Whether you buy existing or make from 80%, the laws are the same. An SBR requires name/city/st & SN for example. A regular rifle does not.Having never looked into it before. I was curious what the laws were regarding an 80% lower that AFAIK doesn't have a SN and no-one but the owner knows if it's going to be a pistol, SBR, or a rifle. I guess the next question is what are the laws regarding a non-serialized rifle, pistol, whatever. I know I have an old 22 with no SN but it's exempted. Do you have to put a SN on it? If so who do you HAVE to tell? If anyone.
If you SBR your 22, you would have to add a SN along with your info. You tell ATF this when you register it on form 1.