You can have an unregistered firearm in your possession as long as you don't leave your own property with it.
That is you must remain on your own property with your unregistered firearm and not leave your private property with it.
Now if you leave your private property and have the unregistered firearm in your possession while in public and get pulled over and it's found that you have an unregistered firearm then you can get fined and/or arrested.
In California you can get arrested for having an unregistered firearm in your possession and could serve 1 year in jail or 2 to 5 years in prison.
If you're a convicted felon and have an unregistered firearm in your possession then you get 15 years to 25 years in prison.
It's always best to register a firearm anyway so that it comes back to you legally and cannot be mistaken as a stolen firearm.
And if the firearm gets stolen it can be easier to trace.