Are 80% lowers and home-built guns legal in California?

Penal Code 29180 sets three duties for a firearm that has no state or federal serial number:

1. Apply to DOJ for a unique serial number before manufacturing or assembling.
2. Engrave or permanently affix that number within 10 days of manufacture or assembly, to the federal marking standard.
3. Notify DOJ that the number has been applied.

DOJ takes applications online through its Unique Serial Number Application process. Under Penal Code 29182, DOJ may deny an application if the described firearm would be an unsafe handgun, an assault weapon, a machinegun, a .50 BMG rifle, a short-barreled rifle or shotgun, or another prohibited firearm. Subdivision (e)(2) states that the chapter "does not authorize a person to manufacture or assemble an unsafe handgun."

A person who builds a firearm also cannot sell it or transfer ownership of it, unless the person is a federally licensed manufacturer. The exception is surrender or sale to a law enforcement agency (Penal Code 29180(d)).

Generally no, unless the part is serialized and treated as a firearm. AB 1621, an urgency law that took effect June 30, 2022, rewrote these rules.

• Definition. A "firearm precursor part" is any forging, casting, printing, extrusion, machined body, or similar article that "may readily be completed, assembled or converted" into a frame or receiver, or that is marketed for that purpose (Penal Code 16531).
• Sales ban. It is unlawful to purchase, sell, offer to sell, or transfer a precursor part that is not a "federally regulated firearm precursor part" (Penal Code 30400). That term means a part federal law treats as a firearm and that carries a serial number from a federal licensee where required (Penal Code 16519).
• Possession. Knowingly possessing a firearm without a valid state or federal serial number is a misdemeanor (Penal Code 23920(b)). New residents have 60 days after arrival to apply for a number (Penal Code 29180(c)).

Penal Code 29185 says that no person other than a state-licensed firearms manufacturer "shall use a computer numerical control (CNC) milling machine or three-dimensional printer to manufacture a firearm." It is also unlawful to sell, transfer, possess, purchase, or receive a CNC milling machine or 3D printer whose "sole or primary function" is manufacturing firearms, unless the recipient is a state-licensed manufacturer. A violation is a misdemeanor.

AB 2047 was signed September 26, 2026 (Chapter 461). It adds the California Firearm Printing Prevention Act to the Civil Code, starting at section 3273.631. The chaptered text has no urgency clause, so under the usual rule it takes effect January 1, 2027.

The act is aimed at printer makers and sellers:

• Starting no later than July 1, 2027, DOJ must check each quarter whether ASTM International has published standards for "firearm blocking technology" in 3D printers.
• If such standards exist and show the technology is feasible, DOJ must publish guidance or regulations within 24 months.
• One year after that guidance, it becomes unlawful to sell a 3D printer in California that lacks qualifying blocking technology.
• If no standard exists as of July 1, 2029, DOJ's duty to keep checking ends.

No printer sales rule is in force yet.

The federal definition at 27 CFR 478.12(c) treats a partially complete frame or receiver, including a parts kit, as a frame or receiver if it "may readily be completed." In Bondi v. VanDerStok, decided March 26, 2025, the U.S. Supreme Court rejected a facial challenge to that rule in a 7 to 2 decision. The California statutes above apply in addition to federal law.