From The Federalist:

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 even 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 3-D metal printers, people can now make weapons that are indistinguishable from those purchased in stores.