Surface Penetration

Wall Penetration is a feature that was first introduced to the Call of Duty series in Call of Duty 4: Modern Warfare and has been included in all subsequent major Call of Duty releases.

How It Works
Wall penetration is an enhancement to the game engine that allows players to engage targets through light cover. Damage done by firing through cover is less than that inflicted by firing on a player directly. A bullet's damage is also its health, and the bullet will lose a certain amount of damage based on the type of material penetrated, the thickness of the material, the angle of penetration, and the wall penetration type of the bullet. There are essentially two penetration types, "Small" and "Medium." "Large" and "Armor Piercing" also exist, but they are exactly the same as "Medium" for all intents and purposes.

Shooting through surfaces has no effect on the bullet's speed or flight path (see Hitscans).

In Call of Duty 4: Modern Warfare and Call of Duty: World at War the damage reduction could be mitigated by using the Perk Deep Impact. This was carried over to Modern Warfare 2 with the weapon attachment FMJ, which is identical to Deep Impact, save for the addition of small explosions and different sound effects when a bullet impacts a surface.

Uses
The technique can be used by experienced players who know the most common spots for players to be in certain buildings, and who would be killed if directly approaching the target - this is common in Search and Destroy. Players using Heartbeat Sensors can use the sensor to work out where a target is through cover, and therefore be confident while firing their weapon.

Trivia

 * Putting a high volume of fire through a wall is often referred to as "wallbanging."