I assume you are talking about the ISU-122, since the SU-122 was more of an assault tank (with the short barrel) firing HE rounds. The Soviet tanks were known to have poor sighting, but to compare the IS-2 is just as accurate and does more damage than the Panther gun. The ISU-122 has the same accuracy as the IS-2 gun with less damage. I did research on penetration values and they weighed in at somewhere between the Panther and Tiger (and leaving all others behind it with the closest being their own SU-85 behind it and the Jagdpanther and King Tiger ahead of it).
sweeten2213
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