It Scares me that a LEO has not given a good response here...
Long guns (rifles, shotguns) must be unloaded when transported in a vehicle. There is no requirement for a locked container with the exception of long guns considered to be "assault weapons". Federal law requires locking containers when inside of a "gun-free school zone." The constitutionality of the federal Gun-Free School Zone Act is in question due to the U.S. v. Lopez ruling.
Assault weapons, as defined by California law, must always be transported in locked containers and may only be transported under certain circumstances.
But like others have stated... Expect some attention from it.. Get a rack that locks (edit: for traveling thru school zones) and you are G2G.
The Constitution does not bestow wisdom. It's up to the body politic to be wise. -Patriot
All that is required for evil to prevail is for good men to do nothing.
I'd much rather go to my grave never needing my gun, than go there wishing I had it.
- Phil Dalmolin
The Battle of Athens was illegal too.