Are 80% Lowers Required to Be Serialized? Federal and State Rules

Under federal law, 80% lowers are not required to be serialized if you finish one for your own personal use and you are legally allowed to own a firearm. No serial number, no registration, no paperwork. That rule is narrower than it sounds, though, because several common situations pull a home-built firearm back into the serialization system, and more than a dozen states have their own requirements that override the federal baseline.

The Federal Personal-Use Rule

The ATF states it plainly: you don’t have to add a serial number or register a privately made firearm if you are not engaged in the business of making firearms for livelihood or profit.¹ Once you machine the trigger pocket, drill the pin holes, and cut the safety selector opening, you have a firearm under 18 USC 921.² You just don’t have to mark it if it stays yours.

Two conditions attach to that freedom. You cannot be a prohibited person, and you cannot be building with the intent to sell. Cross either line and the personal-use exemption disappears.

Why the 80% Kit You Buy Today May Already Be a Serialized Firearm

An 80% lower is a partially machined piece of aluminum or polymer that resembles a firearm’s lower receiver but hasn’t been finished to the point where it can house a fire control group. The “80%” label is an industry term, not a legal standard. What matters is where federal regulations draw the line between an unregulated blank and a “frame or receiver.”

The ATF moved that line in April 2022. The final rule, effective August 24, 2022, redefined “frame or receiver” to include partially complete, disassembled, or nonfunctional frames and receivers, along with weapon parts kits, if they are designed to or may “readily be” completed into a functional firearm.³ Any prior ATF determination that a specific partially complete receiver was not a firearm became invalid on that date.

The rule hit “buy-build-shoot” kits hardest. A partially finished receiver sold together with a compatible jig or template now counts as a frame or receiver, because someone with basic hand tools and online instructions can readily finish the job.⁴ That means it’s a firearm at the point of sale, must be serialized by the manufacturer, and must go through a licensed dealer with a background check. If you bought a kit like that, the lower already has a serial number on it, and the question of whether you have to serialize it never comes up.

A true raw blank that hasn’t reached a stage where it’s clearly identifiable as an unfinished firearm component falls outside the definition.³ That gray area is what fueled years of litigation, and in Bondi v. VanDerStok, decided March 26, 2025, the Supreme Court ruled 7–2 that the ATF’s rule is not facially inconsistent with the Gun Control Act.⁵ The rule is settled law.

So the first thing to check is what you actually bought. A kit with a jig, sold after August 2022, should already be serialized. A bare 80% lower with no jig, sold as an unregulated blank, is the item this article’s core question is really about.

When You Do Have to Serialize a Home Build

You Take It to a Federal Firearms Licensee

This is the trap most people miss. If your home-built firearm enters the inventory of a federal firearms licensee for any reason (consignment, trade-in, transfer, or gunsmith work that requires taking it into inventory) that FFL must add a serial number within seven days of receipt or before the firearm leaves, whichever comes first.¹ The serial number must begin with the dealer’s abbreviated license number followed by a hyphen and a unique identification number.⁶ The gun didn’t need a serial while it sat in your safe, but the moment a dealer takes it in, that dealer has to mark it.

You’re Building an NFA Item

If your project is a short-barreled rifle, a suppressor, or another item regulated under the National Firearms Act, the personal-use exemption doesn’t apply. You must file an ATF Form 1 and wait for approval before doing any work that would create the NFA firearm.⁷ The form requires you to assign a unique serial number and engrave it on the receiver along with your name, city, and state, at a minimum depth of .003 inches and a minimum print height of 1/16 inch.⁸

You Cross Into “Engaged in the Business”

Building with the intent to sell, rather than for personal use, makes you a manufacturer under federal law, and manufacturing firearms without a federal firearms license is a crime.⁹ A 2024 ATF rule listed factors that create a presumption of dealing: reselling firearms within 30 days of buying them, advertising guns for sale repeatedly, keeping profit-and-loss records for gun transactions, and setting up merchant payment services for firearm sales.¹⁰ Building one gun for yourself and later deciding to sell it is a different situation from building guns to flip.

You Sell or Transfer It

Federal law lets an unlicensed person sell a personally made firearm to another unlicensed person in the same state without a serial number, as long as the seller isn’t in the business and has no reason to believe the buyer is prohibited. Interstate transfers between unlicensed individuals have to go through a licensed dealer, which triggers the FFL serialization requirement described above.¹ And many states require every private transfer to run through a dealer or mandate background checks for private sales, which also forces serialization at the dealer stage.

Who Cannot Build a Firearm at All

The personal-use exemption only reaches people who can legally possess a firearm. Federal law bars several categories from possessing or receiving any firearm, serialized or not:

Anyone convicted of a crime punishable by more than one year in prison.
Fugitives from justice.
Unlawful users of, or people addicted to, a controlled substance.
Anyone adjudicated as mentally defective or committed to a mental institution.
Anyone convicted of a misdemeanor crime of domestic violence or subject to a qualifying restraining order involving an intimate partner or their child.
Anyone discharged from the military under dishonorable conditions.
People who have renounced U.S. citizenship.
People under indictment for a crime punishable by more than one year, who may not receive or transport firearms.

A prohibited person who builds a firearm from an 80% lower commits a federal offense whether or not the finished weapon carries a serial number.¹¹

State Laws Can Require Serialization Even When Federal Law Doesn’t

More than a dozen states and the District of Columbia restrict or ban 80% lowers and unserialized home-built firearms, and some of those laws go well beyond the federal baseline. The patterns include:

Requiring you to apply for a serial number from a state agency and engrave it on the receiver before you do any milling, usually with a background check and a fee.
Requiring a serial number on every privately made firearm regardless of whether you plan to sell it.
Banning the sale, transfer, or possession of unserialized firearms outright.
Treating 80% lowers as finished firearms in their unfinished state, so purchase restrictions, background checks, and registration apply from the moment you acquire the blank.

Because the specifics and the penalties vary so much, check your own state’s current law before you buy or machine an 80% lower. Local ordinances can add another layer on top.

Penalties

The federal consequences for getting this wrong are serious. Willfully violating the Gun Control Act, including unlicensed manufacturing, unlicensed dealing, or a licensee’s failure to comply with serialization rules, carries a maximum of five years in federal prison, a fine up to $250,000, or both.¹² Some prohibited devices, like machine gun conversion devices, can bring up to ten years.

State penalties vary but can be equally harsh, and some states treat possession of an unserialized firearm as a felony even if you built it in compliance with federal law. A conviction at either level can permanently strip your right to possess any firearm. Professional engraving to ATF specifications typically runs around $50 at a gunsmith, which is a trivial cost against that exposure if you’re in a state or situation where a serial number is the safer call.

1
Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Privately Made Firearms
2
Office of the Law Revision Counsel. 18 USC 921 – Definitions
3
Federal Register. Definition of “Frame or Receiver” and Identification of Firearms
4
eCFR. 27 CFR 478.12 – Definition of Frame or Receiver
5
Supreme Court of the United States. 23-852 Bondi v. VanDerStok
6
ATF eRegulations. 27 CFR 478.92 – Identification of Firearms and Armor Piercing Ammunition by Licensed Manufacturers and Licensed Importers
7
Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Chapter 6 – Making NFA Firearms by Nonlicensee
8
ATF eRegulations. 27 CFR 479.102 – Identification of Firearms
9
Office of the Law Revision Counsel. 18 USC 922 – Unlawful Acts
10
Federal Register. Definition of “Engaged in the Business” as a Dealer in Firearms
11
Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Identify Prohibited Persons
12
Office of the Law Revision Counsel. 18 USC 924 – Penalties