I've only sent 1 firearm to California and it was an AR10
It was a bit of a pain in the butt because I had the buyer send me a 5rd mag and bullet button. I installed the bullet button and kept the normal mag release. Then I used a TX FFL to send to a CA FFL so he could make sure I was doing everything by the book.
There are many CA residents that will tell you its no big deal to send them AR type weapons without bullet buttons and/or high capacity magazines are ok as long as they're disassembled. Do everything by the book. I understand that their laws are stupid, but they are still laws.