Are 80 Percent Lowers Illegal? Federal Rule, State Laws, Penalties

Owning an 80 percent lower is not automatically illegal, but the rules around buying one have changed. A 2022 ATF rule, upheld by the U.S. Supreme Court in March 2025, treats many unfinished frames and receivers as regulated firearms. That means a commercial purchase now typically runs through a licensed dealer, with a serial number and a background check. Building a firearm for your own use is still legal under federal law in most places, and more than a dozen states have layered on their own restrictions.

What Changed at the Federal Level in 2022

ATF Final Rule 2021R-05F took effect in August 2022. It rewrote the definition of a firearm “frame” or “receiver” to cover partially complete, disassembled, or nonfunctional versions that can quickly and easily be made functional. It also brought weapon parts kits containing those components under the Gun Control Act’s definition of “firearm.” The “buy-build-shoot” kits that packaged an unfinished receiver with jigs, parts, and instructions were the clear target.¹²

Practically, that means a manufacturer or seller of a covered component has to hold a federal firearms license, serialize each unit, and run buyers through the National Instant Criminal Background Check System with an ATF Form 4473. Same process as any completed firearm.³

There is no grandfather clause. The ATF stated that partially complete frames and receivers it hadn’t classified as regulated items before the rule are not grandfathered and have to be re-evaluated under the new definitions. If you already own one and it now fits the rule, transferring it through a commercial channel triggers the same dealer, serialization, and background-check requirements.¹

The Supreme Court Settled the Legal Challenge in 2025

The rule was challenged almost immediately. A federal district court in Texas vacated it, the Fifth Circuit agreed, and the case went to the Supreme Court as Bondi v. Vanderstok. On March 26, 2025, the Court reversed and held that the ATF’s rule is not facially inconsistent with the Gun Control Act. It pointed to kits like the Polymer80 “Buy Build Shoot” product, which the record showed could be assembled into a working pistol in roughly 20 minutes with common tools, as clearly qualifying as firearms. The Court also accepted that the statutory terms “frame” and “receiver” can describe objects that are not yet fully finished.⁴

The ruling has a limit worth knowing. The Court decided only the facial challenge, meaning it did not decide whether the rule can lawfully be applied to every possible product. Specific kits or receivers may still be challenged in later cases. For now, the rule stands as written and is enforceable nationwide.⁴

How the ATF Decides Whether a Blank Is Regulated

The key word in the rule is “readily.” If an unfinished frame or receiver can readily be completed into a functional one, it’s regulated as a firearm. There’s no bright line at 80 percent, or any other percentage. A raw block of aluminum is explicitly excluded. A near-finished receiver clearly falls within the rule. Everywhere in between, the ATF weighs eight factors:⁵

Time the finishing process takes
Ease of the work
Expertise required
Equipment needed
Availability of additional parts
Expense of the conversion
Scope of material to be removed or changed
Feasibility, including whether finishing would damage the part

Anything sold alongside the blank counts too: jigs, templates, tools, instructions, marketing. A bare blank with no guidance is treated differently from the same blank shipped with a drilling jig and a link to a tutorial. The ATF’s own example: a parts kit sold with a compatible jig or template is a frame or receiver, because someone with common hand tools and online instructions can finish it.²

Some vendors have moved their products further from the “readily convertible” line by selling less-finished blocks, dropping the jig, or marketing raw forgings. Whether a redesign actually places a product outside the rule depends on ATF classification decisions, which are made case by case and can shift as the agency evaluates new designs.

Building a Firearm for Your Own Use

Federal law still lets an individual manufacture a firearm for personal use. You don’t need a federal firearms license to do it, and you aren’t required to serialize a gun you build for yourself, as long as you aren’t making it to sell or as a business.⁶

What changed is the front end. You used to be able to order an 80 percent lower online with no paperwork, no background check, and no serial number, then machine it at home. Now, if the component you start with qualifies as a readily convertible frame or receiver, you have to acquire it through a licensed dealer with a background check. The machining you do afterward is still yours to do, and the finished firearm does not need a serial number for personal possession under federal law.

One requirement is easy to overlook. Any home-built firearm must be detectable by standard security screening. If you use a polymer frame or 3D-printed parts, the finished gun still has to contain enough metal to trigger a walk-through metal detector. This comes from the Undetectable Firearms Act and applies to every firearm, not just commercial ones.⁶

Who Cannot Build One at All

The personal-use exception does not override the federal ban on possession by prohibited persons. If you fall under 18 U.S.C. § 922(g), building a firearm is just as illegal as buying one. Those categories include people convicted of a felony, people subject to certain domestic violence restraining orders, unlawful users of controlled substances, and anyone dishonorably discharged from the military, among others. Possession by a prohibited person carries up to 15 years in federal prison.⁷⁸

Building something covered by the National Firearms Act, such as a short-barreled rifle, a short-barreled shotgun, or a suppressor, has its own track. You must file an ATF Form 1, pay a $200 tax, submit fingerprint cards and a passport photo, and wait for approval before you start. Once approved, the receiver has to be engraved with your name, city and state, caliber, and a serial number, meeting the ATF’s minimum depth and size requirements. This applies whether you began with a commercial stripped lower or an 80 percent blank.

States That Go Further Than Federal Law

More than a dozen states have added restrictions on unserialized firearms and unfinished components, and many are stricter than the federal rule. Several require serial numbers and background checks for the purchase of firearm component parts, including unfinished frames and receivers. Some also require owners of existing unserialized firearms to report them to state authorities. A handful restrict 3D-printed firearms, prohibit distribution of digital gun blueprints, or require a state license to make firearms with additive manufacturing.

The states with the most comprehensive restrictions, combining component serialization, background checks, and reporting of existing unserialized firearms, include California, Connecticut, Hawaii, Maryland, Massachusetts, New Jersey, and New York. Colorado, Nevada, Oregon, Rhode Island, Vermont, and Washington have adopted varying pieces of the same framework. These laws change often, so confirm your state’s current statutes before buying, building, or transferring any unserialized firearm or component.

Federal Penalties If You Get It Wrong

Dealing firearms as a business without a federal firearms license: up to five years in prison, a fine up to $250,000, or both, plus forfeiture of the firearms involved.⁹
Willful violations of other federal firearms provisions: up to five years, a fine, or both.⁸
Prohibited person in possession of a firearm: up to 15 years.⁸
Possession of prohibited devices such as machine gun conversion devices or unregistered silencers: up to 10 years, whether the item came from an 80 percent build or a commercial source.⁶

State penalties can stack on top of federal charges. In states that require serialization of personally manufactured firearms, possessing an unserialized gun can be its own criminal offense even if the build was legal under federal law.

Carrying a Home-Built Firearm Across State Lines

The federal safe passage provision at 18 U.S.C. § 926A protects transport through states where local law might otherwise apply, but only under conditions. You must be traveling between two places where you can legally possess the firearm. The gun must be unloaded and not readily accessible from the passenger compartment. In a vehicle with no separate trunk, it has to be in a locked container other than the glove compartment or center console.¹⁰

The protection is narrow. It covers passing through a state, not stopping and staying, and it requires legal possession at both ends of the trip. Building an unserialized firearm in a state that allows it and then driving to a state that bans unserialized firearms puts you outside the safe passage protection once you arrive. If you’re flying, standard TSA rules apply, and the destination state’s law still governs when you land.¹¹

1
Bureau of Alcohol, Tobacco, Firearms and Explosives. Summary of Final Rule 2021R-05F
2
Bureau of Alcohol, Tobacco, Firearms and Explosives. Final Rule 2021R-05F – Definition of Frame or Receiver and Identification of Firearms Overview
3
Federal Register. Definition of Frame or Receiver and Identification of Firearms
4
Supreme Court of the United States. Bondi v. Vanderstok (Opinion)
5
Regulations.gov. Final Rule: Definition of Frame or Receiver and Identification of Firearms
6
Bureau of Alcohol, Tobacco, Firearms and Explosives. Privately Made Firearms
7
Office of the Law Revision Counsel. 18 USC 922 – Unlawful Acts
8
Office of the Law Revision Counsel. 18 USC 924 – Penalties
9
Bureau of Alcohol, Tobacco, Firearms and Explosives. Do I Need a License to Buy and Sell Firearms?
10
GovInfo. 18 USC 926A – Interstate Transportation of Firearms
11
Transportation Security Administration. Transporting Firearms and Ammunition