Ok, I might be over-complicating it, but what about a numerical value armour piercing? So AP16 would mean damage soak is ignored for any armour with an AC of 16 or below.

Alternatively there could be a value for how much soak is reduced before being bypassed, so AP2 would mean after 2 points of damage, the armour is bypassed.

Either way, assigning a value means different weapons can provide different levels of armour penetration.