Silencers are more properly called sound moderators or suppressors and are widely used by hunters to reduce noise levels from the discharge of firearms, particularly sporting rifles and air weapons.
A sound moderator is essentially no more than a series of baffles coupled to an expansion chamber, contained within a tubular attachment which screws on to the end of the firearm’s barrel.
The noise of the discharge of most firearms is made up of two components.
The first comes from the rapid expansion of propellant gases as they leave the muzzle.
The second is the supersonic crack of the bullet. It is not possible to reduce the sound level of a supersonic bullet, but a sound moderator fitted to such a rifle will have some significant effect in reducing the noise signature because it controls the rate of expansion of the propelling gases.
For a sound moderator to be really effective, it must be used with ammunition whose projectiles travel at less than the speed of sound. In such cases, the noise of the discharge is greatly reduced and may not even be recognisable as a gun.
It is not possible to fit a sound moderator to a revolver because the gap between the barrel and the front of the cylinder means that about 5 per cent of the propellant gases escape, contributing to the overall noise of the discharge. Otherwise, sound moderators can be fitted to any type of firearm.
Second World War Sten submachine guns when fitted with a large, integral moderator being fired using special subsonic ammunition are impressive. The only noise that came from the weapon is the clatter of its bolt.
In the public imagination, sound moderators for firearms invariably have a James Bond or underworld image. In reality, they are widely used in the countryside by hunters who wish to play their part in cutting noise pollution.
The first successful silencers were patented in 1910 by the American inventor Hiram P. Maxim, son of Hiram S. Maxim of Maxim machine-gun fame. His devices were of the baffle type, which is still in common use today. A baffle silencer typically consists of a metal cylinder, usually divided into two sections, which is fixed to the muzzle of the firearm.
The first section, which is typically about a third of the silencer’s length, contains an “expansion chamber” into which the hot gases that follow the bullet out of the muzzle can expand to dissipate some of their energy. The expansion chamber may contain a wire mesh cylinder, whose function is to break up the column of gas and to cool it by acting as a heat sink.
The second section consists of a series of metal baffles, with a central hole to allow the passage of the bullet.
The function of the baffles is to progressively deflect and slow the flow of gas emerging from the expansion chamber, so that by the time the gases emerge from the silencer, their flow is cooler, they travel at low velocity and they are silenced. A motorbike silencer works on exactly the same principle.
There are also variations on this theme: some designs consist entirely of baffles, while others are based entirely on one large expansion chamber. In fact, a plastic soft drinks bottle can be made into a fairly efficient silencer that will work for a limited number of shots before it breaks up.
Silencers usually work best with cartridges that fire subsonic ammunition, since this eliminates the sonic crack which is produced by a bullet that goes faster than the speed of sound.
Some silencer designs slow the bullet to subsonic speed by means of ports cut into the barrel, with the ported section extending to protrude into the expansion chamber.
These ports bleed off gas from behind the bullet, thereby reducing bore pressure and, eventually, the velocity of the bullet. In other designs, the baffles are made from an elastic material with a central hole smaller than the bullet.
These “wipes” are pushed open by the passage of the bullet and close when it is past. The idea is that they further slow the exit of gas. Not surprisingly, the wipes can wear out rather quickly and can affect the accuracy of the bullet.
A second, but less common, type of silencer is the “wire mesh” design.
These usually have the same expansion chamber as the baffle type, but the baffles are replaced by a column of knitted wire mesh with a central hole for the bullet. Here, the wire mesh acts to disrupt the column of gas as in the baffle design, while at the same time acting as a heat sink to cool the hot gas and hence quieten it.
Criminals have been known to improvise this type of silencer, using wire wool or steel pan scourers to form the mesh.
The very latest innovation in muzzle-mounted silencers is the so-called “wet can” silencer.
These designs allow the use of water or a lubricating oil. On firing, the hot expanding gases are cooled, and therefore quietened, by the exchange of heat into the liquid. Wet silencers allow the designer to produce much smaller or quieter designs.
An alternative approach to silencer design which dispenses entirely with the muzzle-mounted silencer has appeared from Russia.
Instead it uses a special cartridge in which the bullet is pushed out by a propellant-driven piston. The piston is stopped by the neck of the cartridge, trapping the hot, noisy gas entirely within the chamber of the firearm.
It is fair to say that Hollywood takes great artistic liberties with silencers.
Most real designs are very much larger than the cigar-tube sized ones typically shown on film and usually much less simple to fit and remove. Despite what is shown in films, it is usually impossible to silence a revolver because the gap between the cylinder and the barrel allows gas to escape.
Finally, forget the distinctive “phut” produced by James Bond’s silencer.
Real designs are more likely to produce a muffled crack, or to sound like a car door being slammed.