- Strengthens the tank's armour to resist penetrations.
- Increases the chance of a ricochet.
- Protects the tank's turret from jamming.
- Protects the tank's suspension.
- Strengthens the tank's front for ramming."
Another, much larger, up-armouring was planned for the ISU-122.
"To the People's Commissar of Tank Manufacturing, Comrade Malyshev
Self propelled artillery guns ISU-152 and ISU-122, manufactured at the Kirov factory, differ in mass by 1 ton. The ISU-152 weighs 45.5 tons, and the ISU-122 weighs 44.5 tons. Despite the larger mass, the performance of the ISU-152 is identical to the ISU-122.
As such, it is possible to increase the armour of the ISU-122 within the limits of 45.5 tons.
Preliminary calculations suggest that front armour 150 mm thick and a gun mantlet 160 mm thick will not weigh more than 1 additional ton. This front armour will be able to resist 75 and 88 millimeter enemy shells from a distance of 500 meters and higher.
In order to determine the practicality of this solution, and the necessary amount of work connected with re-balancing the D-25S gun with this additional weight, I deem it reasonable for the Kirov factory to complete the blueprints for this SPG by January 15th, 1945.
After completing that task, a prototype ISU-122 with increased hull armour must be built, in order to carry out trials.
Colonel-General of the Tank Forces, Korobkov"
CAMD RF 38-11369-654
150 mm at 30 degrees is effectively 173 mm of armour! That's quite an amount, giving you a sizeable chance to resist even 88L/71 shells at nearly point blank range.