"How many residents would be affected by the ban is unclear, since California does not require residents to register handguns that are kept in a private residence of business."
CA doesn't require residents to register handguns?
What is this from the CA DOJ website?
Any person who moves into California and who brings any pistol, revolver, or other firearm capable of being concealed upon the person is considered to be a "Personal Handgun Importer" and is required to do one of the following within 60 days:
Complete and submit a NEW RESIDENT HANDGUN OWNERSHIP REPORT form along with $19.00 to the Department of Justice. A separate report form and $19.00 fee is required for each handgun reported. NEW RESIDENT HANDGUN OWNERSHIP REPORT forms can be obtained from California Department of Motor Vehicles' offices, licensed firearms dealers, local police and sheriff's departments, the California Department of Justice Firearms Division at (916) 263-4887, and from the Online Forms Page at this site.
Sell or transfer the handgun(s) to a California licensed firearms dealer or to another individual using a California licensed firearms dealer to conduct the transaction.
or
Sell or transfer the handgun(s) to a California police or sheriff's department. Persons choosing this option should contact the law enforcement agency for instructions prior to transporting the handgun(s) to the agency.