Why Why Why!?!
Welcome.
You DID NOT BRING A 17 ROUND MAG INTO CA. Unless you possessed it in CA BEFORE Jan 1 2000.
If you did not have it in CA before then you may keep it disassembled until you leave. Get some ten rounders until move back to freedom.
Look here first.
http://ag.ca.gov/firearms/
Firearms Information for New California Residents
Handguns
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 Bureau of Firearms 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.
Any person transporting handguns in California is required under California law to transport those handguns unloaded and in a locked container other than the glove compartment or utility compartment of a vehicle.
SHOTGUNS AND RIFLES
California law does not require persons moving into California report rifles or shotguns. One important thing to be aware of though is that certain rifles and shotguns may be defined as assault weapons in California and are prohibited from entering this state under most normal circumstances. Additionally, machine guns and ammunition feeding devices with the capacity to accept greater that 10 rounds may not be transported into California.
Please be aware that failure to comply with these mandated handgun requirements could result in criminal prosecution (Penal Code Section 12072(g)).