In October, Stephen Paddock fired round after round from the window of his Las Vegas hotel room, spraying bullets on thousands of concertgoers below. He killed 58 people and injured more than 500. About a month later, in Sutherland Springs, Texas, gunman Devin Patrick Kelley burst into a church and massacred 26 of its congregants.

A month after that, on December 6, the House of Representatives passed its first major piece of gun legislation — a bill that requires each state to recognize permits for the concealed carry of a handgun issued in other states, making it easier for legal gun owners to travel across state lines.

The Concealed Carry Reciprocity Act of 2017 would force states with stricter gun laws, such as New York and California, to honor out-of-state permits from states with less restrictive requirements. Its passage is a victory for gun rights advocates; the National Rifle Association’s executive director, Chris Cox, called it a “watershed moment.”

The bill, sponsored by Rep. Richard Hudson (R-NC), sailed through the House, 231-198, hewing closing to party lines. (Fourteen Republicans voted against, and six Democrats supported it.) House Republicans tacked on a provision to strengthen criminal background and mental health reporting to the federal database — a move that some critics derided as a cynical effort to put Democrats on the record as against the background check measure.

The legislation also requires the Justice Department to study — but not ban — the use of bump stocks, which can turn semiautomatic firearms into machine gun-type weapons. (The Las Vegas gunman outfitted his gun with this device.)

The Senate will next take up a version of the bill, which will face a tougher hurdle to passage. Democrats are likely to filibuster it, which means it will require 60 votes to pass. (Republicans have a 52-vote majority, and they might be able to siphon off a few red-state Democrats, but it will not be an easy task.)

Proponents of the bill liken concealed carry to a driver’s or marriage license — what works in one state should work in another. Otherwise, its supporters say, different state laws cause confusion for law-abiding gun owners, which puts them unfairly at risk of arrest or prosecution.

Yet the bill’s critics say it undermines state and local laws, effectively forcing states or municipalities with higher restrictions to hew to the loosest laws, regardless of individual states’ public safety needs. The common analogy, which New York City officials immediately brought up: armed tourists in Times Square. On New Year’s Eve.

TO REPEAT. The Manhattan DA and the NYPD say that if the Concealed Carry Reciprocity Act becomes law, they cannot impose restrictions on guns in Times Square on New Years Eve, provided there is a legal permit from another state or the carrier is from a no-permit state. — Alex Silverman (@AlexSilverman) December 6, 2017

Experts say such a law likely won’t affect states that already have lax laws in place but could increase carrying in states with more stringent requirements, potentially undermining policing and law enforcement. There is also no hard evidence that this bill will make Americans safer from gun violence — in fact, data reveals that concealed carry laws (often known as “right to carry,” or RTC laws) correlate with increased violent crime rates.

“What we can tease out from the empirical data is you do worse in terms of more violent crime if you allow anybody to carry concealed handguns outside the home,” said John Donohue, a professor of law at Stanford University who has studied right-to-carry laws. “There are a lot of factors at play. There are some good instances where someone with a gun actually can thwart a crime, so that tends to depress crime. On the other hand, lots of things go wrong when you start carrying guns outside the home.”

The concealed carry reciprocity legislation has a less certain future in the Senate. But it remains a priority for the NRA and other gun rights advocates, and the bill’s victory in the House puts the US one step closer to a potential radical change in its gun laws.

How “concealed carry reciprocity” would work

Concealed carry is basically the right to tote around a weapon, usually a handgun, on or close to the body that isn’t in plain view. All 50 states have laws that allow some form of concealed carry, some with tougher standards than others. It’s different from open carry, which allows gun owners to have firearms in full view. That’s totally banned in five states; other states either allow it or require a permit.

About 9 million US adults carried handguns monthly, and about 3 million did so daily in 2015, according to a 2017 study in the American Journal of Public Health. The pro-gun Crime Prevention Research Center estimates the number of concealed carry permits to have hit approximately 16.3 million in 2017.

The Concealed Carry Reciprocity Act would require states to treat those concealed carry permits similar to a driver’s license, cross-honoring the permits of other states’ residents. This wouldn’t exactly supersede the specific laws within individual states — for example, California currently restricts its residents from carrying concealed handguns in San Francisco. With this potential new law, the state can still enforce that rule.

What California can’t do is stop someone visiting San Francisco from Arizona — which doesn’t require a permit — from carrying a concealed firearm while in the city.

This matters because states vary on how willing they are to hand out permits to carry concealed weapons.

A small number of states — such as New York and California — have what’s known as “may issue” laws, restricting who can carry a concealed handgun. For example, in New York, applicants have to be of “good moral character” and often show they have a need, such as self-defense, to get a permit.

Most states (and Washington, DC) have “shall issue” laws, which means most individuals can get permits if they meet the state and federal requirements for owning a handgun. (They can’t be a convicted felon, for example.) States can add qualifications, such as passing a firearms test. But the permit is granted if the criteria are met.

“Permitless” is exactly that — if someone is a lawful handgun owner, the state says they have a right to concealed carry, no permit necessary. This is also known as “constitutional carry.”

Many states already honor permits for non-residents, usually from jurisdictions that have similar requirements. Some states, such as Arizona, issue concealed carry permits to out-of-state residents. Some states have reciprocity and will honor those non-resident permits. Other states do not, rendering them invalid in those locations.

This creates a mishmash of different rules and regulations among states. This law would simplify the situation — gun owners with a concealed carry permit would know they could continue to carry their gun in every US state without violating any laws.

“Right now, it’s amazingly complex to travel with your concealed-carry weapon because every single state has constantly changing laws, and as you’re traveling from state to state, you never really know if you can carry,” Tim Schmidt, the founder and president of the advocacy and education group US Concealed Carry Association, said, adding that many respgun owners end up choosing to leave their weapons at home.

Gun control advocates, on the other hand, see this simplification as akin to a nullification of the strongest gun laws. Tough states would have to honor permits from visitors who, if they were residents, wouldn’t legally be able to carry a weapon. Under the House (but not Senate) version of the bill, states would also have to honor reciprocity for non-resident permits, meaning those who don’t meet the legal requirements in their home state could now obtain a non-resident permit to carry.

For example, according to data from Everytown for Gun Safety (which opposes the bill), 35 states and DC prevent significant others who are convicted of misdemeanor domestic abuse from obtaining concealed carry permits. Under the House bill, those people could potentially get a permit in a less restrictive state and it would be valid everywhere.

Alex Yablon at the Trace offers another example: Rep. Greg Gianforte (R-MT), who body-slammed a Guardian reporter in May. Gianforte pleaded guilty to misdemeanor assault, which does not disqualify him from obtaining a concealed carry permit in his home state of Montana (a “shall issue” state), should he want one. However, writes Yablon:

... seven other states prohibit anyone with even a misdemeanor assault conviction from owning a gun, much less carrying one concealed. If the reciprocity bill becomes law, those states — Oregon, California, Illinois, Maryland, Delaware, New York and Connecticut — would have to allow someone with Gianforte’s record of aggressive physical behavior to carry a concealed weapon while visiting, even though he would not be able to actually purchase one in those places.

What the data says about concealed carry

“You’ve heard it before: ‘The only thing that stops a bad guy with a gun is a good guy with a gun,’” wrote Rep. Hudson, the sponsor of the bill, in an op-ed in the Washington Examiner.

This is the crux of the argument for the Concealed Carry Reciprocity Act — that law-abiding citizens should have the right to protect themselves, and that more guns will make us safer and are a potential deterrent to violence.

The most often-cited data to support this theory first came from a 1997 study by economist John Lott, who runs the Crime Prevention Research Center. As Vox’s German Lopez and Dara Lind explained, his results and methodology have come under serious scrutiny — and more recent data in the decades since contradicts those findings.

It is true, said Daniel Webster, the director of the Center for Gun Policy and Research at John Hopkins University, that most concealed carry permit holders are relatively law-abiding. But he told Vox in an email that based on studies of right-to-carry laws, a reciprocity act could lead to more violence.

A 2017 analysis led by Michael Siegel of Boston University’s School of Public Heath analyzed “shall issue” and “may issue” states from 1991 to 2015, and found that “may issue” states — those that tend to have stricter standards — had a lower homicide rate than “shall issue” states. “Shall issue” states were associated with 6.5 percent higher total homicide rates, an 8.6 percent higher firearm homicide rate, and a 10.6 percent higher handgun homicide rate than in “may issue” states. (Permitless states were left out of the study.)

Correlation isn’t the same as causation, and Siegel said they can’t know for sure why this is. Still, he has two hypotheses. When states move from “may issue” to “shall issue,” more people successfully obtain permits because the requirements ease up — which also means the number of people with access to firearms increases.

And that means people who might not have qualified under “may issue” laws — which, again, gives officials discretion — may be eligible in “shall issue” locales. For example, all states prohibit a person from carrying (or owning) a weapon if they have committed a violent felony offense — but not all states ban people with misdemeanor convictions, such as assault. In “may issue” states, Siegel said, “less people slip through who otherwise wouldn’t have been able to posses, and those people may be very high risk. So the risk of the pool of the people who are carrying is probably going up.”

Siegel isn’t saying that “shall issue” states open the floodgates to dangerous people; far from it. But less stringent requirements mean more individuals with red flags in their past may slip through.

He added that he and his team found that the increase in homicides was closely tied to firearm deaths; there was no increase in non-firearm homicides. They also looked at handgun versus long-gun homicides and discovered something similar: Handgun homicides went up, but they didn’t see any increase in long-gun homicides. “What this argues for is the validity of the finding,” Siegel said. “We’re not just seeing an increase in homicides. It’s specific to handgun homicides, and that’s consistent with the hypothesis that this is actually related to the ‘shall-issue’ laws.”

Donohue, with his co-authors Abhay Aneja and Kyle D. Weber, also conducted an analysis, published in June 2017, that said states with concealed carry laws have higher rates of violent crime. Donohue and his team relied on a “synthetic control” method, which social scientists use to try to gauge the consequences of policy intervention (in this case, passing an RTC law). So researchers take actual crime data from a state before it passes a right-to-carry law, then combine data from a bunch of other states to create a sort of mirror-image state, and compare the two.

Using this methodology, Donohue and his team found that over 10 years, right-to-carry laws have a 13 to 15 percent higher violent crime rate.

Donohue said a few factors could explain his findings. Simply, things go wrong when you carry a deadly weapon outside the home. “Road rage is one small example of that, and we see that play out in many different scenarios,” he said.

“It also turns out when you take guns outside the home, they’re much, much more likely to be stolen,” Donohue continued, saying he estimated that about 100,000 guns are stolen from permit holders every year. “That’s a case where a good guy is essentially arming a criminal.”

These laws, he said, can also complicate the job of police officers who have to deal with violent episodes — even if they’re, say justified as self-defense — or with guns being stolen.

In total, there have been at least 1,119 deaths via lawful concealed carry permit holders not ruled as self-defense in the past decade, according to the Violence Policy Center. That includes 31 mass shootings carried out by concealed carry permit holders, which the center defines as three or more deaths.

But what of the “more guns, less crime” theory? The oft-cited Lott study is from 1997, and tracked US counties from 1977 to 1994 (and later, from 1977 to 2005). It found states with larger increases in gun ownership were associated with the largest drops in violent crimes. Different studies, including a 2005 study, showed a neutral relationship between concealed carry laws and violent crime — essentially no meaningful increases or decreases in violent crime. But all these studies tend to suffer from old age and older data, which Siegel explained doesn’t really account for the massive shift in both gun culture and changes to concealed carry laws in the past decade and more.

“There’s been a huge increase in the number of people who own handguns, especially pistols, and these weapons have been marketed specifically for their use [for] self-defense,” Siegel said. After 1977 — when the NRA morphed from a group for hunters and sportsmen into the powerful lobby it is today — and states starting moving from “may issue” to “shall issue,” the rest of the public was still catching up. (Lopez has an explainer on the NRA’s shift in the 1970s.)

Now, the consequences are more profound. “The demand for concealed carry permits has risen dramatically,” Siegel said. “There may actually be a change in the impact that these laws are having.” Pair that with the marketing prowess of the gun lobby, which drills down on the idea that people needs guns for self-defense, and old data becomes dogma.

What are the potential consequences?

Congress’s first piece of gun legislation after two back-to-back mass shootings that left dozens of people dead will likely have the most noticeable effects in those “may issue” states.

Reciprocity laws probably won’t affect states that already have more permissive laws, Webster told Vox, but they do have the potential to increase carrying in more restrictive states.

It’s hard to gauge the scope of such changes, but experts and opponents see two major red flags. The first is the obvious. Places like New York, which has extremely restrictive gun laws downstate and in the city, will be forced to accept people lawfully carrying concealed weapons, though some of its actual residents might be barred from obtaining a permit. The biggest concern is that people with violent pasts from states with weak standards will carry concealed firearms into those states, Webster said. “The data suggests that such folks are trouble in their own states, and would likely be so in other states as well,” he explained.

As Donohue points out, this could be a “nightmare” for police officers in California and New York, particularly in big urban areas where cops are constantly trying to take illegal guns off the street (and where, in New York City, for example, they’ve been remarkably successful at it).

The second major concern arises from the fact that some states, such as Arizona, issue non-resident permits. The House bill allows for reciprocity of non-resident permits, and the fear is that an individual who might be convicted for misdemeanor assault in one state and can’t get a concealed-carry permit there might be able to apply for a non-resident license. “What this means is that essentially anybody in any ‘may issue’ state can circumvent the law simply by getting a permit in another state,” Siegel said. “It essentially nullifies these laws.”

A legal challenge on both points is possible — even conservative legal scholars take issue with possible overassertion of federal power.

The bill’s chances in the Senate are narrow

Senate Democrats appear united against the bill, and the filibuster will make that even tougher to overcome. They’re helped by a pretty robust coalition of opposition, which includes advocacy organizations that support survivors of domestic abuse, who fear this will allow abusers to cross state lines and get permits in states where their pasts don’t disqualify them from obtaining firearms; and some law enforcement groups, especially in urban areas, that see this as interfering in how cops police their jurisdictions.

The Major Cities Chiefs Association, which represents police in big cities in the US and Canada, also raised the specter of potential lawsuits that this law could bring. “The ‘Concealed Carry Reciprocity’ bill is both impractical and contrary to the rights of States,” it said in a statement. “Moreover, it raises Constitutional questions about the authority of Congress to direct State officers.”

Meanwhile, officials in “may issue” states such as California and New York condemned the House bill and its passage. "I wouldn't presume to tell Arkansas what its gun laws should be,” Manhattan District Attorney Cy Vance said Wednesday, after the House bill passed. “I don't want Arkansas to tell New York what its gun laws should be."