You can buy 80% lowers, which require no serial numbers on them if you don't transfer or sell, and it is pretty trivial to complete the machining necessary to make it a 100% lower. It does not require a professional CNC machining shop.
Buy an 80%, machine it to finish it, and you now have a completely unregistered long rifle with no serial number, and it is completely legal.
Buy an 80%, machine it to finish it, and you now have a completely unregistered long rifle with no serial number, and it is completely legal.