'ghost gun' is a term created to fear monger the public. before the GCA of 1968 almost all guns were unserialized. Serializatin rarely helps in solving any crime, it just tels yuo who the gun was stolen from. Self made firearms get used in less than 0.01% of crimes.

No where in the Constitution is a requirement for a serial number on a weapon. The requirement for serial numbers on firearms began in 1958, and only applied to licensed manufacturers and importers. Firearms made before the Gun Control Act of 1968 do not need to be marked with a serial number.

I dont see exceptions for ghost weapons in the constitution. "Shall not be infringed" does not include exceptions.