California law prohibits anyone from 3D printing a firearm (or most firearm parts) without first getting a manufacturing license from the state[1]:
> (b) A person, firm, or corporation shall not use a three-dimensional printer to manufacture any firearm, including a frame or receiver, or any firearm precursor part, unless that person, firm, or corporation is licensed pursuant to Chapter 2 (commencing with Section 29030).
To get a license from the state, you must first have a federal firearms manufacturing license. California has additional requirements such as fees ranging from $250-600, yearly background checks of any employees who handle guns, a CA DOJ certificate of eligibility for every business owner, stricter building security measures than a FFL type 07, records of the serial numbers of all firearms produced, and allowing the local police to inspect the facility regularly. Firearm manufacturers are not allowed to sell guns to individuals, so you would not be able to take possession of your 3D printed gun until you got the model approved on California's roster, transferred it to a firearm dealer, then went through California's standard process for buying a gun, which I describe here.[2]
1. https://leginfo.legislature.ca.gov/faces/billTextClient.xhtm...
To be fair, there is a decent chance that law would be deemed unconstitutional in federal court. The only problem is nobody making their own guns at home has the capital to try and bring this case to any larger court. The ones with capital to do so, established gun manufacturers, have no incentive to fight it, a couple hundred bucks is literally nothing to them. They spend more money on office coffee each day.
This is sad to hear. My knowledge of the situation was out of date of what things were like around 2020 in California. Many firearms companies today started off with one person creating an innovative design and then scaling that up. All of these barriers to entry just lock out hobbyists and potential disrupters out of the market.