Chief Justice John Roberts and Justice Amy Coney Barrett joined their three Democrat-appointed colleagues in allowing the Biden administration to enforce these rules while the Biden administration appeals a lower court ruling.

Democrats have pushed for serial numbers on all parts of guns, saying it's a way to protect public safety and prevent violent crime. "The public-safety interests in reversing the flow of ghost guns to dangerous and otherwise prohibited persons easily outweighs the minor costs that respondents will incur," Solicitor General Elizabeth Prelogar wrote in a court filing.

"It will help to ensure that law enforcement officers can retrieve the information they need to solve crimes," U.S. Attorney General Merrick Garland claimed last year. "And it will help reduce the number of untraceable firearms flooding our communities."

Biden's expanded use of serial numbers is aimed at stopping the production of homemade guns, now called "ghost guns" by gun control advocates. Homemade guns have been around since before the United States became a country, and it was never terribly difficult to make a gun with simple machine tools. But now their production has become nearly impossible to regulate. With 3D metal printers, people can now make weapons that are indistinguishable from those purchased in stores.

But, even if homemade guns had serial numbers, that still wouldn't help law enforcement solve crimes. TV shows such as "Law & Order" do not reflect reality.

In theory, if criminals leave registered guns at a crime scene, the serial numbers can be used to trace the weapons back to the perpetrators.