Legal Question in Criminal Law in California

Do not open the package. Do not call any authorities or shippers. Do not say anything to anyone, especially 'authorities. This is the time to exercise your 5th Amendment right to SHUT UP if questioned. Immediately call your local FFL intended to receive this, and ask his advice. He may choose to call the local BATFE office and seek advice. He may choose to simply have you bring the gun to him to log in, which is what I expect him to do. Once logged in by him, with clear description of receipt of the unopened package, you will be ok. You've done nothing wrong, so don't worry about that. The shipping dealer is at fault here, and once you've got your 'package' logged into your local dealer's books, then call the shipping dealer and raise hell with them about being so stupid.

Now, with all that said, if you had already DROS'ed the action in CA, then you are within the range of legal options to send your action to the builder and have him return it after working on it. That is not illegal and neither shipment requires FFL participation. Your problem is that you never took delivery of the action until now.

Feel free to direct any 2nd Amendment rights and firearms law cases my way.