Ghost guns are illegal to possess in California unless the firearm carries a valid serial number issued by the state Department of Justice and is registered with the DOJ. Since January 1, 2024, knowingly possessing any firearm without a state or federal serial number is a misdemeanor, and separate penalties apply to manufacturing, selling, transferring, or milling one. California’s rules are among the strictest in the country, and complying with federal law alone is not enough to stay on the right side of them.
What California Treats as a Ghost Gun
A ghost gun, in practical terms, is any firearm or firearm component that lacks a valid serial number. That includes guns built from kits, 3D-printed firearms, and anything machined from a partially completed receiver, sometimes marketed as an “80% lower.”
California’s firearm definition is broader than many people expect. It covers completed firearms along with frames, receivers, and “firearm precursor parts,” which are partially completed components that can be readily finished into a functioning frame or receiver.1California Legislative Information. AB-1621 Firearms: Unserialized Firearms The marketing label on a product doesn’t control. If an item can be readily converted into a functioning firearm, the state treats it as one.
How to Make a Self-Made Firearm Legal
Before manufacturing or assembling any firearm, you must apply to the California Department of Justice for a unique serial number.2California Legislative Information. California Penal Code 29180 The application asks for your full name, address, date of birth, and a description of the firearm you plan to build. The DOJ runs a background check to confirm you’re legally eligible to possess firearms.3California Department of Justice. Unique Serial Number Application
Three groups must apply:
- Anyone planning to build a firearm. You need the serial number in hand before you start.
- Anyone who already owned an unserialized firearm or precursor part. The deadline to apply was January 1, 2024. If you missed it, the firearm is now illegal to possess.
- New California residents. If you move here with an unserialized firearm, you have 60 days after arriving to apply.
Once the DOJ issues the serial number, you have 10 days after completing the firearm to engrave it and notify the DOJ.2California Legislative Information. California Penal Code 29180 After that, every rule that applies to a commercially manufactured gun applies to yours, including safe storage and transfer restrictions.
Engraving Specifications
The engraving must be at least .003 inches deep, with characters no smaller than 1/16 of an inch, placed conspicuously on the frame or receiver where it can’t be easily removed or altered.4Legal Information Institute. Cal. Code Regs. Tit. 11, 5520 – Procedures to Engrave or Permanently Affix a Serial Number Depth is measured from the flat metal surface, not from the peaks of the engraving.
Firearms made from polymer plastic face an additional requirement. You must embed at least 3.7 ounces of type 17-4 PH stainless steel within the plastic frame or receiver, and the serial number must be engraved on that steel insert.2California Legislative Information. California Penal Code 29180 For a 3D-printed firearm, the stainless steel insert is non-negotiable.
What You Still Can’t Do After Serializing
A state serial number lets you lawfully keep a self-made firearm. It doesn’t unlock everything.
You cannot sell or transfer a firearm unless it carries a serial number placed by a federally licensed manufacturer or importer.1California Legislative Information. AB-1621 Firearms: Unserialized Firearms A DOJ-issued number does not qualify. In practice, that makes privately built firearms essentially non-transferable to other individuals in California.
Buying, selling, or transferring firearm precursor parts is illegal unless those parts are regulated under federal law to the same extent as completed frames or receivers.1California Legislative Information. AB-1621 Firearms: Unserialized Firearms That shuts down most of the “80% lower” market that used to supply home builders.
Only federally licensed firearms manufacturers or importers may use a CNC milling machine to manufacture a firearm, frame, receiver, or precursor part.1California Legislative Information. AB-1621 Firearms: Unserialized Firearms Finishing a receiver on a home CNC machine, which was common before AB 1621, is now off-limits without a federal manufacturing license.
Criminal Penalties
Knowingly possessing a firearm without a valid serial number on or after January 1, 2024, is a misdemeanor.3California Department of Justice. Unique Serial Number Application Each unserialized firearm is a separate offense. Someone with multiple ghost guns faces multiple charges.
Building a firearm without first obtaining a serial number carries penalties that depend on the firearm type. For handguns, the offense is punishable by up to one year in county jail, a fine of up to $1,000, or both. For all other firearms, the maximum jail term is six months, with the same $1,000 fine cap.2California Legislative Information. California Penal Code 29180 Each firearm built in violation is a distinct offense, and prosecutors can bring additional charges under other statutes that carry stiffer penalties.
People who are barred from possessing any firearm face far more serious consequences. Federal prohibited categories include anyone convicted of a crime punishable by more than one year in prison, anyone subject to a domestic violence restraining order, and anyone convicted of misdemeanor domestic violence.5Bureau of Alcohol, Tobacco, Firearms and Explosives. Identify Prohibited Persons California’s prohibited categories are broader. A prohibited person caught with any firearm, serialized or not, faces a potential state prison sentence of up to three years.
If a ghost gun is used during a felony, California’s sentencing enhancements can add years to the base sentence. Personal use of a firearm during a violent felony, and gang-related offenses, carry their own additional enhancements.
Narrow Exemptions
Federally licensed manufacturers and importers may produce unserialized components as part of their business, though they must serialize any firearm before sale or transfer. Law enforcement agencies and forensic laboratories can possess unserialized firearms for official investigations and testing.
Antique firearms are also exempt. California generally defines an antique firearm as one manufactured before January 1, 1899, or one that uses fixed ammunition made in or before 1898 for which ammunition is no longer commercially available.6California Legislative Information. California Penal Code 16170 – Antique Firearm Collectors who own these weapons for display or historical purposes don’t need to obtain serial numbers or register them with the state.
Federal Rules Don’t Replace California Rules
In April 2022, the ATF finalized a rule redefining “frame or receiver” to cover partially complete components that can be readily converted into functional firearms, and updated marking, recordkeeping, and traceability requirements for licensed dealers and manufacturers.7Bureau of Alcohol, Tobacco, Firearms and Explosives. Definition of Frame or Receiver and Identification of Firearms
California goes further. The CNC milling ban, the restrictions on precursor part sales, and the absolute ban on transferring self-made firearms are all state-level additions that don’t exist in federal law. Complying with federal rules does not get you compliance with California’s.
What Changes in 2026
Two laws signed in recent years expand the framework starting January 1, 2026.
Senate Bill 704 requires firearm barrels sold separately (not already attached to a firearm) to be transferred in person through a licensed firearms dealer, similar to firearms themselves. Online retailers shipping barrels to California residents must send them to a licensed dealer for final transfer. A first or second violation is a misdemeanor; a third or subsequent violation can be charged as either a misdemeanor or a felony.8California Department of Justice. Information Bulletin 2025-DLE-18
Assembly Bill 1263 broadens the definition of “firearm manufacturing machine” to include any machine marketed or designed to manufacture firearm components or accessories, not just 3D printers and CNC mills. It also creates the concept of a “digital firearm manufacturing code,” covering CAD files, CAM files, and similar digital instructions that can program a machine to produce firearms, frames, receivers, precursor parts, or accessories like silencers and large-capacity magazines.8California Department of Justice. Information Bulletin 2025-DLE-18 Sharing or distributing digital blueprints for firearms manufacturing will face new legal scrutiny in California once the law takes effect.
Traveling Out of State With a Self-Made Firearm
Federal law provides safe-passage protection for transporting firearms between states where you may lawfully possess them. Under 18 U.S.C. ยง 926A, the firearm must be unloaded and stored where it’s not readily accessible from the passenger compartment. In vehicles without a separate trunk, the firearm and ammunition must be in a locked container other than the glove compartment or center console.9Office of the Law Revision Counsel. 18 U.S. Code 926A – Interstate Transportation of Firearms
The catch for self-made firearms is the “lawfully possess” requirement at both ends. A firearm you built and properly serialized in California is legal here, but the destination state may have its own restrictions on self-made firearms, required registrations, or feature-based bans that make the gun illegal there. Safe-passage protection only works if the firearm is lawful at both origin and destination. Check the destination state’s rules before you drive.