Blow them away!

From RPGwiki

So, which would you rather wear - a bullet proof vest or a chainmail shirt +1? Given the nature of the Fourth World,this problem may actually arise. That's not necesserily a bad thing, but I think we need to have some rules to cover how medievil armour types, and especially natural armour, respond to firearms. Modern weapons are supposed to be rather good against such obsolete protection.

I suggest that we give firearms an armour penetration value. This is the amount of armour or natural armour bonus that is ignored by the weapon. So if a light hand gun has a penetration of 3, a chain mail shirt (Normally +4) would only give a +1 armour bonus.

Modern armour would give both a armour bonus and a ballistic bonus that only works against fire arms. A bullet proof jacket might give +2 armour bonus and a +1 ballistic bonus, granting AC12 against a knife, 13 against an air rifle with no penetration, and 11 against .44 magnum.

Magic armour would still give its enchanment bonus to AC regardless of penetration value, so that chainmail shirt +1 would always give at least a +1 bonus to Ac even against a 75mm field gun.