In CA there aren't any restrictions on non-residents citizens that are above and beyond those of resident citizens.
Maybe I'm misunderstanding the question... There are some minor differences between citizen and non-citizens; but those likely wouldn't hold up anyhow...
Thank mjones for your reply. Actually there can be a BIG difference. This isn't a situation of citizen vs. non-citizen, rather its all about citizens who are U.S. residents (physically residing in one of the 50 States) vs citizens who are not U.S. residents. I'm looking for States that bar simple firearms possession (especially handguns) to non-U.S. residents who are still nevertheless American citizens. Yes we do exist!
This prohibition need not be direct but could be more subtle, for example requiring registration of a handgun (as a condition of possession) via an application form that specifically requires data that a non-U.S. resident may not be able to provide, such as a current U.S. physical address, State ID or Driver's License number, etc.
Even expatriate Americans (living abroad) who still are registerd to vote in the State of their former residence, maintain a home, etc. may still 'lose' their physical residency status in that State if they aren't physically present in that State for a certain amount of time during a calendar year (probably varies by jurisdication but let's say 6 months plus 1 day). Unfortunately, it is something that most Americans living overseas don't even think about or realize and may even be unwittingly committing a felony if they use their stateside residence as a basis for claiming state residency when they are not actually physical residents (arcane legal reasons why this is so).
Despite the 'rareness' of this sitution, I'm still trying to research this issue and would appreciate a better understanding of California's firearm's possession laws (cites to the relevant statutes would be extremely helpful, thanks!).