Silencer is a term that has become a popular replacement for the more appropriate word; suppressor. Suppressors are devices that are used to soften the sound that emits from a firearm's barrel. The suppressor works by allowing the escaping gas that is being forced out from behind bullet to expand into a chamber while the bullet continues through. This gas expansion chamber works very much like a car's muffler does. It is primarily the escaping gases that causes the *bang* from a firearm being discharged. Suppressors can make certain firearms quiet enough that the loudest sound is that of the slide cycling a new round.
Suppressors do not eliminate the super sonic crack that a bullet creates during flight as it continually exceeds the sound barrier. Integral suppressors can cause a bullet to slow down by a small margin due to bleeding off gas in what is normally part of the barrel, whereas attachment suppressors actually functionally increase barrel length and thus muzzle velocity. Most video games make all suppressors decrease power and/or accuracy for balance purposes. With the case of the integral suppressor on the HK MP5SD, the traditionally super sonic 9mm round is slowed enough that it travels below the speed of sound, which eliminates the need for specially made sub-sonic ammunition. The common use of suppressors on the much faster 5.56mm round (M4 for example) do not slow the projectile down below the speed of sound. Thus, there is always the super sonic crack along the bullets path. Suppressors are used on such a round to enable the operator to work without ear protection and also makes locating the shooter's position more difficult since the ballistic crack of a supersonic bullet comes from the bullet's path, not the bullet's origin. While there are sub-sonic loads for most rounds, certain semi-auto and full-auto weapons require more pressure to cycle the action than these slower rounds can create. This results in the action needing to be cycled manually every shot. The super sonic crack is not always represented in games.
While older suppressors from the Cold War era had a poor effect on a weapons accuracy, modern suppressors do not. In fact, many suppressors today increase a firearms accuracy because they have the effect of artificially extending the length of the barrel.