighter than the German heavy Tiger tank series, and was slightly lower than either.