Quote Originally Posted by The Grand Inquisitor
If I recall correctly, armour piercing halves any defence (including bonuses from armoury and possibly valour(not sure on this)) beyond 2.
I don't know where you got that information from but as far as I'm aware that's not true! Having a weapon that is 'armour piercing' only gives a measured bonus to the attack value; it does not directly reduce the opponent's defence.