I have sold into california one time and found it is really not a big deal. Just a little extra work.
The selling FFL dealer has to register as an FFL dealer with the California DOJ. Easily done on-line. Then, when a gun is wanting to be shipped by that dealer to a receiving dealer in California. The shipping dealer requests for approval to ship from the California DOJ. Again, can be done on-line or by mail. You simply state what dealer in California the gun is being shipped to, then the DOJ confirms approval to ship with a letter and a confirmation number. That letter has to acompany the shipment to the receiving dealer. As i recall, when i did it, i received confirmation back via on-line within a few hours. Very fast turnaround.
It is basically a redundant process that allows California to confirm that you are in fact shipping to a licensed dealer.
Now... I do not know what the process might be if you are an individual shipping to California.
