A weapon's Armour Piercing (AP) characteristic represents its ability to cut through armour. This is always presented as a negative modifier, which is applied to the Armour Save roll.

For example, if the target of an attack made by a weapon with an AP characteristic of -1 has an armour value of 5+, it would need to roll a 6 when making its Armour Save roll. A roll of 5 would give a result of 4 once the modifier is applied.