If you read this blog, you're probably already familiar with tests performed with 152 mm guns against King Tiger tanks. Fans of the King Tiger needn't worry about unfairness, since the ISU-152 was in turn tested against a Pak 43. A new welded hull, rather than cast, was adopted by UZTM, which already had experience with using this type of hull on IS-2 tanks. The results were quite impressive.
CAMD RF 38-11369-581 p.12
The upper front plate proved a tough nut to crack. Penetration was not achieved at ranges over 250 meters. There was, however, a problem: after two hits in the immediate vicinity of the welding seam, a crack formed (highlighted with chalk). Even though this performance was much better than the equivalent German armour, work to improve welding technology began. Trials of an IS-3 hull, welded in the same way at the same factory, proved that it could an 88 mm shot to the welding seam without a problem.
"Front of the hull.
The design that was chosen from the front of the IS-3 hull significantly increases the resistance of this component compared to the equivalent section of the IS-2 hull.
The upper front plates of the IS-3 hull completely protect the crew from 88 mm armour piercing rounds at a range of 100 meters or greater at normal.
The upper front plate of the IS-2 tank (Uralmash) can be penetrated from a range of 600 meters and greater.
Photo: damage to the joint between the upper front plates from an 88 mm armour piercing shell."