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A few years ago, just before I first fired an AK-47 that I had built, I worried that it would somehow explode, or jam, or do anything other than what it was supposed to do: shoot a few rounds of 7.62 x 39mm ammunition, hopefully in a straight line.

It was 2013, and I was working on a story for this magazine. I had recently attended, in a garage in Los Angeles, what gun enthusiasts call a “build party,” an event in which a group of people get together to eat pizza, drink soda or beer, and build guns. What struck me most was just how easy it was to actually build a gun, especially after being helped along the way by the hosts of the party. They told me how to bend the receiver—the essential core of a gun that houses the mechanical parts and, in the eyes of the law, makes a gun a gun. They demonstrated how to attach the trunnions, the stock, the barrel, the trigger components, and various other bits and pieces necessary to finish it off.

Days later when I set out to test the gun, I was feeling uneasy. But when I finally pulled the trigger, the AK shot perfectly.

I shouldn’t have been surprised. The guys who helped me assemble it build their own guns all the time. At least to them, there wasn’t anything abnormal about what I was doing. And while it had the feeling of being illicit, it was and continues to be completely legal.

Yet DIY guns are not exclusively built by the hobbyists and gun enthusiasts that helped me back then. The appeal of these guns is, in part, the enjoyment that comes from the build. But for some people, it’s also that they bear no serial numbers, and are, as a result, completely untraceable. No database and certainly no cop will ever know about them. That’s why this class of firearms is commonly referred to as “ghost guns.” And now, it’s becoming more and more clear that ghost guns also provide an avenue for criminals and individuals with mental illness or who are otherwise prohibited from owning guns to get them, undetected. Just last month, for instance, Kevin Janson Neal massacred five people and wounded nine others in Tehama County, California, using two semi-automatic rifles he built himself. A criminal protective order and a separate restraining order that had been placed on Neal in January—he was charged with assault with a deadly weapon after allegedly stabbing and beating two neighbors he held at gunpoint—meant Neal wouldn’t have passed a background check if he’d tried.

“At some point this will be about smuggling tools, information, and knowledge rather than the guns themselves.”

It’s nearly impossible now to keep people with nefarious goals, like Neal, from building their own weapons—the process, from start to finish, increasingly occurs outside of established regulatory schemes. Thanks to various technological advancements, not to mention the lack of background checks on gun parts and tools, this class of guns has the potential to spawn an entirely new front in domestic and global small arms proliferation—and, as evidence suggests, it has already started to do so.

As Mark A. Tallman, an academic who has spent several years studying DIY gunmaking, puts it to Mother Jones, “At some point this will be about smuggling tools, information, and knowledge rather than the guns themselves.”

Under federal law, building an unserialized gun is legal so long as it is intended for personal use and is not sold. And now building one is truly easier than ever; what once required hulking and expensive machinery and expertise can be accomplished with relative ease and on the cheap using tabletop mills and 3D printers. Everything a person would need can be freely purchased and sold online, no questions asked. And since the only part of a gun that does require a background check is the receiver, the companies that sell them have created workarounds by offering partially completed receivers. Known as “80 percent” lowers, or unfinished receivers, they don’t require checks or serial numbers, and they are manufactured so that all a person has to do is drill a few holes, make a few cuts and, depending on the type of gun, bend it into shape for it to be completed. What’s more, some milling machines, small enough to fit on the top of a desk, can now churn out unserialized receivers with just the push of a button.

In the scope of firearms-related crime, the percent committed using ghost guns still remain a statistical outlier, according to Tallman, also an adjunct professor at Colorado State University-Pueblo’s Center for the Study of Homeland Security. “Since 2008,” he says, “trace reports involving DIY crime guns have increased several fold, but the total number is still less than one percent of all gun crimes that are traced.” Tallman warns, though, that the problem is growing; crime connected to ghost guns “ has become an increasing factor in the United States, and certainly other countries.”

While so far there have only been a few glaring, high-profile cases involving these firearms in the United States, each of them has resulted in a shocking massacre. In addition to Neal, in 2013, 23-year-old John Zawahri, who had been kept from purchasing a gun years earlier , used a homebuilt AR-15-style semi-automatic rifle in a rampage in Santa Monica, California, that left five dead. Meanwhile, authorities continue to find ghost guns when busting up trafficking rings. In September 2015, New York Attorney General Eric Schneiderman called ghost guns “the new frontier of illegal firearms trafficking.”

Tallman estimates that, conservatively, there are hundreds of thousands of unmarked receivers that have been sold in America in the past 10 years. However, their inherently hidden nature, and the fact that sales data is proprietary, ensures it’s nearly impossible to get hard numbers. This also makes it nearly impossible to regulate them, and the several attempts to do so over the past five years have all fallen short—and in fact have inspired increasingly creative legal workarounds.

In December 2013, shortly after the first 3D-printed plastic guns captured widespread attention, Congress voted to extend a law prohibiting guns made of plastic, but failed to address whether guns can be 3D printed if they include a section of detectable metal inserted in order to skirt the expected ban. And in California in 2014, Gov. Jerry Brown vetoed a bill that would have retroactively required serial numbers for all home-built guns and to have them registered with the Department of Justice; last year, he vetoed a similar bill that would have expanded the definition of “firearm” to include unfinished receivers, and therefore subject them to background checks.

“It’s like people in California enjoy when they pass a ban because they have an enthusiasm for making a contraption to work around the law.”

Even the legislation Brown did actually sign last July, AB 857—which, like the 2014 measure, will require anyone manufacturing a home-built gun to first apply to the state Department of Justice for a unique serial number and to affix it to the weapon—may not effectively keep ghost guns out of the hands of criminals. “What we do is a way of negating the American gun control effort—especially California’s efforts,” Cody Wilson, co-founder of Defense Distributed, which makes a desktop machine that mills lower receivers out of hunks of aluminum and sells them for $1,675, told the San Francisco Chronicle last month. “The 80 percent culture is fostered because of California’s gun laws. There’s more homemade guns there. There’s more of a home-build culture there. It’s like people in California enjoy when they pass a ban because they have an enthusiasm for making a contraption to work around the law.”

Wilson experienced a moment of infamy in 2013 when he first published online blueprints for a 3D-printable handgun called the “Liberator.” While the media lost its collective mind, the State Department issued a cease-and-desist letter alleging that Wilson’s company may have violated arms export control laws. But Wilson still isn’t deterred; he has since challenged the letter and the issue is making its way through the courts. What’s more, Wilson started a movement. He didn’t invent the concept of a 3D-printable gun, but he was the first to make the plans available to anyone who had an internet connection, anywhere in the world.

The latest attempt to address the ghost guns issue came in late November when the Giffords Law Center to Prevent Gun Violence called on two Internet Service Providers to disable two of the most popular websites—ghostgunner.net and ghostguns.com—that sell these components for homemade weapons and the tools needed to build them. Attacking the two websites, though, will almost certainly fail to adequately address the problem. The internet is replete with websites and marketplaces that legally sell the tools and components needed for DIY guns, not to mention the dark web. Referring to the Giffords group’s tactics, Tallman says, “When it comes to targeting those specific suppliers rather than others, it’s a matter of aesthetics—focusing the attention on the suppliers who are more provocative in their marketing rather than the substance and legality of what they’re doing. I don’t know if it’s likely to produce much in terms of results.”

“Really solving this problem will require more than engaging with these two companies’ ISPs,” Adam Skaggs, senior council at the Giffords organization, allows, emphasizing that the tactic is merely a starting point in his organization’s attempt to tackle the issue. That has included drafting model legislation that has not yet been successful; Skaggs wants the law to treat “devices that are nearly finished gun parts” the same way it treats completed guns, by requiring serial numbers and background checks.

“For me to sell you a gun, I need to get a federal license, I need to comply with certain paperwork obligations, and maintain certain types of records, and I need to submit to oversight by the ATF,” he says. “Yet somebody can sell a device that allows you—with no technical training and no expertise—to manufacture the exact same kind of gun at the click of a button without complying with any of those obligations. It doesn’t make a lot of sense.”

Even if lawmakers are able to mandate background checks for all unfinished receivers and require that they include serial numbers, the phenomenon of modern DIY gunmaking, as Wilson suggests, could eventually make legislation almost completely ineffective. The point he has effectively made, in not so many words, is that for people planning to act illegally , the legal system won’t or can’t do much to regulate their activity until the guns are built and distributed, and, at that point, they’re part of something that’s like any other illicit market—fully clandestine and difficult to combat.

Because these people can manufacture the weapons completely outside of legitimate supply chains, “there is virtually no leverage point to impact it,” Tallman explains. The world of ghost guns is one increasingly built on cheap, easy-to-use tools and open source information—neither of which can we regulated with background checks—rather than the end product. And this stark reality is starting to remake the small arms trade on a worldwide scale, even among organized crime networks.

“Guns have been more or less controlled in the same ways since the rise of industrial production,” says Tallman. “As you clamp down on legal supply chains, you have to ask the question: Are there parallel illegal supply chains that are available substitutes?”

“There is an argument from both the DIY enthusiasts and Second Amendment advocates who will say, ‘You can try to regulate the more benign areas of this, but it’s never gonna touch the more malignant area.’ And to some degree, they’re right.”

For guns, the answer is yes. “It’s not terribly difficult for a semi-skilled person to make a fully functional firearm, either out of available precursor materials or just from scratch, as needed,” says Tallman. “There is an argument from both the DIY enthusiasts and Second Amendment advocates who will say, ‘You can try to regulate the more benign areas of this, but it’s never gonna touch the more malignant area.’ And to some degree, they’re right.”

That’s not to say people won’t get caught trafficking DIY guns. They will, and have. Back in 2013, Michael Yarbrough of Corpus Christi, Texas, was sentenced to 10 years in prison after buying more than 900 parts kits and firearm receivers and selling the AK-47-type rifles to be trafficked across the Southern border. Several of the AK-47s he made were seized inside Mexico, including one at a scene where three men were killed. In September 2015, two men in New York were charged for illegally trafficking at least a dozen ghost guns. The next month, eight men in California were indicted for manufacturing and dealing AR-15-style rifles and silencers without a license—many of them DIY guns.

Tallman also points to the UK, where somewhere between 18 and 40 percent of the illegal firearms the London Metropolitan Police now seize are DIY in some capacity. In Australia, it’s 10 to 20 percent. There, he says, “gunsmiths, criminal groups, motorcycle gangs, and organized crime will hire and train rogue machinists to make fully functional, expedient submachine guns, complete with suppressors and extended magazines—all the bells and whistles.” And in 2015, authorities busted a cartel-backed group in Jalisco, Mexico, that used a $20,000 American-made CNC machine to build AR-15s in its illegal gun lab. It’s anyone’s guess how many others there are. The extent to which organized crime syndicates in Japan are interested in firearms, Tallman notes, “they are building them themselves, they are importing DIY guns from the Philippines, where there is a large illegal artisanal industry, and they are even importing Filipino gunmakers to share their knowledge.”

Kicking websites offline, legislatively mandating background checks and serial numbers for unfinished receivers, and regulating companies that sell the equipment like any other licensed firearms dealer is a certainly a start, but the issue very likely has already grown beyond such fixes.

“There are more sophisticated criminal actors around the world—and here—who are capable of building small arms from scratch with no interaction with the legal supply chain,” Tallman says. “That is a very different kind of problem than trying to regulate the legal industry.”