1S2/IS100
- Yes
- No
In December 1943, two up-gunned variants of the IS-85 were created: the IS-100, armed with the 100mm D-10 gun, and the IS-122, armed with the 122mm A19 gun (later adopted and renamed as the D-25T). The IS-122 was found to be better than the IS-100 in trials, and so the IS-100 was dropped. The IS-122 was renamed to IS-2 and production started with the 1943 model using the IS-85 chassis. First deliveries of IS-2s were in December 1943
By 1943, engineers had succeeded in mounting the 85 mm gun to the T-34 medium tank, making it as well-armed as the KV-85 heavy tank. Efforts to up-gun the IS-85 began in late 1943. Two candidate weapons were the D-25 122 mm tank gun, the ballistic characteristics of which were identical to the A-19 122 mm gun,[10] and the D-10 100 mm gun, which was based on a dual-purpose naval gun. The D-10 had been designed for anti-tank fire and had better armor penetration than the A-19, but the smaller caliber 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. 
Every faction has been asking for game breaking heavy tanks like the Maus and IS3. The IS-100 is a better option a side upgrade same great IS2 armor with a 100MM gun. In competition with what would become the IS2, two guns were in the running. The 122MM gun in the 1S2 and IS2 1944 against the 100MM gun. The 100MM had better penetration but less explosive fill making it better at killing tanks but worse at everything else. The 100MM would have a higher rate of fire and given the same hull more ammo. 
100mm Tanks for Soviet - #2 by OblongHarpy4-live