Honestly, I've found the only way to reliably knock out panzers, or osties, with a panzer AP round is to hit them high, in the turret. Once you hit them in the turret it seems to be better than a 50/50 that they will have the turret knocked out, at most angles and distances. If you hit them in the core of the body, it just bounces off. If you hit them low in the turret, nothing, if you hit them in the face of the hull, nothing, but a hit right up near the top of the turret seems to work 80% of the time.
Just an observation, but I've knocked 10+ AP rounds off a Ostie at point blank, right into the hull, with no effect and the first round that hits high on the turret knocks out his gun. Then I can take my time with him and finish him off with shots to other parts of the hull.
-Soda
The Assassins.