Ok, All the available data I have suggests the 85mm gun will be 4thin penetration beyond 1600m. Closer than that, the mass of the projectile will win through the 75mm gun on the Panzer, but just barely.
The Russian gun technology of the time lagged a bit behind. Most of the tank guns had lower velocities than their German counterparts, therefore had inferior penetration characteristics. To make up the difference they went with weight of shot to provide the force to penetrate the thick armor of the German tanks. Newton said it best ( F=ma ) Force = Mass times Acceleration. The Russians knew they didn't have the acceleration part down, so they upped the mass to apply the same force. How do you get more mass, you get a bigger caliber gun.
This has nothing to so with acceleration. Penetration is a function of either energy, or momentum. What you're describing is momentum, which usually is a poor substitute for kinetic energy when it comes to penetration. Let me give you an example:
9mm (124 gr ball) vs .45 (230 gr ball) against oh, let's say, Level II body armor.
9mm has higher energy, .45 has much higher momentum. Which one gets thru the armor more easily?
9mm. It's all about the energy.

And if you're thinking ,"Hey, but 9mm is smaller in diameter and pointier", you're right. But special light weight .45 rounds (and hence higher energy) can be made to penetrate just as easily as 9mm.