The head of the Bureau of Alcohol, Tobacco, Firearms and Explosives says he’s so inept he “can’t screw in a lightbulb,” but even he was able to build a “ghost” gun from a kit with no problem.

“I made one of those handguns out of the kit, went down to the range, shot it. Shoots just like a traditional firearm, it can wound just like a traditional firearm, it can kill just like a traditional firearm,” ATF Director Steven Dettelbach told CBS News’ “Face the Nation” earlier this year.

He had one of his firearms experts join him for the talk with CBS, and the man figured he would demonstrate how easy it was to combine a gun “receiver,” assembled from a kit purchased online, with a Glock slide, which is how many so-called ghost guns are completed.

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But the slide wouldn’t fit and the expert, still on camera, quietly set the unworkable handgun to the side.

That failure is at the heart of a case before the Supreme Court on Tuesday, as the justices grapple with the Biden administration’s attempt to crack down on ghost guns.

At root, a ghost gun is a firearm fabricated outside the regulated industry, meaning it lacks a serial number that would allow the ATF to track down the initial purchase if the weapon is ever recovered at the scene of a crime.

The modern world of 3D printing and online sales of kits and partial weapons has created an explosion of such weapons. The Biden administration has pushed back with new rules classifying the kits and partial weapons as full firearms, making them subject to the Gun Control Act of 1968 and requiring them to have a serial number.

The case turns on the definition of what makes a firearm, which in turn seems to rest on how easy it is to build a ghost gun.

Rick Vasquez, who at one point ran the ATF’s firearms technology branch, told the justices that compiling a kit gun is far more complicated than Mr. Dettelbach would have them believe.

Mr. Vasquez said it takes a lengthy list of tools and the kind of skill, patience and determination “that eludes most nonexperts.” Not to mention more money than the average street criminal is likely to have lying around.

“In sum, the central premise of the government’s argument — that ‘anyone’ can build a fully functioning Glock-style handgun from a parts kit ‘in a matter of minutes’ — is simply wrong,” Mr. Vasquez and the Center for Human Liberty said in their brief to the justices, argued by their lawyer, Bradley Benbrook.

Making an AR-15-style rifle from a kit would be even “more difficult,” they said.

The case is the latest in a string of major gun controversies to reach the high court in the last few years, with the justices first expanding them seemingly tightening Second Amendment rights.

This new case, Garland v. VanDerStok, doesn’t reach those big constitutional questions. Rather, it turns on the language of the Gun Control Act, which governs the sale and purchase of firearms.

The justices will have to figure out if Congress, in writing that law, intended to also cover gun kits, which don’t include complete firearms or even all the materials needed to fabricate one, or whether the law covers what’s known as “80% receivers” which include the un-machined material that can be made into the middle part of a gun.

“A company that sells kits and parts that can be converted into functional firearms, frames, and receivers in minutes — and that are designed and marketed specifically for that purpose — is selling firearms, frames, and receivers,” Solicitor General Elizabeth Prelogar told the justices.

But opponents say the ATF and Attorney General Merrick Garland, who oversees the agency, are stretching the law beyond what Congress intended.

Besides, they say, the danger of ghost guns is oversold because registration numbers don’t actually help solve very many crimes.

“Look, Merrick Garland, if you want all this, you’re going to have to persuade Congress,” said Stephen Halbrook, a leading Second Amendment scholar. “It’s a matter of law, and if Congress thinks it’s a crime problem, it’s for Congress to deal with.”

For decades, the ATF agreed with that take and allowed sale of 80% receivers without regulation, as part of the country’s long tradition of private firearms manufacturing.

But the Biden administration changed that in 2022, saying ghost guns had grown to be too big of a problem. It said the Gun Control Act’s serial number requirement now applies to gun kits and even to a nonfunctional receiver.

The government says ghost guns are increasingly being found at crime scenes. The Department of Justice said police recovered more than 2,300 a month in early 2023, up from about 1,600 in 2021 and about 160 a month in 2017.

The ATF says it traces about 400,000 guns each year, though critics say that figure includes firearms that weren’t recovered at the scene of any crime.

In announcing the new change President Biden likened guns to an Ikea couch, saying even if it came in pieces and required assembly it was still a couch.

“It’s not hard to put together,” Mr. Biden said. “And, folks, a felon, a terrorist, a domestic abuser can go from a gun kit to a gun in as little as 30 minutes.”

The administration lost in the 5th U.S. Circuit Court of Appeals and asked the justices to overturn that ruling.

Mr. Biden has backing of gun crime victims, including Frank Blackwell and Bryan Muehlberger, each of whom lost a child in a 2019 school shooting in California involving a “ghost” gun.

Exact details are murky because the high school student shooter killed himself and the gun kit was apparently purchased by his father, a firearms aficionado who was also deceased by that point.

Mr. Blackwell and Mr. Muehlberger warned the justices of horrifying consequences should the Biden administration lose the case.

“An increasingly internet-literate generation of children will be able to buy DIY ghost guns online,” they argued in their brief. “And a concrete step taken to slow the epidemic of childhood firearm deaths will be replaced with a regime in which children can order these deadly weapons to their doorsteps — the regime under which Gracie and Dominic were killed.”

Experts, though, questioned the value of registration numbers in the first place.

John R. Lott Jr., president of the Crime Prevention Research Center, said it’s rare that guns are left at the scene of crimes unless the shooter was killed or seriously injured — in which case it’s not so important to trace the purchase anyway. When others do leave guns behind they usually aren’t registered to the perpetrator. 

Mr. Lott said registration is “only useful for confiscating guns.”