The following points supporting the IS-100 were outlined in the letters between Malyshev, Beria, and Fedorenko:
- Increased rate of fire (up to 8 rounds per minute)
- Increased ammunition capacity
- Lack of muzzle brake improves working conditions for the gunner
- The tank is 500-600 kg lighter, which allows for additional armour
- Smaller size, leading to a roomier turret
- Higher penetration
- Factory #9 was ready to produce D-10T guns along with D-10S
- D-25T is already sufficient against German tanks at all combat ranges
- Reduced caliber will reduce HE effectiveness against enemy fortifications and soldiers
- The increased ammunition capacity is negligible (29 rounds instead of 28)
- A large caliber, high rate of fire semi-automatic gun will lead to high concentrations of gases inside the tank
"[The D-25T] penetrates a Panther frontally at 2500 meters, and that is not the limit. The 100 mm D-10T tank gun ... penetrates the front of the Panther at only 1500 meters, which is the maximum distance. ... The results are preliminary, as the D-10T has already fired 400 shells while the D-25T is new, but the difference is so drastic, that it is unlikely to change. "
The report explains this with the lower muzzle velocity of the D-25T: at such a large range, the heavy 25 kg shell has no problem penetrating the Panther's front, while its higher arc lets it impact the front of the Panther at a favourable angle. The D-10T's shells are faster, and therefore fly flatter.
The IS-5 was briefly a platform for very high velocity 85 mm ZiS-1PM (1000 m/s) and S-34-1B (1040 m/s), but those did not pass trials. From the same letter:
"Another question that is prompted by the experiments at Kubinka is the question of large muzzle velocities, namely of 85 mm guns with muzzle velocity of 1000-1100 m/s. Experience shows relatively poor effectiveness of the German 88 mm gun against the German Panther tank. At the same time, we know that such a gun would be of similar effectiveness to a 100 mm gun with muzzle velocity of 900 m/s. Work on these guns is conducted by TsAKB and factory #9, but their effectiveness against a real German tank is increasingly doubtful, especially considering that we cannot place this gun in a turret smaller than one that requires the D-10T or S-34 gun."
Moving on to the IS-6, its initial requirements were pretty lofty. Here are the project requirements: