Ghost Guns
This term is commonly used to refer to home crafted firearms that may not have a serial number. Current Federal law allows the home manufacture of a firearm, but not its sale or transfer to another person . While this legal situation may change, the creation of firearms in a home workshop will always be possible.
One major part of this controversy is that companies have made parts of pistols and AR style rifles that are incomplete. They sell these unfinished parts along with fixtures, jigs and the cutting bits that allow the home craftsman to finish the parts with home workshop tools such as a drill press or router. Banning this sort of thing will still leave options for shop crafted firearms. Additive manufacturing (3D printing) is now widely available, relatively affordable, and more sophisticated than ever. The same is true for table top milling equipment. Both modalities are CNC controlled from a lap top. The software and data files for many firearms prats have been widely distributed. That horse left the barn years ago.
More basic in approach than machining is the field of bolted together home workshop AR receivers. A quick Google search for “Bolted Together Lower” will come up with functioning AR style guns made by cutting flat plates to shape and bolting them together to make the whole lower receiver. Not ideal in terms of functionality, reliability, or durability, yet many of these efforts result in a fully functioning gun.
Legal or not, “Ghost Guns” will never disappear.