I have read accounts of JS-2s in tank to tank fights with the Germans on the Baltic front in late 44/early 45. The Soviets approved the 122mm gun for the JS-2 because during tests, it punched holes in the front glacis of a Panther at 2000 meters. The Soviets only built 3800 JS-2 from the end of 43 through mid 45.
German Army data on the penetration ranges of the 122 mm A-19 gun against the Panther tank showed it to be much less effective when the Panther stood at a side angle of 30 degrees to the incoming round: the A-19 gun was unable to penetrate the glacis plate of the Panther at any distance, and could only penetrate the bottom front plate of the hull at 100 m.It was the large HE shell the gun fired which was its main asset, proving highly useful and destructive in the anti-personnel role. The size of its gun continued to plague the IS-2, and the two-piece ammunition was difficult to handle and slow to reload (the rate of fire was only about two rounds per minute). Another limitation imposed by the size of its ammunition was the payload: only 28 rounds could be carried inside the tank.
From Wiki.