Armor piercing

Armor piercing, or AP, bullets are specifically designed with a single purpose: to penetrate armor.

AP bullets usually have a sharp point, and are coated in copper. They are generally tipped with a solid metal, such as copper or steel, and have cores filled with tungsten carbide, hardened steel or even depleted uranium. In Fallout, AP bullets have the effect of lowering the target's Damage Resistance (DR), while decreasing the overall damage that weapon does.

Fallout 3 doesn't have AP bullets, but has three armor penetrating weapons. These are the player created Unarmed deathclaw gauntlet, the unique ranged microwave emitter, and the unique melee Man Opener. They are not the strongest weapons in the game, but they have the unique property of ignoring target armor, which make them as effective against an adversary in power armor as they would be against an unarmored raider.

Fallout: New Vegas features the return of rounds with varying degrees of armor penetrating abilities. Rounds specifically designed for this purpose reduce a target's Damage Threshold (DT) by 15 at the expense of a slight reduction overall damage and the difficulty in finding AP rounds; however, other types of specialized ammunition have some armor piercing abilities despite not being specifically engineered to do so (.357 magnum JFP, .44 magnum semi-wadcutters and 9mm +P rounds are examples). 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.

Inconsistency over effectiveness

 * Due to a bug in Fallout, AP bullets are identical to JHP or FMJ variants.
 * In Fallout 2, due to a mistake in their design, AP bullets are vastly inferior to JHP or FMJ variants. This is best observed while fighting the assault rifle-toting guards in Vault 15. Both rifles are initially loaded with 5mm JHP, and deal about 22 points of damage in a mid-ranged burst against combat armor. However, the guards only carry 5mm AP to reload with. Subsequent bursts from the same distance often fail to deal any damage at all.
 * Even against targets with high Damage Resistance, the resistance reduction does not make up for the reduced damage. In addition, AP bullets do nothing to reduce the Damage Threshold of the target's armor, making the armor piercing aspect less powerful than intended.
 * This inconsistency is not the case with AP rockets or HN AP Needler Cartridges, which are both substantially better than their "normal" counterparts.

Fallout: New Vegas
"Armor piercing" rounds are reintroduced in Fallout: New Vegas. 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 their what type of enemy they are or how strong their armor is. The types of ammunition listed below negate the Damage Threshold effect for both energy weapons and standard guns.