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I am not sure what JP4's penetration is, but I am certain it is lower than the Panther's. ISU-152 HE lowered to 60 range means that only the JP4 and PaK 43 can not be outranged by the ISU-152. A simple solution, then, would to be raising the frontal armor of the ISU-152 so it bounces more JP4 shots while leaving Panther shots relatively unaffected. Furthermore, allowing the ISU-152 to quickly toggle AP and HE should be more than enough to compensate for the range reduction.