MeeT The Finest

Ability: Increases your weapons bullet penetration, but it doesn't increase bullet damage. This is pretty much like Deep Impact from Call of Duty 4 and FMJ weapon attachment from Modern Warfare 2.
Pro Ability: Your weapons do extra damage to aircraft and turrets. This also reduces flinch when you are shot by someone.

Ability: Enables you to hold your breath longer when steadying a weapons scope. Scout offers a significant advantage to sniper class weapons.
Pro Ability: Allows you to switch your weapon faster.

Ability: Steady aim increases a weapons hip fire accuracy and also reduces shotgun spread. This should be used with a Dual Wield attachment, since Dual Wield significantly reduces a weapons hip fire accuracy.
Pro Ability: When equipped, this enables you to aim faster after sprinting and recover faster from a knife lunge.

Ability: Gives a faster reload time by reducing your weapons reload time in hald. This is extremely useful with weapons that have long reload times, like shotguns and light machine guns.
Pro Ability: Allows you to aim down the sight ~50% faster than normal. This doesn't work with weapons that have scopes.