A silencer reduces the noise level of a discharged gun by trapping and slowing the release of gases inside the gun's muzzle.

In 1908 Hiram Percy Maxim (1869-1936), the son of Hiram Maxim, inventor of the machine gun, invented such a device. Called the Maxim silencer, it consisted of a cylinder screwed onto the gun barrel. Inside the cylinder were several small chambers, separated from each other by metal rings with holes drilled in the center to allow the bullet to pass through. When the escaping gases rushed into the chambers, they expanded and slowed enough to keep from exploding from the gun's muzzle. The United States Army first used Maxim's silencer, with poor results. The military found that the velocity of the bullet, which was faster than the speed of sound, caused a loud noise by itself, thus producing its own shock wave. The only way that army snipers could make their silencers effective was to use low-velocity ammunition which traveled at speeds slightly less than the speed of sound. During World War II, a new variation of the silencer was introduced. Instead of using several chambers separated by metal rings, a simple tube with vent holes was attached to the muzzle of the gun. At the end were several rubber disks through which the bullet passed, and the expanding gases escaped through the holes.

Throughout the years silencers have proved to be less effective when used with most firearms, including revolvers and various types of semi-automatic weapons. Revolvers and semi-automatic weapons have open breeches that allow some gas to escape, and so a silencer on the muzzle does not completely eliminate the noise. Though they tend to make most firearms less effective, silencers have obvious advantages for people who want their gun activity to remain secret. Fearing that silencers contribute to illegal activity, many countries strictly regulate the sale of silencers. Some modern guns, designed specifically for clandestine military operations, are made with built-in silencers. These use vent holes and internal baffles as an integral part of the gun barrel. Like other silenced weapons, these are less accurate and use lower velocity ammunition than conventional guns.