Is your firearm have a capacity of 10 rounds or fewer? I believe CA prohibits normal capacity magazines.
Take a lock box with you and you'll be fine, from my reading of info on handgunlaw.us:
HANDGUNS
California Penal Code section 25400 does not prevent a citizen of the United States over 18 years of age who
is not lawfully prohibited from firearm possession, and who resides or is temporarily in California, from
transporting by motor vehicle any pistol, revolver, or other firearm capable of being concealed upon the
person provided the firearm is unloaded and stored in a locked container.
The term "locked container" means a secure container which is fully enclosed and locked by a padlock, key
lock, combination lock, or similar locking device. This includes the trunk of a motor vehicle, but does not
include the utility or glove compartment. For more information, refer to California Penal Code Section
25610.
and:
2. and when the firearm is carried by the person to or from any motor vehicle for any lawful purpose,
the firearm must be contained in a locked container while being physically carried. (Penal Code §
16850)
and:
Any person over the age of 18 who is not prohibited from possessing firearms, and if otherwise
lawful, may keep and carry a firearm or have a firearm loaded at his or her place of residence,
temporary residence, campsite, or on private property owned or lawfully possessed by the person.
(Penal Code §§ 12026, 12031(h) and (l).) Any person engaged in any lawful business (including
nonprofit organizations) or any officer, employee, or agent authorized for lawful purposes connected
with the business may possess a loaded firearm within the place of business if that person is
over 18 years of age and not otherwise prohibited from possessing firearms. (Penal Code §§ 12026,
12031(h).)
NOTE: A person’s place of business, residence, temporary residence, campsite, or private property
may be located in areas where possession of handguns or other firearms, whether loaded or
unloaded, is otherwise prohibited. Such areas include, but are not limited to, state, federal, or private
game reserves or refuges, federal and state parks, and other public lands. Questions regarding the
applicability of such laws should be directed to your sheriff or chief of police, federal or state fish and game
officers, or federal or state park rangers.