Firearms sold through California Federal Firearms Licensees (FFLs) generally are recorded with the state through the Dealer's Record of Sale process. (Penal Code 12070 and following)
However, sales using an FFL were not required until January 1, 1991. Prior to that date an individual could sell to another individual without using a dealer, and there are probably several million firearms in California which have not changed hands since 1990.
Rifles defined as Assault Weapons had specific time windows before which they had to be registered to maintain possession. Later, .50BMG rifles were also required to be registered by their owners to continue to possess them.
Beginning January 1, 1998, individuals moving to California with handguns are required to register their handguns as "personal handgun importers" (Penal Code 12072(f)(2)). The person must file the "New Resident Handgun Ownership Report".
Prior to 1998, guns moved here were not registered, even by mail. Note that this registration requirement does not apply to "long guns", typically shotguns and rifles. The number of firearms moved here is unknown.
It is illegal to possess a firearm that is defined as an Assault Weapon or a .50BMG rifle without a valid registration or permit. Only those persons with an Assault Weapons Permit from the Department of Justice or sworn law enforcement officers who have obtained a letter from their department can register new Assault Weapons or .50BMG Rifles.
Infrequent sales of long guns over fifty years old are exempt from using an FFL (Penal Code 12078 (t)(2)). (Infrequent means "occasional and without regularity.", Penal Code 12070 (c)(1)) Note that the "infrequent" unlicensed sale of handguns is limited to six transactions per year, Penal Code 12070 (c)(1)(a), ("For pistols, revolvers, and other firearms capable of being concealed upon the person, less than six transactions per calendar year. For this purpose, "transaction" means a single sale, lease, or transfer of any number of pistols, revolvers, or other firearms capable of being concealed upon the person.")
Not all guns are registered
The net effect of these older guns is not all guns in California are registered with the state, and it is not required that they be registered.
What is registered to you?
An individual may ask the California Department of Justice, Bureau of Firearms, for a report of all firearms registered to him or her by filing a "Automated Firearms System (AFS) Records Request Form".
An individual may voluntarily register a firearm with the state by filing a "Firearm Ownership Record".
A benefit of registering an unregistered handgun is that there are certain sentence enhancements for committing a crime with an unregistered handgun. The most relevant example is that illegal concealed carry is a felony if the handgun is not registered to the person illegally concealing the handgun but is a misdemeanor if the handgun is registered to the person carrying the handgun (Penal Code 12025 (a)(6)(B)).
De-Registering a Firearm
Note that when you sell a handgun via Private Party Transfer at a California FFL, the handgun is not automatically removed from your AFS record of registered firearms. One can use the form above to remove a handgun sold from one's AFS record.