A suppressor, sound suppressor, or sound moderator, is a device attached to or part of the barrel of a firearm which reduces the amount of noise, and usually the amount of muzzle flash, generated by firing the weapon. A suppressor can be used both with a firearm and with a compressed air gun. Though it is sometimes referred to as a silencer, this term is a misnomer popularized by cinematic fiction: the sound of a projectile, especially one moving faster than the speed of sound through the air, cannot be eliminated. Using a suppressor in long distance shooting (i.e. greater than 300 meters) can reduce the report at the moment of impact, and create the impression of silence near the target. The noise heard near the muzzle as a projectile leaves the barrel, though subdued, is not silent at all.
Early suppressors were created around the beginning of the 20th century by several inventors. American inventor Hiram Percy Maxim (not to be confused with Hiram Stevens Maxim, Hiram Percy's father, and the inventor of the Maxim Machine Gun), is credited with inventing and selling the first commercially successful models circa 1902 (patented 30 March 1909). Maxim gave his device the trademarked name Maxim Silencer. The muffler for internal combustion engines was developed in parallel with the firearm suppressor by Maxim in the early 20th century, using many of the same techniques to provide quieter-running engines (in many European countries automobile mufflers are still called silencers).
The word silencer in reference to gun suppressor remains in widespread use, although it has since fallen out of favor with some, especially among the firearms industry, being replaced with the more literally accurate term sound suppressor or just suppressor, because a "sound suppressor" does not "silence" any weapon, rather it eliminates muzzle flash and reduces the sonic pressure of a firearm discharging. Common usage and U.S. legislative language favor the historically earlier term, silencer. In U.S. law, the terms "firearm muffler" and "firearm silencer" are synonymous.
Suppressors were regularly used by agents of the United States Office of Strategic Services, who favored the newly designed High Standard HDM .22 Long Rifle pistol during World War II. OSS Director William Joseph "Wild Bill" Donovan demonstrated the pistol for President Franklin D. Roosevelt at the White House. According to OSS research chief Stanley Lovell, Donovan (an old and trusted friend of the President) was waved into the Oval Office, where Roosevelt was dictating a letter. While Roosevelt finished his message, Donovan turned his back and fired ten shots into a sandbag he had brought with him, announced what he had done and handed the smoking gun to the astonished president. The British SOE (Special Operations Executive) Welrod assassination pistol with an integral suppressor was also used by the American OSS on clandestine operations in Nazi occupied Europe during the Second World War.
The suppressor is typically a hollow cylindrical piece of machined metal (steel, aluminium, or titanium) containing expansion chambers that attaches to the muzzle of a pistol, submachine gun or rifle. These "can"-type suppressors (so-called as they resemble a beverage can), may be detached by the user and attached to a different firearm of the same caliber. Another type is the "integral" suppressor, which consists of expansion chambers surrounding the barrel. The barrel is pierced with openings or "ports" which bleed off gases into the chambers. This type of suppressor is part of the firearm, and maintenance of the suppressor requires that the firearm be at least partially disassembled.
Both types of suppressor reduce noise by allowing the rapidly expanding gases from the firing of the cartridge to be briefly diverted or trapped inside a series of hollow chambers. The trapped gas expands and cools, and its pressure and velocity decreases as it exits the suppressor. The chambers are divided by either baffles or wipes (see below). There are typically at least four and up to perhaps fifteen chambers in a suppressor, depending on the intended use and design details. Often, a single, larger expansion chamber is located at the muzzle end of a can-type suppressor, which allows the propellant gas to expand considerably and slow down before it encounters the baffles or wipes. This larger chamber may be "reflexed" toward the rear of the barrel to minimize the overall length of the combined firearm and suppressor, especially with longer weapons such as rifles.
Suppressors vary greatly in size and efficiency. One disposable type developed in the 1980s by the U.S. Navy for 9 mm pistols was 150 mm (5.9 in) long and 45 mm (1.8 in) in outside diameter, and was designed for six shots with standard ammunition or up to thirty shots with subsonic (slower than the speed of sound) ammunition. In contrast, one suppressor designed for rifles firing the powerful .50 caliber cartridge is 509 mm (20.0 in) long and 76 mm (3.0 in) in diameter.
Baffles are usually circular metal dividers which separate the expansion chambers. Each baffle has a hole in its center to permit the passage of the bullet through the suppressor and towards the target. The hole is typically at least 0.04 inch (1 mm) larger than the bullet caliber to minimize the risk of the bullet hitting the baffle ("baffle strike"). Baffles are typically made of stainless steel, aluminium, titanium or alloys such as Inconel, and are either machined out of solid metal or stamped out of sheet metal. A few suppressors for low-powered cartridges such as the .22 Long Rifle have successfully used plastic baffles (certain models by Vaime and others.)
Baffles are separated by spacers, which keep them aligned at a specified distance apart inside the suppressor. Many baffles are manufactured as a single assembly with its spacer, and several suppressor designs have all the baffles attached together with spacers as a one-piece helical baffle stack. Modern baffles are usually carefully shaped to divert the propellant gases effectively into the chambers. This shaping can be a slanted flat surface, canted at an angle to the bore, or a conical or otherwise curved surface. One popular technique is to have alternating angled surfaces through the stack of baffles.
Baffles come in several designs. M, K, Z, Monolithic Core and Ω(Omega) are the most prevalent. M-type is the crudest and composes an inverted cone. K forms slanted obstructions diverging from the sidewalls, creating turbulence across the boreline. Z is expensive to machine and includes "pockets" of dead airspace along the sidewalls which trap expanded gases and hold them thereby lengthening the time that the gases cool before exiting. Omega forms a series of spaced cones drawing gas away from the boreline, incorporates a scalloped mouth creating cross-bore turbulence, which is in turn directed to a "mouse-hole" opening between the baffle stack and sidewall.
Baffles usually last for a significant number of firings. Propellant gas heats and erodes the baffles, causing wear, which is worsened by high rates of fire. Aluminium baffles are seldom used with fully automatic weapons, because service life is unacceptably short. Some modern suppressors using steel or high-temperature alloy baffles can endure extended periods of fully automatic fire without damage. The highest-quality rifle suppressors available today have a claimed service life of greater than 30,000 rounds. Baffles have not been given any specific angles, a specific size, or weight to meet any standards; they are created on a trial and error basis.
Wipes are inner dividers intended to touch the bullet as it passes through the suppressor, and are typically made of rubber, plastic or foam. Each wipe may either have a hole drilled in it before use, a pattern stamped into its surface at the point where the bullet will strike it, or it may simply be punched through by the bullet. Wipes typically last for a small number of firings (perhaps no more than five) before their performance is significantly degraded. While many suppressors used wipes in the Vietnam War era, most modern suppressors do not use them as anything that touches the projectile has significant accuracy implications. Equally important is all "wipes" deteriorate quickly and require disassembly and spare parts replacement.
"Wet" suppressors or "wet cans" use a small quantity of water, oil, grease or gel in the expansion chambers to cool the propellant gases and reduce their volume (see ideal gas law). The coolant lasts only a few shots before it must be replenished, but can greatly increase the effectiveness of the suppressor. Water is most effective, due to its high heat of vaporization, but it can run or evaporate out of the suppressor. Grease, while messier and less effective than water, can be left in the suppressor indefinitely without losing effectiveness. Oil is the least effective and least preferable, as it runs while being as messy as grease, and leaves behind a fine mist of aerosolized oil after each shot. Water-based gels, such as wire-pulling lubricant gel, are a good compromise; they offer the efficacy of water with less mess, as they do not run or drip. However, they take longer to apply, as they must be cleared from the bore of the suppressor to ensure a clear path for the bullet (grease requires this step as well). Generally, only pistol suppressors are shot wet, as rifle suppressors handle such high pressure and heat that the liquid is gone within 1-3 shots. Many manufacturers will not warranty their rifle suppressors for "wet" fire, as some feel this may even result in a dangerous over-pressurization of the silencer.
Packing materials such as metal mesh, steel wool or metal washers may be used to fill the chambers and further dissipate and cool the gases. These are somewhat more effective than empty chambers, but less effective than wet designs. Metal mesh, if properly used, may last for hundreds or thousands of shots of spaced semi-automatic fire, however steel wool usually degrades within ten shots with stainless wool lasting longer than regular steel wool. Like wipes, packing materials are rarely found in modern suppressors.
Wipes, packing materials and purpose-designed wet cans have been generally abandoned in 21st-century suppressor design because they decrease overall accuracy and require excessive cleaning and maintenance. The instructions from several manufacturers state that their suppressors need not be cleaned at all. Furthermore, legal changes in the United States during the 1980s and 1990s made it much more difficult for end-users to legally replace internal silencer parts, and the newer designs reflect this reality.
In addition to containing and slowly releasing the gas pressure associated with muzzle blast or reducing pressure through the use of coolant mediums, advanced suppressor designs attempt to modify the properties of the sound waves generated by the muzzle blast. In these designs, effects known as frequency shifting and phase cancellation (or destructive interference) are used in an attempt to make the suppressor quieter. These effects are achieved by separating the flow of gases and causing them to collide with each other or by venting them through precision-made holes. The intended effect of frequency shifting is to shift audible sound waves frequencies into ultrasound (above 20 kHz), beyond the range of human hearing. The Russian AN-94 assault rifle features a muzzle attachment that claims apparent noise reduction by venting some gases through a "dog-whistle" type channel. Phase cancellation occurs when similar sound wave frequencies encounter each other 180° out of phase, cancelling the amplitude of the wave and eliminating the pressure variations perceived as sound.
Using either property to advantage requires that the suppressor be designed within the specification of the muzzle blast in mind. For example, the velocity of the sound waves is a major factor. This figure can change significantly between different cartridges and barrel lengths.
Thus, in order for maximum effectiveness to be achieved, the suppressor must be "tuned" for a specific cartridge/barrel length combination. This can be done through the use of either a fixed or adjustable baffle design. While it may sound daunting, any weapon that needs to provide such exceptional sound suppression is almost certainly going to be manufactured in small quantities and issued only for mission profiles critical enough to make such efforts worthwhile.
However, these concepts are controversial because muzzle blast creates broadband noise rather than pure tones, and phase cancellation in particular is therefore extremely difficult (if not impossible) to achieve. Some suppressor manufacturers claim to use phase cancellation in their designs, but these claims are generally unsupported from a scientific perspective.
From the practical perspective, supersonic cartridge loads are impractical to suppress past the levels that are merely hearing-safe for the shooter due to the sonic boom emitted by the bullet, and cartridges such as .22 LR and .45 ACP have long been recognized as easy to suppress even if using technology dating back to 1940s when the mission calls for the quietest gun available.
The portrayal of suppressed firearms in popular culture is not always accurate and could lead to the misconception that silencers are capable of completely eliminating the sound of firing, or reducing it to a quiet whistling or "phut" sound. This is unlikely because when a gun is fired, multiple sounds are possibly made. There are five major categories of suppressed fire noise: action, blast, sonic signature, impact, and operator. Some of these are present in all instances, others depend wholly on the specific mechanics of the weapon employed.
In order of timing:
The two loudest sounds in a gunshot are typically the muzzle blast and the sonic signature. Paired reports from the same shot may be observed when the listener is first reached by the shock wave generated by the bullet flying past at supersonic speed, then by the muzzle blast moving at sound speed all the way from the muzzle.
Multiple techniques are used to address each of these sounds, but the suppressor itself is capable of addressing muzzle blast, sonic signature (through integral gas bleed, at the price of reducing projectile speed to subsonic) and the ability to cancel the mechanical action noise through Nielsen device manipulation, canceling the ejection cycle.
Live tests by independent reviewers of numerous commercially available suppressors find that even low caliber unsuppressed .22 LR handguns produce gunshots over 160 decibels. In testing, most of the suppressors reduced the volume to between 130 and 145 dB, with the quietest suppressors metering at 117 dB. The actual suppression of sound ranged from 14.3 to 43 dB, with most data points around the 30 dB mark. A notable example is the De Lisle carbine, a British World War II suppressed rifle used in small numbers by Special Forces. This was recorded at 85.5 dB in official firing tests.
Comparatively, ear protection commonly used while shooting provides 18 to 32 dB of sound reduction at the ear. Further, chainsaws, rock concerts, rocket engines, pneumatic drills, small firecrackers, and ambulance sirens are rated at 100 to 140 dB.
While some consider the noise reduction of a suppressor significant enough to permit safe shooting without hearing protection ("hearing safe"), noise-induced hearing loss may occur at 85 time-weighted-average decibels or above if exposed for a prolonged period, and suppressed gunshots regularly meter above 130 dB. However, the US Occupational Safety and Health Administration uses 140 dB as the "safety cutoff" for impulsive noise, which has led most US manufacturers to advertise sub-140 dB suppressors as "hearing safe." It should be noted that current OSHA standards would allow no more than sub single second exposure to impact noise over 130 dB per 24 hours. That would equate to a single .308 round fired through a very efficient can. This result effectively requires all users of suppressors to wear additional ear hearing protection.
dB testing measures only the peak sound "pressure" noise, not duration or frequency. Limitations of dB testing become apparent in a comparison of sound between a .308 caliber rifle and a .300 Winchester Magnum rifle. The dB meter will show that both rifles produce the same decibel level of noise. Upon firing these rifles, however, it is clear that the .300 Winchester Magnum sounds much louder. What a dB meter does not show is that, although both rifles produce the same peak sound pressure level (SPL), the .300 Winchester Magnum holds its peak duration longer––meaning that the .300 Winchester Magnum sound remains at full value longer, while the .308 peaks and falls off more quickly. Decibel meters fail in this and other regards when being used as the principal means to determine suppressor capability. In a physical sense, dB meters essentially take a short time average (RMS intensity of a sonic signal or impulse) over a specified period of time (sampling rate), and do not take into account the rate of increase of the sound wave packet (first derivative of packet envelope), which would in practice provide a better sense of the human perception of sound. 
The caliber and power of the bullet/cartridge being suppressed is also an important factor. Generally, equal quality suppressors can quieten the report of a smaller caliber bullet more effectively than a larger caliber bullet. This is because stripping the exhaust gases found directly behind the projectile becomes increasingly more challenging. Likewise, cartridges which produce higher pressures and more gases, such as those used in rifles, will also generally be louder than those which produce less pressure and fewer gases, such as handgun cartridges. In a gunshot, the sound of the report (the combination of the sonic boom, the vacuum release, and burn of powder) will almost always be louder than the sound of the action cycling of an auto-loading firearm. Alan C. Paulson, a renowned firearms specialist, claimed to have encountered an integrally suppressed .22 LR that had such a quiet report, although this is somewhat uncommon.
Because of the limited energy and momentum of less powerful cartridges, movie scenes in which an attacker fires a near-silent shot that instantly kills the victim are generally unrealistic; excluding headshots that admit the projectile directly into the cranial cavity and/or chest shots that hit the left ventricle or aorta.
In weapons firing supersonic bullets, the supersonic bullet itself produces a loud and very sharp sound as it leaves the muzzle in excess of the speed of sound and gradually reducing speed as it travels downrange. This is a small sonic boom, and is referred to in the firearm field as "ballistic crack" or "sonic signature." Subsonic ammunition eliminates this sound, but at the cost of lower velocity, resulting in decreased range and lessening effectiveness on the target. Military marksmen and police units may use this ammunition to maximize the effectiveness of their silenced rifles. While the range may be decreased when using subsonic rounds, this may be acceptable for specialized situations, where the absolute minimum amount of noise is required.
However, the numeric effectiveness of subsonic rounds is, again, misrepresented by media. Independent testing of commercially available firearm suppressors with commercially available subsonic rounds has found that .308 subsonic rounds decreased the volume at the muzzle 10 to 12 dB when compared to the same caliber of suppressed supersonic ammunition. When combined with suppressors, the subsonic .308 rounds metered between 121 and 137 dB.
This ballistic crack depends on the speed of sound, which in turn depends mainly on air temperature. At sea level, an ambient temperature of 70 °F (21 °C), and under normal atmospheric conditions, the speed of sound is approximately 1,140 feet (350 m) per second (347 m/s). Bullets that travel near the speed of sound are considered transonic, which means that the airflow over the surface of the bullet, which at points travels faster than the bullet itself, can break the speed of sound. Pointed bullets which gradually displace air can get closer to the speed of sound than round nosed bullets before becoming transonic.
Special cartridges have been developed for use with a suppressor. These cartridges use very heavy bullets to make up for the energy lost by keeping the bullet subsonic. A good example of this is the .300 Whisper cartridge, which is formed from a necked-up .221 Remington Fireball cartridge case. The subsonic .300 Whisper fires up to a 250 grain (16.2 g), .30 caliber bullet at about 980 feet (300 m) per second (298 m/s), generating about 533 ft·lbf (722 J) of energy at the muzzle. While this is similar to the energy available from the .45 ACP pistol cartridge, the reduced diameter and streamlined shape of the heavy .30 caliber bullet provides far better external ballistic performance, improving range substantially.
9×19mm Parabellum, a very popular caliber for suppressed shooting, can use almost any factory-loaded 147 gr (9.5 g) weight round to achieve subsonic performance. These 147 gr weight bullets typically have a velocity between 900 and 980 feet (300 m) per second (275 and 300 m/s), which is less than the common 1140 ft/s speed of sound.
Russian 9×39mm ammo had a high subsonic ballistic coefficient, high retained downrange energy, high sectional density, and moderate recoil. All elements combined make this a very attractive choice for close quarters combat (CQB) firearms.
Instead of using subsonic ammunition, one can also lower the muzzle velocity of a supersonic bullet before it leaves the barrel. Some suppressor designs, referred to as "integrals", do this by allowing gas to bleed off along the length of the barrel before the projectile exits. The MP5SD is the best example of this with holes right after the chamber of the barrel used to reduce a regular 115 or 124 gr ammo to subsonic velocities.
Aside from reductions in volume, suppressors also tend to alter the sound to something that is not identifiable as a gunshot. This reduces or eliminates attention drawn to the shooter (hence the Finnish expression: "A suppressor does not make a marksman silent, but it does make him invisible"). This is especially true in cases where there are other sources of ambient noise, such as in an urban environment. Suppressors are particularly useful in enclosed spaces where the sound, flash and pressure effects of a weapon being fired are amplified. Such effects may disorient the shooter, affecting situational awareness, concentration and accuracy, and can permanently damage hearing very quickly.
As the suppressed sound of firing is overshadowed by ballistic crack, observers can be deceived as to the location of the shooter, often from 90 to 180 degrees from his actual location. However, counter-sniper tactics can include Gunshot Location Detection Systems, where sensitive microphones are coupled to computer algorithms, and use the ballistic crack to detect and localize the origin of the shot. The U.S. Boomerang system is one such example.
There are many advantages in using a suppressor that are not related to the sound.
Hunters using centerfire rifles find suppressors bring various important benefits that outweigh the extra weight and resulting change in the firearm's center of gravity. The most important advantage of a suppressor is the hearing protection for the shooter as well as his/her companions. There are many hunters who have suffered permanent hearing damage due to someone else firing a high-caliber gun too closely without a warning. By reducing noise, recoil and muzzle-blast, it also enables the firer to follow through calmly on his first shot and fire a further carefully aimed shot without delay if necessary. Wildlife of all kinds are often confused as to the direction of the source of a well-suppressed shot. In the field, however, the comparatively large size of a centerfire rifle suppressor can cause unwanted noise if it bumps or rubs against vegetation or rocks, and many users cover them with neoprene sleeves.
Suppressors reduce firing recoil significantly, primarily by diverting and trapping the propellant gas. Propellant gas is generally a fraction of the projectile mass, but it exits the muzzle at multiples of the projectile velocity, and since recoil energy is a function of mass times velocity squared the elimination of the propellant recoil can be significant. Paulson et al., discussing low-velocity pistol calibers, suggest the recoil reduction is around 15%. With high-velocity calibers recoil reduction runs in the range of 20–30%. The added weight of the suppressor—normally 300 to 500 grams—also contributes to the reduction of the recoil. Further, the pressure against the face of each baffle is higher than the pressure on its reverse side, making each baffle a miniature "pneumatic ram" which pulls the suppressor forward on the weapon, contributing a counter recoil force.
A suppressor also cools the hot gases coming out of the barrel enough that most of the lead-laced vapor that leaves the barrel condenses inside the suppressor, reducing the amount of lead that might be inhaled by the shooter and others around them. However, in auto-loading actions this might be offset by increased back pressure which results in propellant gas blowing back into a shooter's face through the chamber during case ejection.
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Legal regulation of suppressors varies widely around the world. In some nations, such as Finland, Norway and France, some or all types of suppressor are essentially unregulated and may be bought "over the counter" in retail stores or by mail-order, as they are considered a great help, along with hearing protection, to preserve the hearing of the user and any onlookers.
In Hong Kong, "any accessory to such arms designed or adapted to diminish the noise or flash" is within the definition of 'arms' under the Firearms and Ammunition Ordinance (HK Laws. Chap 238). As such, a permit is required (as with firearms and ammunition) for possession which would otherwise be illegal and carries penalties up to a fine of HK$100,000 and 14 years in jail.
In Thailand, sound suppressors of any kind are allowed to be used only by law enforcement units or military personnel in operation.
In Turkey, civilian purchase, sale or possession of suppressors are strictly prohibited, with possible jail terms of up to 25 years if convicted. Suppressors can only be purchased by military personnel when approved by the officer in charge of the base armory. Individual law enforcement officers are not eligible to purchase or possess suppressors unless these are issued by a local agency, in which case these would be registered to the General Directorate of Security in Ankara.
In Pakistan, it is strictly prohibited for civilians to possess sound suppressors, with a punishment of life time in prison.
In Austria, the purchase or possession of a suppressor is prohibited according to §17 of the Austrian Weapons Law.
In the Czech Republic suppressors are, according to §4 of Weapons and Ammunition Law, considered an A-class weapon, which means a special exception is needed to possess them. This makes suppressors illegal for any practical purpose.
In Denmark, the Danish Weapons And Explosives Law makes the unlicensed possession of a suppressor illegal. A permit may be acquired from the local police, but permission is almost always denied. Only police and hunters with special permission for the emergency slaughtering of livestock inside buildings are allowed to use them.
In Finland, a firearm suppressor is classified as a firearm part by law. Purchasing a suppressor requires a firearm ownership permit, which is to be shown to the vendor at the moment of purchase.
In Germany suppressors are to be handled in the same way as the guns they are intended to be equipped with. That is, if a firearm requires a specific permit, the corresponding suppressor requires the identical permit as well. For example, suppressors for freely available airguns are also freely available. Suppressors are currently not specified exactly by means of a certain minimum attenuating level in dB which separates them from improved flash hiders or advanced muzzle brakes. Firearm suppressors require a "legal need" to own them, just like the firearms they are designed for, but it is nigh on impossible to legally prove that you need a suppressor, with the exception of large city and graveyard pest control.
In Hungary, the purchase or possession of a suppressor is prohibited for civilians.
Italy prohibited the purchase or possession of a suppressor except for military personnel until 2012. After September 2012, objects that reduce the sound of a firearms are recognised as civilian firearms part, and are legal to own and use with the exception of for hunting.
In Norway, suppressors can be bought by anyone.
In Poland, suppressors are not classified as "important weapon parts". Therefore, they are completely legal in all calibers, requiring no registration or permit. However using suppressors (even installing) with a firearm is prohibited. Only police and military are allowed to use them.
In Romania, buying or possessing suppressors is illegal for civilians. Only military and law enforcement personnel are authorized to use them.
In the Russian Federation, usage of firearm suppressors (legally defined as "devices for noiseless shooting") by civilians is prohibited, and the dealers are prohibited from selling them, but there is no penalty for purchasing or possession of such devices. Also the law lacks any straight definition of what a "device for noiseless shooting" is, or what decibel level is considered to be "noiseless", therefore it is completely up to the expert investigating the device whether it would be considered a "device for noiseless shooting" or not. That concerns not only specifically designed sound suppressors, but also such devices as muzzle compensators and flash suppressors. Sound moderators are very often used for airguns.
In Sweden, suppressors for specified calibers are legal for hunting purposes. A license is required, but is normally always granted.
In the United Kingdom, sales of suppressors fall into four categories of use. For replica and air guns, the purchase of a suppressor requires no license and in most cases, no identification requirement. For shotguns, these will probably require the presentation of the buyer's shotgun certificate but will not be recorded. If the shotgun is classified as a firearm (where capacity exceeds 3 cartridges) the firearm certificate (FAC) will need to show permission for the purchase of a suppressor. For a small- or full-bore rifle, the firearm certificate (FAC) will need to show permission for the purchase of a suppressor and also the gun for which it is intended. All firearms certificates have the firearm and caliber approved by the police and annotated to the document before a suppressor may be purchased. Police forces usually approve applications for a suppressor for hunting and target shooters, as the risks of litigation for personal injury, especially high-tone deafness resulting from shooting-induced hearing loss, are significant; and noise pollution in general is a problem for shooting sports.
In Canada, a device to muffle or stop the sound of a firearm is a "prohibited device" under the Criminal Code. A prohibited device is not inherently illegal in Canada but it does require an uncommon and very specific prohibited device license for its possession, use, and transport. Suppressors cannot be imported into the country by civilians; special licensing is required for businesses to import and sell suppressors, and they are typically only available to law enforcement, conservation agencies, and the military.
In the United States, taxes and strict regulations affect the manufacture and sale of suppressors under the National Firearms Act. They are legal for individuals to possess and use for lawful purposes in thirty-nine of the fifty states. However, a prospective user must go through an application process administered by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), which requires a Federal tax payment of USD $200 and a thorough criminal background check. The USD 200.00 buys a revenue stamp, which is the legal document allowing possession of a suppressor. The market for used suppressors in the U.S. is consequently very poor, which has driven innovations in the field (buyers want the height of technology, because they are basically "stuck" with the purchase). The following jurisdictions have explicitly banned any civilian from possessing a suppressor: California, Delaware, Hawaii, Illinois, Iowa, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, and the District of Columbia.
The federal legal requirements to manufacture a suppressor in the United States are enumerated in Title 26, Chapter 53 of the United States Code. The individual states and several municipalities also have their specific requirements.
Federal law also provides severe penalties for crimes of violence committed using firearms equipped with silencers: a minimum prison sentence of 30 years.
Suppressors are banned in all Australian states.
New Zealand has no restrictions on the manufacture, sale, possession, or use of suppressors.
While suppressors are also referred to as "silencers", the latter is misleading because no firearm can be made completely silent, as the term "silencer" implies. Functionally, a suppressor is meant to diminish the report of a discharged round, or make its sound unrecognizable. Other sounds emanating from the weapon remain unchanged. Even subsonic bullets make distinct sounds by their passage through the air and striking targets, and supersonic bullets produce a small sonic boom, resulting in a "ballistic crack". Semi- and fully automatic firearms also make distinct noises as their actions cycle, ejecting the fired cartridge case and loading a new round. Despite being misleading, the term "silencer" is still widespread.
Both the United States Department of Justice and the ATF (Bureau of Alcohol, Tobacco, Firearms and Explosive) refer to these devices as "silencers". Additionally, Hiram Percy Maxim (the original inventor of the device) marketed them as "Maxim Silencers."
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