I think I'm right with this:
AP shots are just a large bullet shaped chunk of metal.
APC shots are just an AP shot with a shaped cap on the front that helps stop them from shattering on impact.
APCBC shots are APC shot which have a light cap on the front that improves their ballistics.
APCR shots have a dense tungsten penetrating core surrounded by a light metal ballistic cover.
APDS shots are basicly an APCR shot where the outer cover falls away.
I think that PzG39 is APCBC and that PzG40 is APCR.