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Diagrams of guns in a word-bubble answer to a question. Javier Zarracina/Vox

Caliber, cartridges, and bump stocks: guns, explained for non-gun people

As new gun control plans fly, here’s a guide to the weapons at the heart of the debate.

This past Saturday brought yet another mass shooting in the United States, yet another push for aggressive gun control from presidential candidates and the public alike, and reminders from President Donald Trump and Sen. Ted Cruz (R-TX) that they will resist efforts to implement new background checks or gun limitations.

Gun control supporters are getting more ambitious in their rhetoric; presidential candidate and former Texas Rep. Beto O’Rourke told a reporter that under his plan, if you own an AK-47 or AR-15, “you’ll have to sell them to the government.” That promises new debates about banning “assault weapons,” and with them discussions about what an “assault weapon” even is.

New attempts at gun control often prompt a specific frustration among gun rights supporters: that their opponents in this debate just don’t get how guns work.

They think it’s ridiculous that former Rep. Carolyn McCarthy (D-NY), who lost her husband in a mass shooting in 1993 and became a vocal advocate for gun regulation, didn’t know what a barrel shroud was, despite wanting to ban them. They rolled their eyes when then-California state Sen. Kevin de León described a gun as having a “30-caliber clip” when he meant a 30-round magazine, and when Rep. Diana DeGette (D-CO) appeared to believe that gun magazines can’t be reloaded.

If these are the people who want to restrict guns, firearms advocates argue, why in the world should we trust them?

I don’t totally buy this argument — for instance, I think you can have an opinion on the death penalty without knowing exactly how lethal injections work — but I understand the frustration behind it.

More importantly, I do worry that ignorance about the particulars of guns can lead to bad, or insufficient, policy. The 1994 federal assault weapons ban appears to have done some good in preventing drug-related violence in Mexico, but there’s not much evidence suggesting it saved many lives in America.

Many of the details of the ban were somewhat arbitrary, based on factors like whether the weapon has a mount for attaching a bayonet. These factors say more about whether a weapon looks especially dangerous than whether or not it is especially dangerous. Even defenders of the law tend to focus on more substantive provisions it included as opposed to the weapons ban itself, like a ban on magazines holding more than 10 rounds.

With that in mind, here’s an overview of the basics of how common guns work, what role each plays in America’s gun violence problem, and how different gun regulations would affect them.

Ammunition, explained

From left, three pistol cartridges, three rifle cartridges, and shotgun shells.

In the 18th century, infantry in wars such as the American Revolution used muzzle-loading, smoothbore muskets: The barrels of the guns were smooth, and you jammed a large, round piece of ammunition down them with gunpowder, added more powder at the start of the barrel, and ignited the powder using a flintlock mechanism, as illustrated in this video (and explained in more detail by HowStuffWorks):

Muskets had a lot of problems: They were slow to load, and the bullet would bounce around in the barrel, causing it to fly off in a random direction, which in turn meant the weapons were very difficult to aim.

By the mid-19th century, muskets were replaced by rifles, whose major innovation was, well, rifling: The carving of grooves in the barrel of the gun that caused the bullet to spin in a precise way, making the bullet more stable and accurate. Gunmakers had been doing that for hundreds of years, but it was expensive to do the necessary metalwork and the weapons were slow to load, so in the American Revolution, rifles were typically reserved for sharpshooters.

Now they’re standard, and handguns, like pistols and revolvers, feature rifling too. These days, most shotguns still use a smoothbore barrel, because they’re intended to shoot from a shorter distance. Further, shotgun shells, the most common kind of shotgun ammunition, contain many small metal pellets called “shot,” which mitigates the reduction in accuracy by covering a larger area than a single bullet could.

A diagram showing the location of gun powder and shot in a shotgun shell.
Gunpowder (left) and shot in a shotgun cartridge, as depicted in a 1908 illustration.
Chambers’s Twentieth Century Dictionary of the English Language

(Shotguns can also fire single “slugs,” which are large metal projectiles, instead of shells containing lots of tiny pellets.)

Meanwhile, rifles and handguns use cartridges, metal containers topped with a small lead bullet poking out and containing a propellant (like gunpowder) and a primer (to be set off by the gun’s firing pin) that can ignite the gun powder and send the bullet down the barrel. The bullet itself is sized to be a tiny bit wider than the barrel, ensuring that the rifling on the sides will cut into it and send it on a straight path:

A diagram showing components of cartridges Javier Zarracina/Vox; Washington Hunter Ed Course

Cartridges are usually distinguished by the diameter of their bullets, or of the barrels of guns for which the bullets are intended; this is known as the “caliber.” Caliber is usually expressed either in metric (as with 9mm handgun cartridges) or imperial measurements (.45 automatic Colt pistol cartridges have bullets about 0.45 inches wide). It’s often used to refer both to the ammo (“a .45 caliber cartridge”) and the gun (“a .45 caliber pistol”).

The force of a bullet is a function of both how fast it’s going and how big the projectile is, so generally speaking, bigger bullets do more damage and have more “stopping power” (the firearms term for a bullet’s ability to incapacitate the person or animal it’s targeting).

Rifle cartridges tend to hold more powder and have the benefit of longer barrels than handgun cartridges, and the longer a bullet is traveling down the barrel, the more speed it can build up due to the burning of the propellant. That means rifles tend to fire dramatically faster bullets than handguns do, which can cause them to do more damage as well as to go through targets, potentially hitting things or people the shooter didn’t intend to hit.

Gun control debates about ammo generally revolve around either a) limitations on specific types of ammunition, in particular hollow-point bullets and armor-piercing bullets; or b) limitations on how much ammunition a firearm can hold at once, so as to slow down the rate of fire in mass shootings.

Hollow-point bullets, before and after firing.
Michael E. Cumpston

Hollow-point bullets are meant to expand on impact, so named because they’re typically shaped to have a pit toward their tip. They are less likely to “over-penetrate” by going through a target and onto other objects or people, but can cause more internal damage in the target they hit. Only New Jersey and San Francisco restrict the use of hollow-point bullets, and New Jersey has numerous exceptions for target shooting, hunting, and the like.

Armor-piercing rounds generally either have a hard metal bullet made of a material like steel that’s covered in a softer metal like copper, to better penetrate the target, or have a weighty hard metal jacket surrounding a soft lead bullet.

Under federal law, armor-piercing ammunition has been illegal for handguns since 1986, but not for rifles; the Bureau of Alcohol, Tobacco, Firearms, and Explosives during the Obama administration attempted to classify certain currently legal rounds as armor-piercing, only to back down after strenuous objections from gun rights advocates. Twenty states and Washington, DC, have their own bans, including some unlikely ones such as Texas and Mississippi; the association of armor-piercing bullets with crimes against law enforcement (the main group in the US wearing Kevlar vests of the kind armor-penetrating rounds can cut through) scrambles the politics of the issue a bit.

Finally, there’s the magazine. The terms “clip” and “magazine” are often used interchangeably, which annoys gun enthusiasts, as they’re different devices. Magazine refers to a part of a gun holding ammunition before it’s ready to be fired; it can be detachable, as on AR-15s and many semiautomatic pistols, or it can be internal, as in many hunting rifles. Clips are simple devices that connect together rounds of ammunition, so they’re easier to load.

Rounds loaded into a Zastava M48 bolt-action rifle are connected by a clip.

A Zastava M48 Yugoslavian bolt-action rifle being loaded using a clip.

A Heckler & Koch P9 pistol, on the other hand, has a detachable magazine, which is a component of the gun itself.

Photo of a Heckler & Koch P9 pistol with magazine detached. Hecklerfan

Between 1994 and 2004, the federal assault weapons ban prohibited the manufacture, import, and possession of “large capacity ammunition feeding device[s],” which effectively banned magazines capable of holding more than 10 rounds. Such devices produced before the ban took effect remained legal. Nine states (California, Colorado, Connecticut, Hawaii, Maryland, Massachusetts, New Jersey, New York, and Vermont) and DC ban high-capacity magazines; the cap in Colorado (and in Vermont for long guns) is 15 rounds, and in all other states is 10.

Research on whether such bans save lives is sparse. The share of crimes involving high-capacity magazines in Virginia fell when large magazines were federally banned, and then rebounded, according to an analysis by the Washington Post, but that doesn’t necessarily mean the ban reduced the lethality of crimes during that period. A 2017 study found that the federal ban’s expiration increased the share of crimes committed with high-capacity semiautomatic weapons, but again, it doesn’t necessarily follow that the ban saved lives. Perhaps the same crimes would have taken place with different weapons; perhaps those weapons would have been less lethal, but it’s hard to know.

A CNN analysis conducted by Boston University public health researcher Michael Siegel found that states with bans on high-capacity magazines had 63 percent lower rates of mass shootings, and a New York Times poll of gun experts found that it was tied for first as the policy most experts thought would reduce mass shooting deaths. But there’s a lot of research left to be done on what exactly these bans do.

Rifles, explained

Chart of gun sales in America by category; rifles and pistols have dominated over revolvers and shotguns; the gap has widened in the past 10 years. Small Arms Survey, 2018

Some years in the US, people buy more pistols than rifles; other years, they buy more rifles than pistols; but usually, as the above chart demonstrates, it’s one of the two, and other categories like shotguns and revolvers are left far behind.

These large categories can often obscure important changes within them, like the shift toward semiautomatic weapons, which fire a new round with every trigger pull without requiring the shooter to take any other steps. In the United States, gun researcher Aaron Karp writes for the Small Arms Survey (an international organization compiling data on gun ownership worldwide), there have been “dramatic shifts in public gun-purchasing patterns in the past decade, as pistols and semi-automatic rifles became increasingly dominant, influenced by the expiry of the Federal Assault Weapons Ban in 2004 and changing consumer preferences.”

A survey conducted by the National Shooting Sports Foundation, an industry group advocating for gun manufacturers, suggested that in 2012, 20.3 percent of guns sold were “modern sporting rifles” — NSSF’s terminology for semiautomatic rifles like the AR-15 — while only 14 percent were “traditionally styled” rifles, and 13 percent were shotguns, making semiautos easily the most popular kind of long gun. NSSF is hardly a disinterested party, but nonetheless this is one of the few glimpses at the details of the gun market the public has.

AR-15-style rifles have become infamous among gun opponents because of their use in some high-profile mass shootings, but they’re also arguably the most popular gun variety in America. AR-15 is not a specific model — it gets its name from ArmaLite, the company that originally manufactured the rifle, and while Colt still produces a line of semiautomatic rifles called AR-15s, the gun is no longer patented, and most major gun companies produce variants on it, including the SIG Sauer SIG516, the Smith & Wesson M&P15, and the Bushmaster XM-15 (used in the Beltway sniper attacks and the Sandy Hook shooting).

Fans of AR-15s sometimes call them “grown-up Legos” due to their huge degree of customizability. They’re a modular design; you can sub out the barrel and replace it with a new one, and same for the stock (the part of the gun you push against your shoulder), the sight, the grip, the magazine, and on and on. Wired’s Andy Greenberg has a useful video showing how he built a “ghost gun,” a homemade AR-15 with no serial number that you can build without going through any background checks or other legal hurdles.

As Greenberg explains, AR-15s can be broken down so completely that the part of the AR-15 that’s legally a firearm and regulated as such is the lower receiver, the central body of the gun. In the below image, the lower receiver is the part connected to the pistol grip, while the upper receiver is the detached portion above it connected to the barrel of the gun. In a fully disassembled AR-15, the lower receiver would be detached from both the pistol grip and the stock.

Photo of an AR-15, with upper and lower receivers disconnected. Steve Rainwater

AR-15-style guns are semiautomatic: Each trigger pull fires a new round. But historically, semiauto rifles have been less popular for hunting and sporting purposes than rifles that require additional action by the shooter to manually eject fired cartridges and reload new ones.

Those rifles took a number of forms. The simplest are bolt-action rifles, which are still popular for hunting and remain the gold standard for sniper rifles in the military. In a Remington 700, for instance, the gun can hold three cartridges in its internal magazine (you couldn’t detach it and stick in a new one, the way you can with an AR-15). The top one rests in the chamber, ready to be fired. The bolt is the part of the rifle behind the chamber, closing off that end of the barrel. With a bolt-action rifle, after firing a round, you unhook the bolt, pull it back (which ejects the spent cartridge and lets a new one come up from the magazine), and then push it back to seal the barrel up again.

The video below is a useful animation of the process:

There are other methods rifles can use to eject spent cartridges and load new ones for firing. There are lever-action rifles, where that process happens through pulling a lever on the bottom of the rifle; and pump-action rifles where pulling a pump does the same.

In semiautomatic rifles, some force has to accomplish what other rifles make the user do manually. That’s typically done by redirecting the gases created by the explosion of the cartridge propellant. If you’re interested in the details, at minute 3:00 of the video below, the animation shows how this process works in “direct impingement,” one of the two main gas operation methods used in AR-15s and other semiautomatic rifles:

All else being equal, bolt-action rifles tend to have longer range and be more accurate, because semiautomatic weapons have more moving parts and more moments of recoil (because the gun is directing gas back at the shooter), and use up some propellant to load the next cartridge rather than concentrating it on firing the current bullet. But bolt-actions fire considerably more slowly.

Bushmaster has estimated that the XM-15, its variant on the AR-15, can fire 45 rounds per minute; Wired’s Greenberg puts the number closer to 80 to 100. A shooter using a 10- or 30-round magazine might shoot fewer due to the time spent reloading. Magazines holding 50, 60, or even 100 rounds are also legally available; the gunman in the Aurora, Colorado, mass shooting in 2012 used a 100-round magazine.

The Bushmaster number, though, is much lower than the rate of fire that can be attained in short bursts. While typical users might not be able to fire more than 45 rounds a minute, they can go much faster for short periods. According to analysis by the New York Times, in the Orlando Pulse shooting, the gunman used a semiautomatic Sig Sauer MCX rifle and fired 24 shots in nine seconds, for a rate of fire of 160 rounds per minute.

For comparison, a fully automatic M16, the military equivalent of an AR-15, can fire 98 shots in seven seconds, or 840 rounds per minute. On the other extreme, British forces in World War I were expected to fire “15 aimed rounds a minute” from their Lee-Enfield bolt-action rifles with 10-round magazines, and “up to double that number [with] rapid fire.” Even at the high end of 30 rounds a minute, that’s significantly less than a semiautomatic can fire, and bolt-actions without detachable magazines and with small magazines are even slower.

If someone in the US wants to fire faster than a normal AR-15, there are a few options. You can buy a pre-1986 machine gun; it’s been illegal to manufacture new fully automatic weapons since then, but ones built before 1986 are legal to own. Fully automatic weapons are regulated under the National Firearms Act of 1934 and must be registered with the federal government; purchases have to be cleared through the ATF, and applying for approval is costly, cumbersome, and time-consuming.

Also, machine guns are ridiculously expensive; at, dealers are selling an Uzi with silencer for $15,000, a full-auto AK-47 for $30,000, and a couple of FAL G1s (formerly a common infantry rifle in Western Europe) for $8,000 a pop. In other words, they’re an order of magnitude costlier than semiautomatic weapons.

It’s possible to modify a semiautomatic weapon to fire automatically, but doing so requires either a conversion kit (which are regulated as machine guns under the National Firearms Act) or alterations that are illegal to perform.

There is, however, one currently legal way to greatly increase a semiautomatic weapon’s rate of fire to near-automatic speeds. You can buy a gadget known as a “Gat crank,” which turns AR-15s into crank-operated firearms like the Gatling gun, one of the first rapid-firing weapons used by the US military.

Each turn of the crank fires three rounds, enabling an extremely rapid rate of fire. The shooter above records a rate of 681 rounds per minute, nearly as fast as a fully automatic M16.

Until March 2019, bump stocks offered another option for legal rapid fire. The gunman in the 2017 Las Vegas shooting used these modifications, which replace the stock (the part of the gun abutting the shoulder) with a moving component that, driven by recoil, leads the trigger to be pulled very, very rapidly, for a much higher rate of fire:

The Las Vegas gunman fired 90 shots in 10 seconds, for a rate of fire of 540 rounds per minute. That’s much faster than the 160 rounds per minute of the Orlando shooter, who was using a conventional semiautomatic, and closer to the 840 rounds per minute one can attain with a fully automatic M16 rifle.

However, President Trump’s Department of Justice enacted a regulation classifying bump stocks as machine guns, subject to the NFA, effective in March 2019. Even before the federal ban, several states had banned bump stocks since the Las Vegas shooting, and the largest manufacturer of bump stocks has stopped making them.

Other than the bump stock ban and restrictions on fully automatic weapons, regulations targeting rifles usually take the form of assault weapons bans. How an assault weapon is defined varies widely from state to state and law to law, and gun advocates typically resent the term “assault weapon” when applied to semiautomatic weapons, as it evokes the more widely used term “assault rifle,” which refers to a class of fully automatic rifles like the AK-47 or M16.

Let’s take as our main example the national ban that expired in 2004. The law explicitly outlawed some gun models by name, like the Colt AR-15 and the Uzi, and further banned any “semiautomatic rifle that has an ability to accept a detachable magazine and has at least 2 of”:

  1. A folding or telescoping stock
  2. A pistol grip that protrudes conspicuously beneath the action of the weapon
  3. A bayonet mount
  4. A flash suppressor or threaded barrel designed to accommodate a flash suppressor
  5. A grenade launcher

Semiautomatic pistols with at least two features from a slightly different five-item list, and semiautomatic shotguns with at least two items of yet another list, were also banned. Semiautomatic rifles and shotguns that couldn’t accept detachable magazines of more than five rounds were categorically exempted.

Critics of the law liked to note that these provisions — like flash suppressors, which try to prevent shooters from being temporarily blinded by the light of the gunshot, or bayonet mounts, or pistol grips — are essentially cosmetic. While they may enable a higher rate of fire by making guns more comfortable to use, they’re not core to the weapons’ functionality. That undermined how effective the ban could be.

“Relatively cosmetic changes, such as removing a flash hider or bayonet mount, were thus sufficient to transform a banned weapon into a legal substitute,” George Mason University criminologist Christopher Koper writes. “In this sense, the law is perhaps best understood not as a gun ban but as a law that restricted weapon accessories. … In other respects (e.g., type of firing mechanism, ammunition fired, and the ability to accept a detachable magazine), the banned AWs did not differ from other legal semi-automatic weapons.”

Koper ultimately concluded that the ban had no significant effect on gun crime while it was in effect, though it might have done more given more time. My colleague German Lopez reviewed some more of the evidence and concluded that assault weapons bans are overhyped in their effects and might not accomplish much in terms of reducing violence and deaths.

Handguns, explained

Between 2012 and 2016, according to FBI statistics (which rely on police department self-reporting, so caveat emptor), about 70 percent of homicides in the US were committed using firearms. And of firearm homicides, about two-thirds are committed using handguns.

Chart of gun homicides by type of firearm; handguns are involved in the great majority of homicides from 2012-1016 Javier Zarracina/Vox

Handguns are also the primary guns used in suicides. A 1980s study using data from California found that in 65 percent of male firearm suicides and 88 percent of female firearm suicides, the deceased used a handgun. A 2001 study in Bexar County, Texas (including San Antonio), covering suicides from 1984 to 1998, found that 78 percent of victims used a handgun. A 1993 study in Iowa found lower numbers, which corresponds to the greater popularity of rifles and shotguns in rural states, but also found that as handguns became more popular, their share of suicides increased.

Handguns come in two main forms: revolvers and semiautomatic pistols. Revolvers have multiple chambers (usually six, but sometimes five or seven) which rotate, or revolve, between shots to place a new round in firing position, while semiautomatic pistols load ammunition from a magazine, which is sometimes detachable. With each shot, the slide (the component of the pistol containing the bolt sealing the back end of the barrel) recoils, enabling a new cartridge to come into the gun’s chamber to be fired.

Both revolvers and semiautomatic pistols can be single-action, double-action only, or double-action/single-action. The terms refer to how much work a trigger pull does. In a single-action gun, pulling the trigger fires a bullet but does not ready the firing pin for the next round.

If you’ve ever seen a Western where a cowboy has to pull back or “cock” the hammer (the part of the gun that that strikes the cartridge to fire it) to his revolver in between every shot, he’s shooting a single action. Single-action semiautos are slightly different, in that you have to “rack” the first round, but subsequent rounds are readied automatically by the action of firing the round before them, and require no manual operation beyond a trigger pull.

In double-action guns, pulling the trigger cocks the hammer to prepare a round to be fired and fires that round. In double-action/single-action semiautos, the first firing of the trigger works like that, but subsequent trigger pulls work like single-action trigger pulls. Double-action/single-action revolvers give you a choice of how to use the gun: either fire with one trigger pull or a trigger pull plus manual hammer cocking.

In double-action-only revolvers and semiautos, there isn’t a hammer for the user to manually pull back, and every time you pull the trigger, the pull both cocks the hammer and fires the round.

In practical terms, double action means that the trigger pull takes more effort, as the pull is doing more work. That reduces accuracy but improves safety by reducing the risk of accidental firing. Double-action-only guns are sometimes popular for concealed carry for that reason. But it’s also common to carry double/single-action semiautomatic pistols in “cocked and locked” mode: A round is in the chamber, the slide is racked and the round is ready to fire, but a safety is engaged to prevent the round from being fired accidentally.

The trend in recent decades has been toward more semiautomatic pistols and fewer revolvers; between 1980 and 1994, the share of handguns produced in the US that are semiautomatic grew from 32 percent to 77 percent.

“There isn’t a huge initial difference between a semiautomatic pistol and a semiautomatic assault weapon,” my colleague Alvin Chang points out. “Both allow you to fire multiple bullets in rapid succession.” The rounds from a semiautomatic pistol are, to be sure, smaller and less powerful than rounds from a rifle. Pistols typically can’t hold as large magazines as rifles, either. But the difference in rate of fire isn’t that big, and as Chang further notes, mass shootings have been carried out using only semiautomatic handguns, including the 2007 Virginia Tech killings.

Specific regulations on handguns are relatively rare, outside of bans on certain high-capacity handguns under assault weapons bans. Democrats’ latest proposal, the Assault Weapons Ban of 2019, would ban pistols with a “fixed magazine that has the capacity to accept more than 10 rounds,” pistols with a “threaded barrel” (which makes it easier to attach a suppressor), a barrel shroud (a covering around the barrel meant to protect users from barrels that are hot post-firing), a second pistol grip, or a place to load a magazine besides the pistol’s grip. These are, once again, largely if not entirely cosmetic distinctions.

DC had, until 2008, a fairly comprehensive ban on all handguns, which the Supreme Court struck down in DC v. Heller as a violation of the Second Amendment. In McDonald v. Chicago in 2010, the Court struck down Chicago’s handgun ban on similar grounds, arguing the Second Amendment applies to states and localities as well.

So handguns are in a strange place: They are both the most important weapons in terms of homicide and suicide deaths and the ones that the Supreme Court has put the most explicit restrictions on in terms of regulation. They cannot be banned entirely, but some more modest regulations might be possible.

Thanks to Lois Beckett, Alex Yablon, and especially Massad Ayoob for their help researching and fact-checking this piece. All errors and opinions are my own.

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