Problem with AP attribute is that that not all arms and armors are created equal against each other.
A blunt weapon will not do so great against a padded protection, but a sharp one will.
An iron tip can, in theory, punch a hole in a bronze plate. But it won't do much against an iron one. It can burst a chainmail link with enough force, though.
People in the game don't wear uniform armors even in the same unit.