By law, if you want to transport an out of state gun to CA, you have to send it to an FFL by mail. There's an exception if you're licensed in another state, but you won't be that unless you're a resident of that state and actually moving to or visiting CA. There's also an exemption if you inherit or are given the gun by a direct family member, but I think that only applies if you both are CA residents.

I've also been told that gun shows and stores in neighboring states will deny you a sale if they see that you have CA plates or DL.