Armor Piercing

Armor Piercing, or AP, bullets have the effect of lowering the target's Damage Resistance (DR), while decreasing the overall damage that a weapon does.

Fallout: New Vegas
With the version 1.02 patch, all energy weapon ammunition gains innate armor-piercing capabilities, with Over Charge and Max Charge cells offering better AP characteristics. As of patch 1.5, standard 5mm Rounds reduce DT by 10 and AP 5mm rounds reduce it by 25.