This is a rather good read on physics of kinetic energy projectiles:
http://ciar.org/shotmagnet/Armor%20a...rojectiles.pdf
However, they're testing modern, 105-140 mm KE penetrators. Old ammunition might have been worse and glanced off or broken easier by slanted armor.
I could try and find if there's any material in Finnish related to this gun. Finns have a tendency of keeping all the records somewhere and then unclassifying them. However, I suspect that the issue of slanted armor effectiveness from gun to gun will provide way too complex for the game and this will be mostly an academic exercise.