I believe California has some laws that specifically apply to semiautomatic firearms, which an AR15 lower is not. The lower is incapable of semiautomatic fire (as in the gas blowback/piston system is in the upper).
They sell single-shot .50 BMG uppers, non-semiautomatic AR15 uppers do exist.
I believe the majority of stuff California regulates attaches to the upper anyways, which isn’t a firearm under federal law (unsure if Cali is weird about that). Bump stocks and responsive triggers are the only things I can think of California might regulate that go on the lower, and last I heard the ATF was tracking those down as an NFA violation.
The lower is the serialized part, and is regulated in isolation as a firearm in every instance I am aware of (but I am not familiar with California gun laws specifically).
Unless a CA-licensed attorney knowledgeable in California gun laws and ATF regulations specifically told you that a particular CA law applying to "semiautomatic firearms" does not apply to your AR-15 lower in isolation, do not listen to random internet comments about it especially when they begin with "I believe." Regardless of where you fall on the issue politically or ideologically, it's an objective fact that California's government is hostile toward its citizens possessing and bearing firearms, and being fuzzy on what is or is not a felony is a risky proposition.
If you're wrong, best case scenario you will lose all your guns, worst case you will end up in federal prison.
>it's an objective fact that California's government is hostile toward it's citizens possessing and bearing firearms
A state puts restrictions on semi-automatic weapons (one model which has been used to kill hundreds or thousands of people in random shootings) and here it's described as "being hostile to people possessing firearms". For most non-americans (which is my case) this will always sounds so strange
It's relative. Think of the US as a collection of small countries, comparable to the EU, and it's a little easier to understand.
I know relatively little about European gun laws, but I imagine that the gun laws in Romania and the Czech Republic are quite different, and that the Czech would argue Romania is "hostile toward it's citizens possessing and bearing firearms."
Texas would argue California is. California would argue the UK is.
Texas has more regulations than people think. It's the only "gun-friendly"[0] state I can think of that not only requires classes, but also a certain level of marksmanship - if memory serves you have to put 7 rounds in a piece of paper at 7 yards within 10 seconds or something.
I am certainly not suggesting anyone rely on me for legal advice, to be clear.
But no, my understanding in CA is unlike the Feds, California does not have the concept of constructive possession as it applies to assault weapons. As such, separated parts cannot constitute a CA AW, unless the lower is already registered as such, or said lower is on the list of named CA AWs.
It is technically not an assault weapon in that configuration; however, depending on the DA, they may still come after you under PC 12280(a), stating that you are attempting to possess an assault weapon. The sticking point for them is showing intent, but they have convicted on possession and research of how to assemble an assault weapon.
They certainly could charge someone, and maybe even win; I’m not suggesting anyone rely on this as a defense, but that is what the law states. Short of any other aggravated reasons to charge you, it’s unlikely a DA would have any interest.
CA regulates magazines and pistol grips (in combination with other parts) which attach to the lower receiver. Not sure you are fully correct in saying the lower is not a semiautomatic firearm. While the lower by itself is clearly not a complete and functional firearm, and not capable of firing without an upper, the sear/disconnector, which determines semi/full auto is part of the lower.
They sell single-shot .50 BMG uppers, non-semiautomatic AR15 uppers do exist.
I believe the majority of stuff California regulates attaches to the upper anyways, which isn’t a firearm under federal law (unsure if Cali is weird about that). Bump stocks and responsive triggers are the only things I can think of California might regulate that go on the lower, and last I heard the ATF was tracking those down as an NFA violation.