Currently, it's legal to buy and possess a P80 kit. However, doing anything else is a bit more difficult. California law regulates self-manufacturing of firearms, including a requirement of applying for and being issued a serial number from the California Department of Justice. Slightly problematic as part of that process is engraving the serial number on 3.7 ounces of 17-4 stainless steel that is embedded in the polymer frame. Also problematic is that a self-manufactured handgun must comply with all of the safe handgun roster requirements, including microstamping. And then on top of that, starting in 2022, "firearm precursor parts," which probably includes any 80% receiver will be treated like a firearm and have to be transferred through a Californian gun dealer.