Silencer

A suppressor is a device fixed on the end of a gun's barrel to greatly suppress, hence the name, the sound of a shot going off from the weapon. Suppressors are mistakenly called silencers, this is an incorrect term because it is almost impossible to totally silence a weapon, and have it be effective. They reduce the sound of gunfire by allowing the rapidly expanding gases from the firing of the bullet to be temporarily diverted or trapped in a series of hollow chambers, known as expansion chambers. The trapped gas expands and cools, and it's pressure and velocity decrease as it exits the chambers. Suppressors can fire super-sonic ammunition but this wears out the suppressor quicker and is not as quiet as using sub-sonic ammunition.

In-game
In Call of Duty 4, a suppressor can used on pistols, assault rifles, and sub-machine guns. It is effective when ambushing an enemy, or firing from a distance. The less sound makes it harder to locate the origin of the firing, so an enemy may have trouble shooting back accurately.

A player start out with the suppressor on the pistol, but you have to complete the second Marksman's Challenge to get it for other weapons( You'll need 75 kills.)

It is a very popular attachment in Search and Destroy.

It is effective when used with the Perk UAV Jammer.