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Most suppressors work by allowing the pressurized gases in the barrel to slowly expand before they exit the muzzle of the firearm, resulting in a "phum" sound rather than a bang. The process is comparable to slowly opening a soda can and hearing a "ffft" sound rather than a "pop". Some suppressors are also structured like motorcycle mufflers in order for the sound waves to cancel themselves out as the reflect off of the sides of the casing. Such suppressors are inherently more difficult to produce since they require more precise cutting and assembly. For this reason, they are often bigger and are mounted on large-caliber rifles where the added sound suppression is needed most.

When mounted on pistols and submachine guns, the effective noise reduction of a military grade suppressor is astounding. On the Heckler & Koch MP5 and Mark 23, the weapon's noise is reduced to the point that the click of the action is louder than the muzzle burst. On sniper rifles, and large caliber assault rifles, the noise reduction is still immense, but a distinctive whistle is still present. Another important factor in sound signature suppression is the muzzle velocity of the ammunition. The MP5 and Mark 23 chamber 9 mm Luger and .45 ACP rounds, respectively, both of which are available with subsonic muzzle velocities. In large caliber assault rifles and precision rifles, the bullet itself produces a noticeable crackling sound as it travels through the surrounding air. For this reason, it is more difficult to effectively lower the sound signature of these firearms.

One solution is to lower the muzzle velocity of the cartridge. Some suppressor designs do this by allowing gas to bleed out of the barrel before the bullet exits, others do it with rubber "wipes" that use friction to slow the bullet. While this is effective, it dramatically reduces the range and stopping power of the cartridge. In addition, the wipes in velocity reducing suppressors that use them tend to adversely effect the accuracy, since they come in direct contact with the bullet after it leaves the barrel, and the wipes generally wear out and lose effectiveness after only a few shots.

Oftentimes, a subsonic cartridge may even be more powerful than a suppressed high-power weapon, because the bullet mass may be increased to make up for the energy lost by lowering the velocity. A good example of this is the .300 Whisper cartridge, which is formed from a shortened, necked up 5.56 x 45mm cartridge. The subsonic .300 Whisper fires up to a 240 grain, .30 caliber bullet at about 950 feet per second, generating about 480 ft·lbf of energy. By comparison, the standard SS109 62 grain bullet at 950 feet per second would generate only 125 ft·lbf of energy, only as much as a .22 Long Rifle cartridge. Since the .300 Whisper is based on the 5.56 x 45mm cartridge, firearms such as the M-16 rifle can be changed to .300 Whisper by simply changing the barrel.

Most suppressors can be removed by unscrewing them off the barrel, but others, such as suppressors that bleed off powder gasses to reduce the ammunition velocity, are built into the barrel and can only be removed by removing the barrel. Integral suppressors are also more robust than detatchable suppressors, because they attach to the barrel over much of the barrel's length, making them less susceptable to bending if subjected to torque, such as when the firearm is dropped. A detachable suppressor that is even slightly misaligned can come into contact with the bullet, which will significantly degrade the accuracy at best, and at worst can result in the suppressor being ripped off the firearm by the impact.

History

Diagram from Maxim's 1908 patent 916,885 on the suppressorEarly suppressors were created around the beginning of the 20th century by a number of inventors. American inventor Hiram Percy Maxim is credited with inventing and selling the first commercially successful models circa 1902. The suppressor was first introduced into the United States Army Air Force before World War II. Office of Strategic Services agents during World War II favored the newly-designed .22 caliber pistol. The addition of a sound suppressor baffle to the barrel absorbed 90 percent of the noise. William Donovan, Director of the OSS, demonstrated the pistol for President Roosevelt while visiting the White House. Donovan fired ten shots into a sandbag without interrupting the President as he dictated a letter.

Legal status

Legal regulation of suppressors varies widely around the world. In some nations, such as the United Kingdom or Finland, some or all types of suppressor are practically unregulated and may be bought "over the counter" in retail stores or by mail order, though in many such countries the firearms themselves are strictly controlled. Other nations, such as Canada, practically forbid their private citizens from owning suppressors, while yet others, such as the USA, tax and regulate their manufacture and sale. In the United States, it is legal for an individual to possess and use a suppressor, however the BATFE must approve the transfer; such transfers also require payment of a $200 tax and a significant background check. Some states go further, and explicitly ban any civilian possession of suppressors.

Despite common misconception that suppressors violate the laws of war, special forces have made use of suppressed firearms in warfare worldwide since their invention. One of the more famous, and most effective, suppressed firearms was the British De Lisle carbine developed in World War II. During the late 1950s the People's Liberation Army procured millions of domestically produced clones of the PPSh-41 submachine gun with integrated silencer for infantry use.

Suppressor design and construction

The suppressor is typically a cylindrical piece of machined metal that attaches to the muzzle of the pistol or rifle. Some others are designed as an integral part of the weapon, and may include an expansion chamber that partially surrounds the barrel (These are often called "telescoping" designs, a reference to old-fashioned collapsing telescopes). The suppressor reduces noise by allowing the rapidly expanding gasses from the detonation of the round to be briefly diverted or trapped inside a series of hollow chambers called baffles. Some advanced designs use baffles to shift the frequency of the remaining sound beyond the range of human hearing, further reducing noise. Other types, called "wet" suppressors or "wet cans", use a small quantity of water, oil, or grease in the first chamber to cool the powder gasses and reduce the volume (See ideal gas law). The coolant lasts only a few shots before it must be replenished, but while it lasts it can greatly increase the effectiveness of the suppressor. One manufacturer claims a 30% improvement in sound suppression for "4 magazines" (32 to 68 rounds) with the addition of 5 ml. of water or light oil to their suppressor. Water is most effective, due it its high heat of vaporization, but it can leak 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 preferable, as well), as it leaks like water and is as messy as grease, leaving behind a fine mist of condensed oil after each shot.

Suppressors vary greatly in size and efficiency. A type developed in the 1980s by the US Navy for 9 mm pistols is 150 x 45 mm (5.9 x 1.77 in) and is good for six shots with standard ammunition or up to thirty with low-powered, subsonic ammunition. The British Sterling suppressor is 350 mm (13.78 in) long and 75 mm (2.95 in) in diameter and will work effectively for hundreds of shots with standard ammunition. Generally the longevity of a suppressor is based on the material used for the baffles. Rubber baffles that are smaller than the bullet diameter are best,as they trap the most gas, but they wear quickly and lose effectiveness. Steel or aluminum baffles last far longer, but are less efficient to begin with.

Suppressors can be improvised with any baffling material (Pillow, potato, plastic bottle, etc); These are only marginally useful, and often dangerous to the user. In areas where suppressors are regulated, improvised suppressors may also be illegal, whether they actually reduce the sound of the firearm or not. This law is controversial, however, since improvised suppressors are usually regular household items.

Ammunition for use with suppressors

Suppressors are most effective when the bullet's velocity does not exceed the speed of sound. A bullet that breaks the sound barrier creates loud flight noise, or a "sonic boom". As velocity increases further beyond 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.

Firearms for use with suppressors

HK USP 45 with a SuppressorThe type of gun also affects suppressor efficiency. Guns with the least 'leakage' are best, so a sealed breech (e.g. bolt 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 autoloading firearms still produce a significant amount of noise from the gun cycling (A video is available here) and the leak of high velocity gas from the breech. Revolvers, due to their 'loose' structure, cannot be made quiet, with few exceptions: The Nagant M1895 revolver, used an unusual gas-sealed cylinder that made it suitable for use with a suppressor.

While it seems that any semiautomatic pistol could be fitted with a suppressor, it's not as easy as just threading the barrel and screwing one on. Most semiautomatic pistols in larger calibers, 9 x 19 mm Luger and larger, use a short recoil action. This means that the slide and barrel both recoil backwards for a short distance before the slide unlocks from the barrel, allowing the round to be ejected. This is done to keep the breech sealed until the chamber pressure drops to safe levels. Adding the mass of a suppressor to the mass of the recoiling parts means that it will significantly alter the operation of the gun; in most cases, it stops the slide from unlocking at all, and effectively turns the semiautomatic pistol into a bolt action. This is not always a bad thing, however, as the sound of the action cycling is often louder than the suppressed shot. In addition to this, nearly all short recoil designs are based on the John Browning designed tilting barrel lockup, as used in the M1911. This system uses a tilting barrel, which means that in addition adding mass, the suppressor also adds rotational inertia, greatly resisting the force that tilts the barrel. Because of the high pressures and close tolerances required, the suppressor cannot be allowed to bend at the joint, or the bullet would hit the baffles rather than passing through the middle. Special mechanisms, called "recoil enhancers" or "Nielsen devices", are used to decouple the mass of the suppressor from the barrel. These consist of a sliding baffle 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 one of these mechanisms increases the complexity and cost of the suppressor, but perhaps also efficiency.

Because of the difficulties of suppressing short action designs, suppressors are easiest to add to smaller caliber pistols. .380 ACP and .22 Long Rifle are both usually blowback designs with fixed barrels, and are easy to suppress. The most commonly suppressed firearms are .22 Long Rifle semiautomatic pistols and rifles; suppressing the firearms allows them to be fired without use of hearing protection. Subsonic rounds are readily available in .22 Long Rifle, and even with supersonic rounds the crack of firing is not uncomfortable. The small powder charges of the .22 Long Rifle keeps suppressors small; many, like those available for the Ruger 10/22, are no larger than a heavy barrel.

Specially designed firearms with integral suppressors 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. .45 ACP is an ideal choice, since the standard 230 grain (15 g) loading is both powerful and subsonic. Special cartridges are also available designed for use with suppressors. The .300 Whisper is probably the most common of these. It is made by cutting a 5.56 x 45 mm NATO cartridge down to a shorter length, then increasing the neck to accommodate a .30 caliber (7.62 mm) bullet. It is loaded with a heavy bullet, over 200 grains (13 g), at subsonic velocities. This gives it the muzzle energy of the .45 ACP with a far better ballistic coefficient (see external ballistics) for better long range performance. The use of the 5.56 mm NATO as a base case means that it can be chambered in M16 derived rifles with a simple barrel change. In addition to special applications in suppressed firearms, the .300 Whisper has become popular with metallic silhouette shooters due to its low recoil, good long range performance, and very high accuracy.

While suppressors are most accurate with subsonic cartridges, they can be used effectively with supersonic cartridges. The crack of the supersonic bullet cannot be avoided, but the suppressor will reduce the sound of the muzzle blast, and make it more difficult to locate the source of the shot by sound. Suppressors are most effective to the side and rear of the shooter, so a suppressor could be used by a sniper effectively. Observers not in the target area would hear the least, and the crack of the arriving bullet will tend to obscure the location from the area of the target.

An exhaust pipe is usually a pipe used to guide waste exhaust gases away from a controlled combustion inside an engine or stove.

An exhaust pipe must be carefully designed to carry toxic and/or noxious gases away from the users of the machine. Indoor generators and furnaces can quickly fill an enclosed space with carbon monoxide or other poisonous exhaust if they are not properly vented to the outdoors. Also, the exhaust gases from most types of machine are very hot; the pipe must be heat-resistant, and it must not pass through or near any combustible materials.

An automobile's exhaust system usually connects to the exhaust manifold and usually includes a muffler (British English: silencer) to reduce engine noise, and often in recent years a catalytic converter to reduce the emissions that contribute to air pollution.

In a two-stroke engine, such as that used on dirt bikes, a bulge in the exhaust pipe known as an expansion chamber uses the pressure of the exhaust to create a pump that squeezes more air and fuel into the cylinder during the intake stroke. This provides greater power and fuel efficiency.