Damage

Damage is a shotgun-only Weapon Proficiency in Call of Duty: Modern Warfare 3. It increases the damage of the weapon on which is attached by 40%. This proficiency, although late to unlock, is highly recommended for all shotguns and is practically a necessity for the slower firing shotguns.​

Trivia Bullet, suggesting it may have also been a proficiency for sniper rifles, though this may have been removed for balancing reasons.
 * As previously stated, it is a shotgun only Proficiency, but ironicly, It's create a class icon depicts a .50 Caliber