"The ISU-152's 90 mm of sloped frontal armor, in contrast to the SU-152's 65 mm, provided excellent frontal protection from the 75mm KwK 40 gun of the ubiquitous Panzer IV and StuG family at all but the closest ranges, while also forcing the original Tiger I, with its vaunted 88mm KwK 36 gun, to close to medium ranges in order to successfully penetrate the vehicle, negating its traditional long-range superiority and exposing more of its vulnerable flanks to the 85mm ZiS-S gun of the Soviet T-34-85."
https://en.wikipedia.org/wiki/ISU-152
"The ISU-152 self-propelled gun combined three battle roles : heavy assault gun, heavy tank destroyer and heavy self-propelled artillery. The 152.4 mm gun used a number of powerful (shell and charge) ammunition. Some of these ammunition had a 43.56 kg high-explosive shell, or a 48.78 kg armour-piercing shell, or the heaviest of all, the 53-G-545 (53-Г-545) long-range concrete-piercing ammunition with a 56 kg shell. The ISU-152 was used for infantry and tank support, and attack on fortified positions in a direct fire role, for support on the battlefield in an indirect fire role, and for fight against tanks with a direct fire."
