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.
Additional Information: en.wikipedia.org/wiki/IS-2