Most "ghost guns" use factory made parts for pressure bearing components.

US is a rare country that treats only serialized parts as "guns" and don't regulate actual gun parts. This allow Americans to trivially "build guns at home", but easd of actually building guns is extremely exaggerated.

Also, there's little in 3D printers that actually makes it easy to build gun parts. It's all cosmetics. Real illegal ghost guns are always made with plumbing supplies.