You can take advantage of subtle weaknesses in your opponents’ defenses.
Prerequisites: Int 13, Combat Expertise, Weapon Focus (chosen weapon).
Benefit: Whenever you attack with a weapon with which you have selected the Weapon Focus feat, you may ignore up to 5 points of the target’s damage reduction (regardless of the material or enhancement bonus of your weapon) or hardness. This benefit can’t reduce the effective damage reduction or hardness of a target to less than 0.
Source: Draconomicon, page 106.