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
"Armor piercing" rounds are reintroduced $$ The armor piercing capabilities are different in New Vegas as some rounds are actually labeled as armor piercing while others have the ability to negate Damage Threshold, which gives a target a certain amount of protection, depending on what type of enemy they are or how strong their armor is. The types of the ammunition listed below negate the Damage Threshold effect for both energy weapons and standard guns.

With the version 1.02 patch, all energy weapon ammunition gain 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.