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
Points against the IS-100 in the same series of letters:
- 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:
CAMD RF 38-11355-2243
The mass was incredibly high compared to any IS tank before it, 65 tons. The armour was initially supposed to resist all German 75 and 88 mm guns, firing at up to 1500 m/s (I wonder where they managed to find those), but that requirement is struck out in the document. The tank was to be armed with a 122 mm gun, capable of 4-6 RPM, with one-piece ammunition. and 1100 m/s muzzle velocity. One 50 mm breech-loading mortar was planned (a feature experimented with on all IS tanks), but also crossed out. The tank would carry 35-40 shells for its main gun, 1500 rounds for its 7.62 mm coaxial machine gun, 500 rounds for its 12.7 mm AA machine gun, 1000 rounds for SMGs, 25 grenades, 27 signal flares, and 12 smoke bombs. All of this was planned to move at 40 kph, powered by a 1000 hp engine.
The IS-6 didn't quite measure up to these requirements, but, several years later, the IS-7 did.