A suppressor, sometimes called a "silencer", can be attached to the muzzle of a firearm to reduce the sound of firing the weapon. Suppressors attached to the muzzle will not reduce the sound of high velocity gas released from other locations like the gap between the cylinder and barrel of a revolver. A muzzle silencer is also ineffective in reducing the snap of a supersonic bullet, or the noise produced by the mechanical action of a self-loading firearm. Use of silencers is rare in United States crimes. A 2007 study estimated unlawful silencers were involved in only 0.05 percent (1 in 2,000) federal criminal prosecutions. Also, these include cases where the silencer was not used, but simply in the possession of the defendant for 92% of prosecutions involving a silencer.