A silencer is a device used to reduce the sound and visible muzzle flash created by a weapon firing; usually a series of baffles and chambers that allow expanding propellant gases to slow and cool down, reducing the noise when they finally exit the device.
Silencers are also marketed as suppressors — see below. Their use and effectiveness differs quite a bit from the Hollywood Silencer.
Silencers vary in capability depending on what they're attached to and what it's firing; for anything above a pistol firing normal ammunition, the most you can hope for is "hearing safe," with the goal being to reduce the sound of the weapon's discharge below the human pain threshold. SWAT teams, for example, use silenced weapons to avoid deafening each other when working indoors. An additional goal may be to mask the shot's point of origin.
Even with a handgun, supersonic ammunition is effectively impossible to silence, as the crack of the bullet breaking the sound barrier will still be quite audible. Subsonic ammunition produces a sound similar to a very loud stapler, with much of that noise being from the motion of the slide as the weapon cycles. That a shot has been fired will still be pretty obvious to everyone with functional ears, as long as they are close enough: the sound of the action cycling travels a much shorter distance than the sound of a bullet firing.
Revolvers are problematic: normal revolvers don't form a gas seal between the cylinder and barrel; while some revolvers exist that have the seal, such as the Nagant M1895, they are a very tiny minority. Hollywood tends to ignore this.
Shotguns are just barely possible to silence by using slugs or shotshells with plastic shot cups to protect the inside of the device; conventional shotshells would wreck a silencer in short order. Their primary use is to eliminate the need for ear protection, meaning you can preserve your situational awareness while hunting without risking long-term hearing damage.
One reason for the relatively restricted use of silencers is their tendency to foul up and wear out: firing a gun with a typical silencer will damage the interior of the device. Some older or less expensive silencers have very limited use, and wear out after just a few shots (about a dozen at most). A purpose-built silencer such as those used by special forces, however, will last for quite a long time with proper maintenance.
Another technique to improve sound suppression is shooting a silencer "wet", which is literally dunking it in water beforehand. The water absorbs some of the energy of expanding gases, but dries quickly, meaning that this method is mainly used by civilian suppressor enthusiasts. Some use a water-based gel to prevent the device from drying out as quickly.
Another unexpected feature of firing a weapon with a silencer is that the shooter might suffer a bad onion-eye. Silencers and suppressors work by slowing and trapping the gas exhaust; therefore, much of the "dirty" combustion byproducts that normally exit via the muzzle can blast right back through the action instead, blowing noxious powder fumes and even unburnt grains of propellant right into the shooter's face. This can cause anything from mild discomfort to a serious detriment to performance. There is a video in which an experienced Russian SWAT officer gets a powder grain in his eye - although in the video he simply keeps firing as he was taught, he still had to use eye-drops for a week after. The best way to mitigate this issue is to select a firearm, ammunition, and a silencer that will go well together.
The idea of the Hollywood silencer is so prevalent that real silencers are often marketed as "suppressors" to emphasize that they do not make weapons completely silent, and gun enthusiasts typically discourage use of the term "silencer" due to the association. However, it isn't true that "suppressor" is the correct or original term; Hiram Percy Maxim (whose father, Sir Hiram Stevens Maxim, had invented and marketed one of the first practical automatic machine guns and gone deaf from all his backyard test-firing) patented his original device as the "Maxim Silencer".
Only a handful of real weapons can really be called "silent"; these are purpose-built to prevent gas escaping from any part of the action during firing (and are thus typically manually operated), and typically have silencers that shroud almost the entire barrel; examples include the De Lisle Carbine, a rather odd-looking hybrid that accepted Colt M1911 magazines and used a Lee-Enfield bolt action mated to a Thompson SMG (a.k.a. "Tommy Gun") barrel, and pistols like the Welrod. In both cases, it was said operating the weapon's action was louder than firing it. Both are bolt-action weapons, meaning that unless a second shot is needed, the primary sound of firing it is the click of the trigger. Also, most guns chambered in .22 LR can be made rather quiet with a decent silencer since they don't make that much noise in the first place; the High Standard HDM is a good example that was first used during the Second World War and is still in US military service.
Sometimes the most unlikely objects are used as silencers, such as as so-called "bottle silencers" which can be fashioned from household items, though their effectiveness is significantly lower than a purpose-built silencer's.
Sticking a silencer on a gun increases bench accuracy (though it can make the weapon harder to actually aim), and can increase the muzzle velocity slightly, though much less than an equivalent length of actual barrel; an exception to the latter is that some silencers are specifically designed to reduce the velocity of the fired round. Obviously, in the latter case or if the shooter switches to subsonic ammo, his range will suffer for it in comparison to the supersonic ammo he was using before.
An additional and often ignored function of the silencer is to reduce muzzle flash. This greatly aids any shooter attempting to stay concealed (read: any shooter that doesn't want things shooting back at him), especially in low-light conditions where muzzle flash is the easiest way to spot a gunman in action.
One of the more surprising facts about suppressor use: US Navy SEALs on scouting and sabotage missions in Vietnam almost never used suppressed pistols for eliminating sentries or combat in general; the most useful function of their issued silenced S&W Mk 22 pistols turned out to be simply killing various animals who otherwise might have been dangerous or compromising to the team. Hence the name "Hush Puppy" given to the gun.
Due to the Hollywood-fueled perception that silencers are primarily used to aid and abet crime, private ownership or possession of a silencer is illegal in several U.S. states, including California and New Jersey. In the states where silencers aren't illegal, there is still a Federal approval procedure that has to be followed (which can take anywhere from months to more than a year, because only a small number of officials are assigned to approving purchases of silencers and other devices regulated under the National Firearms Actnote ) and a $200 Federal tax that has to be paid in order to purchase one. This is why the average Hollywood silencer is just a metal tube and still produces a large muzzle flash; Hollywood prop masters don't want to pay thousands of dollars replacing and registering worn-out working silencers. It is also a common topic of complaint among gun advocates; suppressors only reduce the sound of gunshots to that of a nail gun, and would be far simpler, cheaper and more effective (a metal tube and rubber washers) than $5 earplugs or $12-$35 earmuffs at preventing ear damage - if not for the $200 tax stamp and a ludicrously long waiting period (up to an entire year after getting grilled by the Feds). Societal ignorance of why suppressors were regulated has to do with the actual reasons behind the regulation: "Silenced" hunting rifles were used to poach deer during the Great Depression as desperate people with guns looked for ways to get food. As the poachers were hunting on federally controlled property, their crimes became high-degree felonies. To prevent smuggling and serious depopulation of deer, the NFA simply made suppressors extremely impractical for ordinary people to buy. The masses of the time took it to mean that the Feds were trying to weed out Mobsters and arrest them for tax evasion, thus suppressors are nowadays associated with gang-related crime despite the impractical nature of attempting to conceal a suppressed weapon on one's person in the first place.
Interestingly, despite the generally stricter gun control laws in Europe, it's a lot easier to buy a silencer in the UK and other European countries than in the U.S., due to them having never been painted with the "criminality" brush.
But now you know too much ☞ .