(Pzgr39 striking at <650m/s (?) with 704m/s muzzle velocity)
that less than 650m/s was my estimate for 600m distance, which I cannot verify here but Mobius seems to have been able to find the reference velocity (i.e. 638) in the meantime. By no way I intent to attribute this exact velocity to the -IVl at this range.
The difference in penetration between both data is subtle, and the higher velocity data was also choosen to guard against my own error.
Therefore, the refernce penetration at 638 is even lower, yet the projectile defeated the frontal aspect in two out of two cases (I presume under ideal conditions in regard to target angle), where it was at least 120mm thick -in possession of sufficient section thickness to theoretically completely keep out the 75mm Pzgr39, had the cast armor been of the same quality as german RHA.
OKH mentioned that the armor quality of the IS2 (presumably the early, stepped front type) cast was too low, allowing successful penetrations by PAK 40 and KWK40 from usual "battle range". "Quality" however, can have multiple charakters. Speculating here, but I use to think partly because a later remedy was attempted by substituting softer plates in IS2 production, that overly hard armor in original production (good against soviet AP projectiles but a poor choice vs non-deformable (under those conditions) Pzgr39) was part of the problem. Of course, it could also have included one or more of the following issues, which are commonly encountered in thick cast armor:
inferior non-metallic inclusions,
inferior quantity of gas bubbles in the cast matrix,
improper heat treatment, or
britellness caused by differential cooling rates between surface and centre of thick sections subjected to air cooling