Thus, a suppressor is a simple canister attached to the gun muzzle. Inside the canister are baffles, which make the gas swirl, producing less sound.

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The first sound moderator came to market in 1909, under the name “Maxim Silencer.” The name was marketing hyperbole, like calling a flannel shirt an “Arctic coat.” The inventor, Hiram Percy Maxim, also applied his noise-moderating system to automobile mufflers and other machines whose gas emissions create noise. Today, the company Maxim Silencers does not make firearms silencers, but it does produce noise reducers for many other applications.

Noise reduction made shooting more pleasant in the short run, and protected hearing in the long run. President Theodore Roosevelt put a Maxim Silencer on his 1894 Winchester lever-action rifle.

How much is the noise reduced? By up to 30 decibels, depending on the type of gun, ammunition and suppressor. Currently, gun control lobbies are claiming that if “silencers” are available, people will not be able to hear a mass shooting that is going on nearby. To test the claim, let’s consider last week’s attack on Republicans who were practicing baseball in Alexandria.

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The criminal used a SKS rifle, with 7.62mm ammunition. Without a suppressor, the sound of a shot from such a gun is 165 decibels. This is more than twice as loud as a jet take-off, if you are 25 meters from the jet. With a suppressor, the SKS would be about 140db. That’s equivalent to being on an active aircraft carrier deck.

The would-be assassin also had a Smith & Wesson 9mm handgun. In handguns, 9mm is an intermediate caliber — smaller and quieter than larger calibers such as .44 or .45 (inches). Without a suppressor, the S&W handgun is about 157 to 160 db. With a suppressor, that handgun would be around 127 to 130 db. That’s about the same as a jackhammer. Thus, the assertions that people will not be able to hear criminal gunfire are not well supported by physics, although the assertions are consistent with how “silencers” are portrayed in movies.

(The specific decibel levels for particular guns were supplied by Jeremy Mallette, director of social media for Silencer Shop, a company in Austin.)

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Advocacy for banning silencers. In the early 20th century, the most influential advocate for banning many firearms and accessories was William T. Hornaday, director of the Bronx Zoo. Using the resources of the Bronx Zoo and others for conservation, Hornaday helped save the American bison from extinction.

Hornaday’s 1913 book, “Our Vanishing Wildlife: Its Extermination and Preservation,” warned that over-hunting was wiping out American wildlife. According to Hornaday, one problem was that modern guns were too accurate. Also, hunters now had better scopes and binoculars. In Wyoming, hunters were using silencers so one shot didn’t frighten away other game.

Even worse, in Hornaday’s view, was who was hunting. Namely, lower-class Americans and immigrants. He urged new laws to “prohibit the use of firearms by any naturalized alien from southern Europe until after a 10-years’ residence in America.” Wildlife was vanishing because “the Italians are spreading, spreading, spreading. If you are without them to-day, to-morrow they will be around you. Meet them at the threshold with drastic laws, thoroughly enforced.”

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In the South, Hornaday argued, the problem was hunting by “poor white trash” and blacks. In an earlier time, black Americans “were too poor to own guns.” But “the time came when . . . single-shot breech-loading guns went down to five dollars a piece. The negro had money now, and the merchants . . . sold him the guns, a gun for every black idler, man and boy, in all the South.” Hornaday favored an Alabama proposal for an annual tax of at least $5 a year on every firearm, to prevent poor people from owning inexpensive guns.

Hornaday argued that all pump-action guns should be banned, as should all semi-automatics and all “silencers.” Some of Hornaday’s proposals did become law, in attenuated form. For example, states did not ban hunting by immigrant citizens, but they did enact laws against hunting or firearms possession by legal resident aliens. North Carolina enacted a statute (later repealed) that required a license for purchasing a pump action gun. (These laws are described in my article in the Harvard Journal on Legislation, Background Checks for Firearms Sales and Loans: Law, History, and Policy.)

The National Firearms Act of 1934. In 1934, Congress enacted the National Firearms Act, which used the tax power to set up a tax and registration system for certain arms and accessories. As enacted, the NFA applies to machine guns, short-barreled shotguns and rifles, “silencers,” grenades, mortars and various other devices.

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As introduced in Congress, the NFA also covered handguns, which prompted a tremendous debate. Once handguns were removed from the bill, the NFA passed with little opposition.

In the legislative history, there was no discussion of “silencers.” We simply have no idea what (if anything) Congress thought it was doing about them. See Stephen P. Halbrook, Firearm Sound Moderators: Issues of Criminalization and the Second Amendment, 46 Cumberland Law Rev. 33 (2016).

Pursuant to the NFA, purchasing a suppressor today requires a $200 tax. Before a person takes possession of a suppressor, the suppressor must go through a months-long registration process with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

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The Gun Control Act of 1968. As amended, the 1968 Act is the main federal law for ordinary firearms. For the GCA, suppressors are treated the same as ordinary firearms. Thus, the retail purchaser must fill out the dozens of questions in the four-page federal Form 4473, which includes questions about the purchaser’s race, and whether the purchaser is Hispanic. A false answer on the 4473 is punishable by five years in federal prison. 18 U.S. Code 924(a)(1).

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The Form 4473 functions as a registration system, since the dealer must retain the form. The dealer’s registration forms may be examined by law enforcement officials in the course of criminal investigations or during regulatory compliance inspections. 18 U.S. Code 923(g).

Once the 4473 has been completed, the retailer contacts the FBI or a state counterpart by Internet or by telephone. The law enforcement agency conducts a background check by comparing the buyer’s name to various lists of “prohibited persons.” These include persons with felony convictions, domestic violence misdemeanors, illegal aliens, dishonorable discharges from the military, and so on.

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Sometimes the “instant check” is completed in less than 20 minutes. Other times, the wait time to initiate the check can be hours or days. If the lawful buyer has the same name as a prohibited person, there may be delays of days or months.

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To repeat: A suppressor purchaser must go through the same procedures as an ordinary firearms buyer (Gun Control Act) and the same procedures as a machine gun buyer (National Firearms Act).

This is an unusually stringent pair of systems. Most firearms accessories (e.g., scopes) have no special rules for purchase. The default rule is to have controls on the firearm, and not to bother with extra rules for accessories.

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State laws. As long as a person complies with the NFA and the GCA, suppressor ownership is legal in almost all states. The exceptions are Hawaii, California, Illinois, New York, New Jersey, Delaware, Rhode Island and Massachusetts. In the 42 states where suppressors are legal, they are allowed for hunting in all but two (Connecticut and Vermont). (See this map from the American Suppressor Association.) The number of states that allow possession of suppressors, including for hunting, has grown in recent years, in part due to the lobbying of the NFA Freedom Alliance, a group that concentrates on items covered by the NFA.

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Why do people own suppressors? There are three main reasons: reduction of noise pollution, hearing protection, and safety training. As for the first, hunting sometimes take place in state or national forests or other locations near where people live. During hunting season, nearby residents may be annoyed by the frequent sound of gunfire. Likewise, some people have built houses near established target ranges; when people at the range use suppressors, the ambient noise is reduced, although certainly not eliminated.

In the 1950s, many shooters did not use hearing protection. As people have become more health conscious, they have recognized that gunfire can damage the inner ear. Accordingly, use of hearing protection when shooting has become standard and is mandatory at public ranges. Earmuffs reduce the felt decibel level by about 20 to 30 decibels, depending on the model. Today, best practices are to supplement over-the-ear muffs with foam inserts into the ear. By themselves, ear plugs are usually not as effective as earmuffs, but they do provide some additional protection.

Suppressors reduce noise by about as much as earmuffs do. No one would ever suggest that a suppressor is an acceptable replacement for muffs, but suppressors are a very good supplement to reduce the sound the reaches the inner ear. Using a suppressor + earmuffs + ear plugs can reduce the perceived sound to around 100 decibels, the same as a power lawn mower.

Finally, firearms safety instructors often prefer that their students use suppressors. First of all, suppressors reduce overall noise, which makes it easier for students to hear the instructor. Second, some new shooters flinch because of the sharp noise of a gun when it is fired. A suppressor can prevent a flinch from developing and thus help students progress more quickly to proper and safe shooting form.

How many people own suppressors? As of November 2006, the number of suppressors in the ATF’s registry was 150,364. By February 2016, the number had risen to 902,805. These numbers are not as precise as they might appear, since the ATF has acknowledged that its National Firearms Registration and Transfer Record registry (NFRTR) is riddled with errors; items that were properly registered with the ATF at some point may not appear on the current ATF registry. Regardless, there is no doubt that suppressors have become much more popular, especially with hunters, as CNN has reported.

What is the rate of crime with suppressors? A study of federal prosecutions for 1995-2005 , using Westlaw and Lexis, found 153 total “silencer” prosecutions. “[M]ore than 80 percent of federal silencer charges are for non-violent, victimless crimes.” Paul A. Clark, Criminal Use of Firearms Silencers, 8 Western Criminology Review 44 (2007). In other words, the possessor was a prohibited person (not allowed to possess firearms, ammunition or silencers), but the possessor was not misusing the item. In only 2 percent of the cases was the firearm discharged. The author noted that most prosecutions involved improvised, home-made silencers, rather than the commercially manufactured kind that can be purchased in gun stores. The article’s analysis of California cases from 2000-2005 found similar results. Thus, the misuse rate for lawfully purchased suppressors appears to be very low.

What are the laws in other countries? American suppressor law is anomalous, because suppressors are accessories yet are treated the same as firearms (Gun Control Act). On top of that, they are also treated like machine guns (National Firearms Act). As Halbrook’s article details, in European nations such as Finland, France, Germany, Italy and Britain, among others, an individual who is licensed to own a firearm is always allowed the appropriate suppressor. Many European guns are sold with suppressors already attached. The policy is that if a person is legally authorized to possess a firearm, then it is generally preferable for that firearm to have a suppressor.

What is the Hearing Protection Act? In the current Congress, the Hearing Protection Act (HPA) is H.R. 367 in the House (sponsored by Rep. Jeff Duncan, R-S.C.) and S, 59 in the Senate (sponsored by Sen. Mike Crapo, R-Idaho). The HPA retains all of the Gun Control Act’s provisions on suppressors. In other words, purchasing a suppressor would continue to be subject to all the rules that apply to purchasing or possessing an ordinary firearm.

The HPA removes suppressors from the National Firearms Act, which means buyers would not have to pay a $200 tax and would not have to go through a months-long federal registration process.