It's not a problem. "Ghost gun" refers to any gun without a serial number, whether it was manufactured without one or simply scratched off. Even under that exceedingly broad definition, these unserialized firearms represent less than 0.1% of all guns in existence.

3D printers aren't the only way to make a gun. Traditional tooling also works.

Manufacturing guns for personal use is completely legal in most US jurisdictions.

It's already illegal to manufacture and distribute or sell guns without a license.

"Ghost guns" are a boogeyman invented to sell more surveillance.