The Supreme Court has revived, at least for now, the Biden administration’s regulation of “ghost guns,” drawing a spotlight to the kits that can be assembled into homemade firearms and the new front in the fight over gun control that they have opened up.

Administration officials say the popularity of the weapons has soared in recent years, particularly among criminals barred from buying ordinary guns. The regulation, issued in 2022 by the Bureau of Alcohol, Tobacco, Firearms and Explosives, broadened the bureau’s interpretation of the definition of “firearm” in the Gun Control Act of 1968.

It did not ban the sale or possession of parts that can be assembled to make homemade guns, but it did require manufacturers and sellers to obtain licenses, mark their products with serial numbers and conduct background checks.

The rule was a centerpiece of President Biden’s broader initiative to address the proliferation of illegal weapons. Mr. Biden said last year that the rule would “help save lives, reduce crime and get more criminals off the streets.”

But the federal effort to regulate ghost guns has been troubled by uneven enforcement, significant loopholes and ongoing court challenges. A lower court judge had struck down the regulation, but the Supreme Court said it could remain in place while a challenge moves forward.

Here’s what you need to know about the weapons.

Traditional firearms are made by licensed companies and then bought from licensed gun dealers. All guns manufactured in the United States, as well as those imported from abroad, are legally required to have serial numbers that are typically displayed on the back of the frame.

In contrast, a ghost gun is sold in parts, and can be assembled at the home of an unlicensed buyer. Before federal regulations were put in place under the Biden administration, there was no need to pass a background check to obtain the components of a ghost gun. They are sold online as D.I.Y. kits, and typically shipped as “80 percent receivers.” That means the gun is 80 percent complete, and buyers have to assemble the final 20 percent themselves.

The key selling point for many buyers was that ghost guns were not required to have serial numbers, the critical piece of information that law enforcement agencies use to trace the gun from the manufacturer to the gun dealer to the original buyer.

It’s easy and relatively inexpensive.

According to a report by Everytown for Gun Safety, a gun violence prevention organization, an AR-15 build kit costs as little as $345.

The sales pitches usually promise little work for the buyer. One online purveyor assured that “building time doesn’t take too long,” adding, “Within an hour or two, you should be breaking it in at the range.”

The kits usually come with directions on how to finish the gun or link to YouTube tutorials. Typically, the only tool needed is a drill, and the kits are often sold with the necessary drill bits.

Many ghost guns are also sold with a “jig,” which fits around the frame or receiver to make assembly easier. One site said the jig could be used to complete a gun “in under 15 minutes with excellent results.”

According to Everytown for Gun Safety, the top five instructional videos on YouTube for building a ghost gun have drawn more than three million views.

Ghost guns aren’t new, but they are a growing problem. Even though kits to assemble guns have been sold since the 1990s, the market did not really take off until around 2009. That is when firearm sellers in California began offering unfinished receivers for the AR-15 and AK-47 series of guns, in an attempt to circumvent the state’s assault weapons laws, according to T. Christian Heyne, the vice president for policy at Brady, a gun violence prevention organization.

The problem of ghost guns did not become well known until 2013, when one was linked to a rampage by a gunman who killed five people in the vicinity of Santa Monica College in California.

Sales of ghost guns started to rise substantially around 2016, as people began buying kits to re-create a firearm based on the Glock 9-millimeter semiautomatic pistol.

There is no way to know how many ghost guns were in circulation before last year’s rule, because of the lack of requirements for serial numbers or a background check.

But data shows that prevalence of the weapons appears to be growing every year, especially in coastal blue states with strict firearm laws like California. Law enforcement officials in the state said ghost guns accounted for 25 to 50 percent of firearms recovered at crime scenes in an 18-month period from 2020 to 2021, and the vast majority of suspects caught with them were legally prohibited from having guns.

In 2021, the Justice Department reported that law enforcement agencies had recovered 19,300 homemade guns, about five times the number confiscated or found at crime scenes in 2018.

Proponents of stricter gun laws have been pushing for action on ghost guns to address the problem before it reaches epidemic proportions. Opponents have questioned the data showing their growth.

Some other mass shootings have been linked to ghost guns, like a 2019 incident at a high school in California where a 16-year-old killed two students. A ghost gun was also linked to a 2017 rampage in which a man killed his wife and four others in Northern California.

But analysts said ghost guns were not disproportionately linked to mass shootings. The bigger issue is that they have an outsized effect on day-to-day gun violence in communities of color across the country, gun safety groups said.

There are significant limits to how much Mr. Biden can do through executive action to regulate ghost guns.

Officials and gun control groups have previously said that the regulation had done little to stop the sale of key parts used to make ghost guns, in part because the rule was created through executive action, rather than a statute validated by Congress.

The Biden administration has since moved to close a major loophole in the rule by directing vendors to label some partially finished handguns with serial numbers, and to require that buyers of these parts undergo criminal background checks.

Democrats have pushed the A.T.F. to more aggressively enforce the rule, but officials have cautioned that pushing too far could jeopardize the rule entirely by opening it to more court challenges. The powerful gun lobby has strongly opposed the rule, and several conservative legal groups have already challenged it, arguing that it violates existing firearms laws and Second Amendment protections.

Adam Liptak contributed reporting.

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