This would require going through a dealer for many standard parts you order online now. They would also get a serial number and require a background check as if it were a firearm. Barrels, drop in triggers, bolt carriers etc. Here is a quick form Gun Owners of America has to fill out.
Last Chance to Stop ATF | GOA
The ATF is Rewriting the Definition of a Firearm All comments must reference this document’s docket number ATF 2021R-05, be legible, and include the commenter’s complete first and last name and full mailing address. ATF may not consider, or respond to, comments that do not meet these...
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