Gun Control & RKBA

A ghost gun has no serial number and cannot be traced to an owner if it's found by law enforcement.

Answer: A ghost gun, as the name implies, is a slippery item -- hard to pin down and track. Here's how a ghost gun is made.

The gun's maker begins with a "receiver," the part of a gun that projects the bullet and houses the mechanical components of the firearm itself. A finished receiver is considered a working gun and is subject to regulation and control, while an unfinished receiver is not. An individual can convert an unfinished receiver into a working gun, without having to register the gun and provide a serial number. Without registration, the gun can't be traced; hence the industry term for a firearm created from an unfinished receiver: “ghost gun.” It is possible to create “ghost” handguns all the way up to “ghost” semi-automatic weapons.

http://www.criminaldefenselawyer.com/resources/what-a-ghost-gun.htm