Firearm Precursor Parts: Purchase, Home Builds, and Penalties

Firearm precursor parts laws treat unfinished frames and receivers that can readily be completed into working firearms as firearms themselves under federal law. That means buying one goes through a licensed dealer and a background check, building one for your own use is allowed without a serial number, and building them to sell without a license is a felony. The Supreme Court upheld the ATF rule behind this framework in March 2025.¹

Which Unfinished Parts Are Regulated

A raw block of metal or a puddle of polymer is not a firearm. A partially machined lower receiver that needs only a few cuts or holes to function usually is. The line in 27 CFR 478.12 turns on whether the item can “readily” be finished into a working frame or receiver.²

Under the revised rule, a frame is the part of a handgun that houses the sear or an equivalent component; a receiver is the part of a rifle, shotgun, or similar weapon that houses the bolt, breechblock, or breech-sealing component. Only one part of a given firearm is the regulated one and carries the serial number.³

“Readily” does the work in close cases. The ATF looks at how much additional machining the item needs, and at what comes packaged with it. Jigs, templates, drill bits, step-by-step instructions, and marketing that shows how to finish the part all count toward the determination. A kit that would be borderline on its own can cross the line once the tools and directions travel with it.²

The reverse also applies. A frame or receiver that has been cut apart or otherwise destroyed so that it cannot function and cannot efficiently be restored stops being a regulated item.⁴

Buying a Precursor Part

Because a regulated precursor part is legally a firearm, the purchase runs through a Federal Firearms Licensee. The FFL runs you through the FBI’s National Instant Criminal Background Check System before completing the sale.⁵ It cannot ship to your house. It ships to the FFL, where you fill out the paperwork and clear the check in person before taking it home.

The minimum buying age is 21. Federal law lets 18-to-20-year-olds buy rifles and shotguns from a dealer, but a bare frame or receiver is neither, so the handgun-level age floor applies.⁶ Dealer transfer fees typically run $25 to $50, and some states add their own background-check fees on top.

Building a Firearm for Yourself

Federal law lets you make a firearm for your own personal use without a manufacturer’s license, and the 2022 rule did not change that. A self-built gun you intend to keep does not need a serial number, and you do not register it with any federal agency.⁷ ATF marking rules apply to licensed manufacturers and dealers, not to private individuals building for their own collections.⁸ The federal excise tax on firearms also does not apply to a personal build.⁹

The limits are strict. You cannot build with the intent to sell or distribute. That puts you in “engaged in the business” territory, which requires a Type 07 Federal Firearms License. The Bipartisan Safer Communities Act of 2022 tightened the definition: the test is now whether your intent is “predominantly one of obtaining pecuniary gain,” not whether firearm-making is your main livelihood.¹⁰ Building one gun and later selling it is not the same thing as repeatedly building guns to flip, but anyone making firearms with the regular goal of making money needs the license.

The Undetectable Firearms Act

Every home-built firearm has to be detectable. The Undetectable Firearms Act makes it illegal to manufacture, possess, or transfer a firearm that a standard walk-through metal detector cannot pick up, and each major component (barrel, slide or cylinder, and frame or receiver) must produce a recognizable image on an airport X-ray machine.⁶

The benchmark is the “Security Exemplar,” an object containing 3.7 ounces of 17-4 PH stainless steel shaped like a handgun. If your firearm gives a weaker signal than the exemplar on a properly calibrated detector, it fails the test and is illegal. Polymer-framed builds and 3D-printed firearms have to carry enough metal to clear this bar.

Configurations You Cannot Build Freely

Home builders also have to steer clear of items regulated under the National Firearms Act unless they have gone through the NFA process first: short-barreled rifles (barrels under 16 inches), short-barreled shotguns (barrels under 18 inches), and silencers all qualify. Machine-gun conversion devices such as drop-in auto sears and switches are illegal to possess without proper licensing regardless of whether they were made commercially or at home.⁷

When a Home-Built Gun Has to Be Serialized

A privately made firearm has to be marked the moment it enters commercial channels. If you bring an unserialized home build to a dealer for consignment, trade-in, or sale, the FFL must serialize it before selling it or even adding it to inventory. The serial number format is the dealer’s abbreviated license number (first three and last five digits), a hyphen, and a unique identifier.¹¹ The dealer has seven days from receipt to mark it, or must do so before transferring it, whichever comes first. If you already placed a serial number on the firearm, the dealer can adopt it by prepending their license prefix and hyphen.³

Gunsmiths have their own trigger. Drop off a PMF for a same-day repair and no serialization is required. Leave it overnight and the gunsmith has to mark it within seven days or before returning it, whichever comes first.³

Building NFA Items at Home

Making a short-barreled rifle, short-barreled shotgun, silencer, or any other NFA-covered item at home requires ATF approval before any work starts. You file ATF Form 5320.1 (Form 1), including a description of the item with caliber and dimensions and your identifying information, and wait for approval.¹² Applications can be submitted through the ATF’s eForms system.¹³ As of 2026, recent legislation eliminated the $200 making tax that used to accompany each application for silencers, short-barreled rifles, short-barreled shotguns, and certain other NFA items, but the application and approval process itself is still in place. Starting work before you have approval is a federal offense.

Once approved, you engrave the firearm with your name, city, and state as listed on the application and the serial number you assigned on the form. You cannot reuse a serial number you have used on another firearm.¹²

Federal Penalties

The penalties for precursor-part violations run through the same firearm statutes as finished guns. Knowingly selling or transferring a firearm to someone who is legally prohibited from owning one carries up to 10 years in federal prison. Prohibited persons, including convicted felons, people under domestic violence restraining orders, and those convicted of misdemeanor domestic violence, face the same 10-year maximum for possession alone. Repeat violent offenders with three or more prior felony convictions for crimes of violence or drug trafficking face a mandatory minimum of 15 years without parole.¹⁴

Manufacturing firearms for sale without the required federal license is a separate felony under 18 U.S.C. § 922(a)(1).⁶ Convictions can bring imprisonment, fines up to $250,000 under general federal sentencing provisions, property seizure, and permanent loss of firearms rights. Transferring a firearm to a minor is punishable by up to one year, or up to 10 years if the seller had reason to believe the minor would use the weapon to commit a violent crime.¹⁴

State Laws Often Add More

Federal law sets the floor. Roughly 16 states and the District of Columbia have their own laws targeting unserialized firearms and precursor parts, and many go further than the federal baseline. Common additions include serializing every privately made firearm even when kept for personal use, requiring background checks to buy any unfinished frame or receiver, requiring owners to report existing unserialized firearms to state authorities, and banning 3D-printed firearms or the distribution of digital blueprints.

Some states ban polymer or plastic firearms outright, beyond the federal detectability rule. Others require a state-issued serial number instead of one you assign yourself. Because these rules vary and carry their own criminal penalties, check state and local law before starting a build. Clearing federal requirements does not by itself make a project legal where you live.

1
Congressional Research Service. Supreme Court Upholds ATF Ghost Gun Regulation in Bondi v. VanDerStok
2
eCFR. 27 CFR 478.12 – Definition of Frame or Receiver
3
Bureau of Alcohol, Tobacco, Firearms and Explosives. Final Rule 2021R-05F – Definition of Frame or Receiver and Identification of Firearms
4
Bureau of Alcohol, Tobacco, Firearms and Explosives. Summary of Final Rule 2021R-05F
5
Federal Bureau of Investigation. National Instant Criminal Background Check System
6
Office of the Law Revision Counsel. 18 US Code 922 – Unlawful Acts
7
Bureau of Alcohol, Tobacco, Firearms and Explosives. Privately Made Firearms
8
Congressional Research Service. Privately Made and Unmarked Firearms – Overview of ATF Ghost Gun Rule
9
eCFR. 27 CFR Part 53 – Manufacturers Excise Taxes, Firearms and Ammunition
10
Congress.gov. Bipartisan Safer Communities Act – Text
11
Bureau of Alcohol, Tobacco, Firearms and Explosives. 27 CFR 478.92 – Identification of Firearms and Armor Piercing Ammunition by Licensed Manufacturers and Licensed Importers
12
Bureau of Alcohol, Tobacco, Firearms and Explosives. Application to Make and Register NFA Firearm
13
Bureau of Alcohol, Tobacco, Firearms and Explosives. eForms Applications
14
U.S. Department of Justice. Quick Reference to Federal Firearms Laws