Armor Piercing Weapon (3.5e Variant Rule)
From D&D Wiki
Some weapons are built for the purpose of piercing armor, such as powerful cannons or a rifle. In the olden days, weapons like the arbalest were considered 'dishonorable', because with it, even a peasant untrained for war could take down an armored knight. However, when it comes to armor and piercing it, D&D has always been rather sketchy on the execution. As a result, many of such attacks or powers are channeled as a touch attack, which is a simple but unelegant solution to the problem. In this section I attempt to describe a way to implement an armor piercing factor to certain weapons without sacrificing much of the simplicity you would desire from D&D.
Weapons that pierce armor have the 'Armor Piercing' quality. This quality can only be given to weapons that deal piercing damage. Armor piercing weapons are almost always ranged weapons, although certain weapons that deal extra damage when used in a charge, such as the lance, could apply for the armor piercing quality also.
An armor piercing weapon bypasses varying types of armoring. There is a limit to how much armor can be bypassed; armor piercing weapons bypass armor bonus, shield bonus and natural armor bonus to AC, up to a total maximum of the effective hardness of the weapon (or ammunition). An armor-piercing projectile fashioned from steel bypasses a maximum of 10 points of armor bonus, shield bonus, and natural armor. That same projectile fashioned from adamantine bypasses a maximum of 20 points of these kinds of armor. Regardless of the type of material the weapon or its ammunition is made of, an armor piercing weapon cannot bypass more armor than a creature has.
- A fighter attacks a hill giant (AC 20, of which +9 is natural armor bonus and +3 comes from a worn hide armor; total armor +12) with an armor-piercing weapon. A steel projectile bypasses 10 points of armor, leaving the hill giant with a total armor bonus of +2, and an AC of 10.
- The same fighter attacks the same hill giant, but now with adamantine ammunition. Adamantine has a hardness of 20, and can bypass up to 20 points of armor. The hill giant only has 12 points of armor, so the amount of armor bypassed is 12. The fighter may attack the hill giant at an AC of 8.