First and foremost, we are not lawyers. We do not give legal advice. That said, depending on where you live and how your finished build is configured, there is no need to serialize your 80% lower. Mind you, when I say serialize, I also imply registering. Hop onto the ATF’s webpage and take a stroll through the Q&A section as well as the publications regarding firearms. The ATF is constantly changing and adding to their rules. Courts are constantly upholding or striking down those rules as reasonable or unconstitutional, respectively.
The great part about the publication section is that they archive all of their decisions and all of their responses to public question. Being that there is “limited” space in the Q&A section before the options start to look cluttered, the ATF regularly shuffles the Q&A section making it more resemble a “Frequently asked question” section.
Specifically, take a walk through the firearms Q&A, click on the “General” category, and click on the question “Does an individual need a license to make a firearm for personal use?” The answer there is…. No. Check this link to confirm for yourself: https://www.atf.gov/firearms/qa/does-individual-need-license-make-firearm-personal-use. Then there are document references at the bottom of the response; in this case [18 U.S.C. 922(o), (p); 26 U.S.C. 5822; etc, etc. Don’t just gloss over them, actually look up those documents on ATF’s page and read what they say about your rights. They are the context behind how your rights are established and may end up (God forbid) as the basis of your defense.
It is also worth your time to study up on the Gun Control Act (GCA). In it are restrictions and laws pertaining to the manufacture and sale of firearms. But it only applies to businesses. It makes no mention of the private citizen, in fact, it specifically classifies the targeted audience as “engaged in the business” of dealing in firearms, ammunition, and so on. So if you are reloading at home or making your own firearm for personal use then absolutely none of the document pertains to you. That means, in a nutshell; that you don’t have to conduct a background check on yourself (I would anyway, you can find out if anyone has ever used your name when they got busted), you don’t have to wait 10 days, and you don’t have to serialize and register the firearm.
I think it would be pretty cool to have your own makers mark made out of carbon steel so that you could stamp your AR-15 Lower receiver. Then you could give all of your firearms custom serial codes like names of historical bad-asses. That’s just me though.