Can an out of state traveler legally enter California with a pistol in car?

The pistol must be California legal. A list can be obtained from the California Department of Justice website as to which firearms are permitted in the state. Legally, you can keep it unloaded in the front seat with the ammo stowed separately, but California cops not heeding the legality of the practice is not unheard of, and you're better off to keep it secured and out of reach, again with the ammo stored separately.

Magazines with a capacity of more than ten rouds are prohibited in California. If you do have one, you can transport it within the state, fully disassembled.