It's not illegal to own or possess an unregistered handgun in California.
However it is illegal to carry and possess an unregistered handgun outside your home and off your own property in California.
To carry a handgun outside your home and off your land in California you must have a concealed carry license.
Simply having an unregistered gun in your possession is not a criminal offense in California.
However, being caught with an unregistered gun in public may result in criminal charges.
California law makes it illegal to carry a gun in public unless one of the following applies: You have a valid California firearm permit.
A U.S. citizen or legal resident over age 18 may generally carry a handgun anywhere within his or her place of residence, place of business, or on private property owned or lawfully possessed by the citizen or legal resident.
A permit or license is not required for a person to carry within these locations.
There's 9 states that I know of that do not require a person to have a permit to carry a handgun either concealed or opening but you must abide by all local, state and even Federal laws when carrying the gun without a permit and you must be legally able to have a firearm in your possession and it must also be registered legally in your name.
The states of West Virginia,Vermont, New Hampshire, Missouri, Mississippi,Kansas, Maine, Alaska and Arizona don't require a permit to carry a firearm opening or concealed.