Have you ever heard of an "80% lower receiver"?
An 80% lower is not a fully built AR receiver and therefore has no serial number. It is just a hunk of metal in the eyes of the law so it is legal.
You drill your own original holes yourself and build the gun.
I have 4 more on order along with a new CNC mill. I'm wondering which will get here first. I'm a smith and to be a smith you have to be a pretty fair machinist.