IS-2

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By 1943 engineers had succeeded in mounting the 85 mm gun to the T-34-85, making the IS-85 superfluous. Efforts to up-gun the IS-85 began in late 1943. Two candidate weapons were the D-25 122 mm tank gun, ballistic characteristics of which were identical to the A-19 122 mm gun, and the D-10 100 mm gun, based on a naval dual-purpose gun. The D-10 had been designed for anti-tank fire and had better armor penetration than the A-19, 185 mm compared to 160 mm but the smaller calibre meant it had a less useful high explosive round. Also, the D-10 was a relatively new weapon in short supply, while there was excess production capacity for the A-19 and its ammunition. Compared to the older F-34 76.2 mm tank gun, the D-25 delivered 5.37 times the muzzle energy, while it had similar accuracy to that of the 88 mm L/71 gun mounted on the Tiger Ausf B.

After testing both guns on the IS-122 and IS-100 respectively, the former was selected as the main armament of the new tank, because of its availability and the effect of its large high-explosive shell against German fortifications. The D-25 used a separate shell and powder charge, resulting in a lower rate of fire and reduced ammunition capacity, both serious disadvantages in tank-to-tank engagements. Soviet proving-ground tests claimed that the D-25 could penetrate the front armor of the German Panther tank at 2,500 metres (2,700 yd) while the D-10 could do so at a maximum range of 1,500 metres (1,600 yd). It was therefore considered an adequate anti-tank gun.

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. It was slightly lighter and faster than the heaviest KV model 1942 tank, with thicker front armor and a much-improved turret design. The tank could carry thicker armor than the KV series, while remaining lighter, due to the better layout of the armor envelope. The KV's armor was less well-shaped and featured heavy armor even on the rear, while the IS series concentrated its armor at the front. The IS-2 was slightly lighter than the Panther, much lighter than the Tiger I and Tiger II and had a lower silhouette than both. Western bservers tended to criticize Soviet tanks for their lack of finish and crude construction. The Soviets argued that it was warranted, considering the need for wartime expediency and the typically short battlefield life of their tanks.

Early IS-2s can be identified by the 'stepped' front hull casting with its small, opening driver's visor. The early tanks lacked gun tube travel locks or anti-aircraft machine guns and had narrow mantlets. According to Steven Zaloga, the IS-2 and Tiger I could knock each other out in normal combat distances below 1,000 m (1,100 yd). At any range, the performance of each tank against each other was dependent on the crew and combat situation.