Weapon Penetration and Armor Piercing

The penetration mechanic (going through walls) has no impact by itself on the amount of damage a weapon does against armored enemies (the yellow health bar). The Javelin, for example, has a penetration value of 100 = 1 meter so it can penetrate through a total of 1 meter of solid objects but it doesn't do extra damage to the yellow armor health bar. The Javelin will, however, penetrate through other solid objects like the detachable armor plating on the Brutes, Husks, and Cannibals, and the Guardian shield. Whenever a weapon hits a target after going through a solid object, the amount of damage done is reduced by a flat amount.

The weapon mods, equipment (MP), and Armor Piercing Ammo can all increase the distance that weapons can penetrate through solid objects. So if you add a penetration mod on the Javelin, the penetration distance gets even larger, but that won't increase the damage done to enemies after penetrating through objects.

The penetration mods and Armor Piercing Ammo have another mechanic called 'piercing' which affects the damage done to armored targets. By default, enemies with the yellow armor bar will chop off a set amount of damage done to them from weapons. This makes low damage weapons less effective against armor and high damage weapons very effective against them. So even if a weapon has a really fast fire rate but each bullet does a low amount of damage it will get penalized for hitting armor since a big chunk of every bullet's damage is ignored. When equipping penetration mods and using Armor Piercing Ammo, the weapon's "armor piercing" goes up, which means a lot less damage is cut off. For example, equipping rank 1 Armor Piercing Ammo will reduce the amount of damage that armor cuts off by 50%. By default, weapons do not have any base armor "piercing."

Armor can never negate weapon damage completely. There is a minimum damage value that weapons will always do to armor.