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ITT: Many people make the incorrect assumption that, aside from some specific state- and city-level ordinances, there is such a thing as "firearm registration" in the US.
This is a myth largely promulgated by TV shows about cops. There is no nationwide firearms registry in the United States.
Regulation != Registration.
What does happen is that when a firearm is legally purchased from a dealer (FFL), the buyer must submit to and pass a federal background check. Records of these are not retained centrally, but each FFL dealer must maintain their own records of their own sales, indefinitely, as long as they remain in business. Ready to be reviewed at any time by the cops or ATF. Failure to do so can land the dealer in very deep shit. Centralized collection of firearm transaction records is prohibited by federal law, under the assumption that such a central record would be used to target, harass, and confiscate arms from their owners whenever the government felt like it (which is probably about a 50/50 mix of paranoia and accurate prediction).
Some states also require their own more strict background checks. States also vary in how strict or lax they are in requiring background checks for transfers between private individuals, and not a dealer. There is no federal requirement for private sellers to conduct a background check to transfer ownership of a firearm except across state lines, but many states themselves do have such a requirement. Further, transfers and sales of handguns often have stricter state level requirements vs. long guns (rifles and shotguns).
3D printing a firearm (receiver) does not allow any individual to "evade" any type of mythical "registration," which by and large does not exist -- as above. It does, however, allow a suitably motivated individual who could not pass a federal or state background check to get their hands on a presumably functional firearm.
It is perfectly legal for a person who is not prohibited from possessing a firearm to begin with to manufacture their own firearm, via 3D printer or otherwise, on a federal level. Some states have already enacted restrictions on this, however.
It is already illegal for a person prohibited from possessing a firearm to A) manufacture a firearm, B) possess any firearm (duh), or C) possess ammunition for any firearm, whether they are found to have a firearm to put it in or not.
It is already illegal for a person to manufacture firearm(s) for the purposes of selling, trading, giving away, or otherwise putting into the possession of others, if they are not a federally licensed firearms manufacturer.
It is already illegal to provide access to a firearm to a person prohibited.
It is already illegal to use any manufacturing method (even a 3D printer) to produce a firearm or component that is otherwise illegal or restricted NFA item such as a machine gun, suppressor, short barreled rifle, etc., etc.