I suggest starting here:I'm sorry I can't answer your question but I do have one to add.
I've got a BIL in FBI and we go visit him last year. While there we go shooting at a range and store front is stacked with hand guns WAY beyond what is here in CA (supports your point). They also had silencers and I said, can't buy one or a nice handgun cause I'm from CA. Shop owner says he's not sure why I couldn't buy one, I dismiss it and keep browsing. Later at BIL house he says he's not sure what would prevent me from buying whatever handgun I wanted and returning (via air) to CA with it. He was shooting from the hip so to speak and didn't have the law in front of him. I said well, when I hand him my CA Driver License for identification he would run it and not be able to sell to me. He countered and said (in that State) when they run my DL it is just ran through a Fed background check and not a state so I would pass that and then buy my gun. Again, no law book in front of him but that's what he was supposing and I imagine was also why shop owner wanted to sell to me.
Now, transport via air and TSA is a whole other issue, or NOT an issue as I found out. They have NO idea what they are looking at and merely need to know the firearms aren't loaded so passing TSA would have been no problem.
So, is it really that easy to buy a gun out of state and then walk back into CA with it? I understand it may be illegal to posses it but so were mags at one point and the evil AR's, I'm mainly asking about the purchase.
Am I secretly making plans for my next trip? NO WAY, NEVER, wouldn't even think about it.
Thats the hard part with the handguns, there are so many other ways to get them.The "challenge" could occur at a range or public shooting area in CA. I could only imagine the interaction with a BLM or FS Ranger were they to encounter some CA non-compliant weapons in your possession during a routine check at a public shooting area.