A suppressor, sound moderator, or silencer is a device either attached to or part of the barrel of a firearm to reduce the amount of noise and flash generated by firing the weapon. It generally takes the form of a cylindrically-shaped metal tube with various internal mechanisms to reduce the sound of firing by slowing the escaping propellant gas, and sometimes by reducing the velocity of the bullet. Suppressors are also popularly known as silencers, though no suppressor completely eliminates the noise of discharging a firearm.
Early suppressors were created around the beginning of the 20th century by several inventors. American inventor Hiram Maxim is credited with inventing and selling the first commercially successful models circa 1902. Maxim gave his device the trademarked name Maxim Silencer. Today all rights to the Maxim name, rights to produce under it, and an archive of historical documents are the property of U.S. Fire Arms Manufacturing Company of Hartford, CT. 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. Indeed, in many European countries, automobile mufflers are still referred to as "silencers." The term silencer has since fallen out of favor among the firearms industry, being replaced with the more accurate term sound suppressor or just suppressor. 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 American Office of Strategic Services (OSS) agents during World War II, who favored the newly-designed High Standard HDM .22 Long Rifle pistol. "Wild Bill" Donovan, director of the OSS, demonstrated the pistol for President 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 letter, 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 suppressor is typically a hollow cylindrical piece of machined metal (usually steel or aluminum) containing expansion chambers that attaches to the muzzle of a pistol, submachinegun 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 bullet 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 long and 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 long and in diameter.
Baffles are 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, aluminum, 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 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. Aluminum 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.
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 to minimize disassembly and parts replacement.
"Wet" suppressors or "wet cans" use a small quantity of water, oil, grease or water-based wire-pulling lubricant 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. Water-based wire-pulling lubricant gel is more convenient as it does not run or drip. 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.
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. However, steel wool degrades very rapidly, usually within ten shots (stainless-steel wool is a better choice), while metal mesh may last for hundreds or thousands of shots of semi-automatic fire, or significantly less for full-automatic fire. However, 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 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. The intended effect of frequency shifting is to shift audible sound waves frequencies into ultrasound (above 20 kHz), beyond the range of human hearing. Phase cancellation occurs when similar sound wave frequencies encounter each other 180° out of phase, canceling the amplitude of the wave and eliminating the pressure variations perceived as sound.
Utilizing either effect to an advantage requires that the suppressor be designed with specific properties of the muzzle blast in mind. For example, the velocity of the sound waves are 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.
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 utilize phase cancellation in their designs, but these claims are generally unsupported from a scientific perspective.
The portrayal of silenced firearms in movies 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. Some films even depict silenced .50 caliber sniper rifles making only a barely audible sound. In reality this is not always accurate. Depending on the suppressor and the surrounding conditions (such as background noise, echoes from nearby structures or terrain, etc.), the decreased noise may still be heard for some distance. The "quiet whistling sound" commonly seen in the movies is actually more comparable with the noise made by firing a suppressed air gun. However, and as described below, it is possible to render a firearm near-silent under special circumstances.
When mounted on pistols and submachine guns with subsonic (slower than the speed of sound) ammunition, a good suppressor can reduce the sound to a loud clacking noise, roughly comparable to a staple gun. Often the sound of the gun's bolt cycling is louder than the actual report. On centerfire rifles, the noise reduction is significant enough to permit safe shooting without hearing protection ("hearing safe"); however, the noise of firing is still loud enough to be heard for hundreds of meters. Also significant is the alteration of the firing sound to something that is not identifiable as a gun shot, reducing or eliminating attention drawn to the shooter (hence the Finnish expression: "A silencer 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.
Another important factor in sound signature suppression is the muzzle velocity of the ammunition. In weapons firing supersonic bullets, most often rifles, the supersonic bullet itself produces a loud and very sharp sound (a tiny sonic boom) as it travels downrange, referred to as "ballistic crack". For this reason, it is more difficult to reduce the total sound signature of these firearms effectively. Subsonic ammunition reduces sound report, but at the cost of lower velocity, often resulting in decreased range and effectiveness on the target. Military marksmen and police counter-terror units may use this ammunition to maximize the effectiveness of their silenced rifles. While this significantly decreases the effective range of the rifles, this may be acceptable for specialized situations, where the absolute minimum amount of noise is required.
Another solution is to lower the muzzle velocity of a supersonic bullet before it leaves the barrel. Some suppressor designs do this by allowing gas to bleed off along the length of the barrel before the projectile exits; others contain wipes that use friction to slow the bullet before exiting. However, wipes generally wear out and lose effectiveness after relatively few shots, and the bleed-off designs require periodic cleaning.
A suppressor also cools the hot gases coming out of the barrel enough that most of the lead 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.
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. By reducing noise, recoil and muzzle-blast, it 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.
There are many benefits with suppressors on military rifles. Suppressors can increase the precision of a rifle, as they strip away hot gases from around the projectile in a uniform fashion. The suppressor can reduce the recoil significantly as it traps the escaping gas. This gas mass is a little less than one-half the projectile mass (approximately 1.6 grams vs 4 grams for NATO 5.56x45 mm ammunition), with the gas exiting the muzzle at about twice the projectile's velocity, thus giving a reduction in the felt recoil of approximately 15%. The added weight of the suppressor - normally 300 to 500 grams - also contributes to the reduction of the recoil, though a significantly heavy suppressor would unbalance a weapon. Further, the pressure against the face of each baffle is higher than the pressure on its reverse side, making each baffle a miniature "hydraulic ram" which pulls the suppressor forward on the weapon, which can contribute an immense force to counter recoil.
Some manufacturer data suggests that suppressors can reduce a rifle's recoil by over 60%. The suppressor also has the often-neglected benefit of reducing muzzle flash by as much as 90%. This is very important as combat frequently takes place at night, and soldiers are commonly trained to identify and shoot at enemy muzzle flashes. Silencers are also useful for target shooting, as the reduced noise can prevent hearing damage to the user of a firearm, and prevents complaints about noise from neighbors when using an outdoor range.
Suppressors are most effective when the bullet's velocity does not exceed the speed of sound. At sea level, at an ambient temperature of 70 °F (21 °C) and under normal atmospheric conditions, the speed of sound is approximately 1100 feet per second (340 m/s). A bullet that breaks the sound barrier creates a sonic boom. For any further increase in velocity higher than the speed of sound, flight noise does not increase significantly. Supersonic flight noise may be reduced somewhat by using a projectile of smaller caliber. 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 specifically to maximize the energy available when used 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 Fireball cartridge case. The subsonic .300 Whisper fires up to a 250 grain (16.2 g), .30 caliber bullet at about 980 feet 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.
The type of firearm to be suppressed also affects suppressor efficiency. Guns with the least gas leakage are best, so a sealed breech (e.g. bolt action or lever action) is preferable and can be suppressed to the point that the click as the striker or hammer falls is the loudest sound of firing. Most semiautomatic and fully-automatic firearms still produce a significant amount of noise from the gun cycling and the leak of high-velocity gas from the breech. Revolvers, due to the gap between the cylinder and barrel, cannot be made quiet. There are however, a few exceptions: The Nagant M1895 revolver uses an unusual cylinder that moves forward upon firing, and a special extended cartridge case which seals the gap between cylinder and barrel, making it suitable for use with a suppressor.
While it seems that any semi-automatic pistol can be fitted with a suppressor, it is not as simple as threading the barrel and installing a suppressor. Most semiautomatic pistols of 9 mm Luger caliber or larger use a short recoil action. In this system, the slide and barrel both recoil for a short distance before the slide unlocks from the barrel and opens the breech. This keeps the breech sealed until the chamber pressure drops to a safe level. Adding the mass of a suppressor to the barrel/slide combination will significantly alter the operation of the gun; in most cases, the added mass stops the slide from unlocking at all, and effectively turns the semiautomatic pistol into a single-shot weapon. This is not always undesirable, as the sound of the action cycling is often louder than the suppressed report. Nearly all short recoil designs are based on the John Browning-designed tilting barrel lockup, as used in the M1911, Browning Hi-Power and Glock pistols. This system uses a tilting barrel, which means that in addition to adding mass, the suppressor also adds rotational inertia, greatly resisting the force that tilts the barrel. Special mechanisms, called recoil boosters or Nielsen devices (shown in the photo gallery below), are used to decouple the mass of the suppressor from the barrel. These devices consist of a sliding piston in the rear of the suppressor that is forced back under the pressure of the powder gas, thus forcing the barrel backwards and unlocking the short recoil mechanism. Adding a recoil booster increases the complexity and cost of the suppressor, but enhances its ability to function in the semiautomatic mode. Many companies include an indexing system in the design of the Nielsen device which allows the suppressor to be oriented in a number of different longitudinal positions. This allows the user to fine-tune the weapon's point of aim; typically the user selects the setting which minimizes the impact shift between the suppressed and unsuppressed states.
Due to the difficulties of suppressing short recoil designs, suppressors are easier to add to smaller-caliber pistols, such as those chambered in .380 ACP, .32 ACP and .22 Long Rifle (.22 LR). Pistols using these cartridges are usually blowback designs with fixed barrels, which are easier to suppress. The most commonly suppressed firearms are .22 LR semiautomatic pistols and rifles, which allows them to be fired without the use of hearing protection, even with supersonic rounds. Specially-designed firearms with integral suppressors (e.g. the Welrod or De Lisle Carbine) provide the best overall result, as the suppressor can be fully telescoped to reduce the overall length of the gun, and the caliber can be chosen for maximum performance with the suppressor. The .45 ACP is an excellent choice, since the standard 230 grain (15 g) loading is both powerful and subsonic.
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 these same countries, however, the firearms themselves are strictly (by US standards) controlled.
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.
The United States taxes and strictly regulates 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-eight of the fifty states. However, a prospective user must go through an application process administered by the Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE), which requires a Federal tax payment of US$200.00 and a thorough criminal background check. The market for used suppressors in the US 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). Primitive suppressors are available in other countries for under US$40, but they are usually of crude construction, using cheap materials and baffle designs that were obsolete in the United States by the 1970s. While suppressors in the US are more expensive (hundreds to thousands of dollars), they are generally built with highly advanced baffle stacks and exotic materials like Inconel and high-grade heat-treated stainless steels. Several states and municipalities explicitly ban any civilian possession of suppressors.
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.
In the United Kingdom, sales of suppressors fall into four categories of use. For replica and air weapons, 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. 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 silencer may be purchased. Police forces usually approve applications for a silencer 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 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 Sweden, suppressors for specified calibers are legal for hunting purposes. A license is required, but is normally always granted.
In Austria, the purchase or possession of a suppressor is prohibited according to §17 of the Austrian Weapons Law.
Italy prohibits the purchase or possession of a suppressor except for military personnel.
New Zealand does not require permits for the manufacture, sale, possession, or use of a suppressor. In Turkey, civilian purchase, sale or possession of suppressors are strictly prohibited, with possible jail terms of up to 25 years for 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.
Many suppressor users prefer the term "suppressor" to "silencer", as no firearm suppressor is truly silent. Others believe that "suppressor" is more politically correct, and does not carry the same "hitmen and mobsters" stigma that the general public has applied to "silencers".
Functionally, a suppressor is not meant to completely silence a firearm, but to make its sound unrecognizable. 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". In addition, the sounds may reflect off adjacent structures or terrain. Semi-automatic and automatic firearms also make distinct noises as their actions cycle, ejecting the fired cartridge case and loading a new round.
The ideal suppressed weapon would therefore be either a single-shot or a manually-operated repeating firearm such as a bolt-action rifle (see Firearm types above). Effective suppressors either use a large total suppressor volume, or a moderately large volume plus many baffles, or wipes. It is possible to design a very small and compact suppressor with wipes which effectively silences a pistol; these suppressors have a lifetime of as few as five shots and typically no more than a few magazines of ammunition. Most suppressor designs trade reduced total volume and weight for somewhat louder noise, which is still tactically useful. The optimum point for any particular design depends on the suppressor's intended use.
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