From Bearing Arms:

So-called “ghost guns” are really just any firearm that’s created without a serial number. Making such weapons isn’t illegal. It’s not even illegal in many places that have supposedly banned these kinds of weapons–mostly because the bans only impact kits and not the act itself. Making a firearm yourself is something that a lot of Americans enjoy.

Yet some people are completely uncomfortable with the idea that someone could build a firearm and not have to get permission from the government to do so.