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 HollywoodSilencer.
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 where the shot has come from.
Even with a handgun, supersonic ammunition is impossible to effectively silence, as the crack of the bullet breaking the sound barrier will still be heard. 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. The fact that shooting has happened will still be pretty obvious to anyone 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 [[http://en.wikipedia.org/wiki/Nagant_M1895 Nagant M1895]], they are a very tiny minority. Hollywood tends to ignore this. Shotguns are just about 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. The resulting silencer would tend to be so enormous as to render the weapon impractical for any conceivable use, however.
One reason for the relatively restricted use of silencers is their tendency to become fouled 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.
The idea of the HollywoodSilencer is so prevalent that real silencers are often marketed as "suppressors" to emphasise they do not make weapons completely silent, and gun enthusiasts discourage use of the term "silencer" due to the association. However, it isn't true that "suppressor" is the correct or original term; Hiram Maxim 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 [[http://en.wikipedia.org/wiki/De_Lisle_carbine 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 barrel, and pistols like the [[http://en.wikipedia.org/wiki/Welrod 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, while not "silent", be made rather quiet with a decent silencer since they don't make that much noise in the first place; the [[http://en.wikipedia.org/wiki/High_Standard_HDM 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 during low light conditions where muzzle flash is the easiest way to spot a gunman in action.
Due to the 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 those states where silencers aren't illegal, there is still a Federal approval procedure that has to be followed (which can take months, because only a small number of officials are assigned to approving purchases of silencers and other devices regulated under the National Firearms Act[[note]]Other NFA items include machine guns, short-barreled rifles and [[SawedOffShotgun shotguns]], and very high-caliber (as in larger than .50 caliber) firearms. Silencers, however, are the the most common NFA items, probably because they're less expensive and fewer states outright ban them.[[/note]]) and a $200 Federal tax that has to be paid in order to purchase one. This is why the average HollywoodSilencer is just a metal tube and still produces a large muzzle flash; Hollywood propmasters don't want to pay thousands of dollars replacing and registering worn-out working silencers. Interestingly despite the stricter gun controls it's a lot easier to buy a silencer in the UK and other European than the US due to having never been painted with the brush of criminality.
But now [[HeKnowsTooMuch you know too much]] [[labelnote:☞]]fwip![[/labelnote]].