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The AR-15, the gun behind some of the worst mass shootings in America, explained

Updated by Libby Nelson@libbyanelson Jun 14, 2016, 1:00pm EDT

An AR-15 from Rock River Arms, one of the many companies that makes its own version of the rifle. Joe Raedle/Getty Images

Some of the worst mass shootings, the ones that turn the names of American towns into a universal shorthand for tragedy, have all used similar guns: versions of the AR-15 assault rifle(Ed: The AR-15 is not an assault rifle).

The 20 children and six adults who died in Newtown were killed, in part, by a version of the AR-15. So were the 12 who died in an Aurora, Colorado, movie theater. So were the 14 who died in San Bernardino.

After Sunday's shooting in Orlando, carried out with the Sig Sauer MCX, a military-style assault rifle that looks much like the AR-15, calls to ban the AR-15 are surging. The Boston Globe described it as "mass shooters' weapon of choice." The New York Daily News called it "a mass murderer's best friend."

The AR-15 has come to stand in for an entire category of military-style assault rifles(Ed: The AR-15 is not an assault rifle) developed for combat that many believe have no place in civilian life. The irony is that the outrage will almost certainly boost AR-15 sales. Historically, the more the AR-15 and other assault weapons(Ed: assault weapon is a term invented by politicians who know little about guns) are criticized, the more guns their supporters buy. And the more AR-15s there are in American hands, the harder it gets to regulate them, both politically and practically.

The AR-15 is a military-style rifle that civilians can use

AR-15 at Florida gun store

An AR-15 for sale at a gun store in Florida. Joe Raedle/Getty Images

The AR-15, a trademark owned by Colt but applied to wide range of near-identical guns from different manufacturers, is a civilian version of the M16, a weapon designed for military combat in Vietnam.

It's very difficult for civilians to own an M16. The gun can shoot up to three bullets in a single burst, and older versions were fully automatic, meaning they keep firing as long as you hold the trigger. As such, the gun is strictly regulated by the National Firearms Act, the 1934 law that restricted the possession of machine guns. The AR-15, though, is semiautomatic — you have to pull the trigger for each bullet — and so has no such restrictions.

Since 2009, gun manufacturers have preferred to call these guns "modern sporting rifles," a name that conjures up hunters in the woods rather than soldiers shooting at the enemy. But even enthusiasts agree that the AR-15 doesn't shine while hunting. It's better at killing people than it is animals.

So there's another name for AR-15s and guns like them: assault weapons. While exact definitions of the term vary, it typically means a military-style weapon adapted for civilian use, with modifications that can make it more lethal. This graphic shows some of those modifications on a non-AR-15 assault weapon:

Javier Zarracina/Vox

And since the federal assault weapons ban expired in 2004, AR-15-style rifles and similar guns, including the gun that Orlando shooter Omar Mateen used, have become astronomically popular.

The more you try to ban the AR-15, the more popular it gets

The federal assault weapons ban, which went into effect in 1994, banned the sale of AR-15-style guns and high-capacity magazines(Ed: the 1994 assault weapons ban had little or no effect on crime). It was the first example of a phenomenon that's driven the AR-15's popularity for two decades: The more gun owners fear their guns will be taken away, the more assault rifles(Ed: The AR-15 is not an assault rifle) they want to own.

As Congress debated the assault weapons ban in 1993, production of assault rifles(Ed: The AR-15 is not an assault rifle) soared, according to a 2004 report from the University of Pennsylvania:

Even while the ban was in effect, barring the production of certain kinds of assault weapons, gun manufacturers got around it by creating new "postban" models of guns that went by different names.

Olympic Arms began selling a version known as the PCR — "politically correct rifle" — that lacked a threaded barrel and a bayonet lug, two of the features Congress objected to. Colt stopped making the AR-15, but it made the Colt Match Target, which looks pretty similar.

This has happened at the state level, too: California has an assault weapons ban, but entrepreneurial gun manufacturers have found ways to modify the AR-15 to get around it.

The 1994 assault weapons ban banned the AR-15 by name, including identical versions made by other manufacturers. So as calls to ban the AR-15 intensified this week, some gun defenders argued that the Sig Sauer MCX that Mateen used wasn't an AR-15 at all due to slight technical differences. (Orlando's police chief, John Mina, initially described the weapon as an AR-15.) Reviews of the gun from before the Orlando shooting often compare it to AR-15-style weapons but also highlight the ways in which it differs from standard AR-15s, such as firing several calibers of bullets.)

That's unlikely to convince anyone whose real concern is civilian access to military-style weapons. But when it comes to writing precise legislation, those differences matter, which is one reason the movement to protect guns has seized on them.

Obama's election made the AR-15 and other assault weapons a lot more popular

Gun manufacturers started making more AR-15s again after the assault weapons ban expired. They became more popular as part of a trend for military-style weapons, in part because they're endlessly customizable.

But sales didn't really take off for four more years, until President Barack Obama was elected in 2008, when gun sales of all kinds began to soar.

Smith and Wesson manufactured nearly 10 times as many M&P15s, its AR-15 equivalent, in 2008 than in 2006. Their output tripled again in the next year. By December 2012, when the AR-15 became the focus of a debate after the Newtown shootings, there were about 3.8 million AR-15s in circulation in the US, Slate's Justin Peters estimated.

Then, for a brief moment after the Newtown shootings, additional gun control in the US seemed possible. Obama called for new gun control legislation, including a renewal of the federal assault weapons ban. Connecticut, New York, and Maryland expanded their own state bans.

And sales of AR-15s went nuts. Retail stores doubled or tripled prices in the first months of 2013, finding that desperate gun owners, worried the weapons would be banned, would pay.

The more popular the guns get, the harder it becomes to actually ban them. There are so many AR-15s now owned in the US that gun aficionados argued in summer 2013 that the market was saturated. That means that if a new assault weapons ban, like the 1994 version, grandfathered in guns that people already owned, there would still be millions of AR-15s available in the US. A buyback could get some of those guns off the street, but it would also be expensive.

Still, AR-15 owners are still outnumbered. Assault rifles(Ed: The AR-15 is not an assault rifle) are a tiny slice of the more than 300 million guns in circulation in the US. The majority of Americans don't own guns at all, and the majority of Americans favor stricter gun laws.

But the rush to buy AR-15s every time a politician mentions gun control illustrates why the National Rifle Association has so much political clout. Its 5 million members care deeply about their guns, a combination of campaign money, political power, and a single focus that gun control supporters don't have.

The other hurdle to banning AR-15s: the Supreme Court

Assault rifles(Ed: The AR-15 is not an assault rifle) aren't the only weapon of choice in mass shootings: The Virginia Tech and Charleston mass shootings were tragic and traumatizing, and the shooters in each only used handguns. But they get special attention both because those weapons formerly were banned and because the number of mass shootings appears to have risen since the assault weapons ban expired.

Between 1984 and 1993, according to the 2004 University of Pennsylvania study on the assault weapons ban, there were 15 mass shootings that killed at least six people or wounded at least 12. Assault weapons were used in only six of these shootings, but those shootings killed and injured more people than the other nine combined.

Mother Jones, which tracks mass shootings that meet specific criteria, counts 46 mass shootings since 2004, when the assault weapons ban expired. Of those 46 shootings, 14 featured assault rifles(Ed: no assault rifles were used. An assault rifle is select fire, able to fire morfe than one round per trigger pull), including Newtown, Aurora, and San Bernardino.

And assault rifles(Ed: The AR-15 is not an assault rifle) appear to be becoming more popular among mass shooters. In 2015, five of the seven mass shootings Mother Jones logged featured a shooter carrying an assault rifle(Ed: The AR-15 is not an assault rifle).

In the wake of Newtown, a few states did tighten their gun laws, including bans on AR-15s. The Supreme Court might soon have to weigh in on whether those bans are constitutional.

A case against Maryland's assault weapons ban is making its way through the courts, and in the most recent development, the Fourth Circuit sent that case, Kolbe v. Hogan, back for stricter review at the trial court level. Two conservative justices seemed very skeptical that banning the AR-15 was constitutional.

The reason the Fourth Circuit judges cited was from District of Columbia v. Heller — the 2008 decision that found a constitutional right to own a handgun for self-defense — which included a line saying that, constitutionally, "dangerous and unusual" weapons could still be prohibited.

The AR-15 is certainly dangerous. But it isn't necessarily unusual. If there were nearly 4 million AR-15s in the hands of American gun owners in 2012, before the Sandy Hook-induced panic, there are almost certainly more now. The specter of a federal ban could make them more popular still.

So the AR-15 is caught in a cycle. The more it's used in high-profile mass shooting cases, the more people want to ban it. The more people want to ban it, the more AR-15s are sold. And the more AR-15s are sold, the harder it becomes to create a ban that would be able to stop the next tragedy.

Correction: A previous version of this article misstated the year the National Firearms Act was passed.