Register.... .... ..... ......?
Strictly speaking for California:
The only exception to the below is the city of San Francisco (I believe) wherein you can not possess an unregistered firearm in the city.
Before 1991 it was not required to register small arms. Any small arms from before 91' doesn't need to be registered unless it transfers ownership. That's as far as the law goes on that matter. You cant even get it as a gift/inheritance from a family member without doing an FFL transfer. The grey area in this note is if you say that you used your own $$$ or were given the firearm before 1991.... Then registration can be ignored.
Cops wont like it; they will want to take your gun if they themselves don't know the law; But you can easily save yourself by giving a statement of how it was under your ownership/possession before 1991, and having the manufacturer serial number by date sheet to verify it was manufactured before 1991.