CA has a list of 'named', so-called, assault weapons as well as definitions 'by evil features.' before bringing any firearm into CA be certain it clears the 3 assault weapon flowcharts. You can find the flowcharts at the top of every page at www.calguns.net (there is one each for pistols, rifles and shotguns)
1) It is illegal to import "large capacity ammunition feeding devices" (mags >10 rounds) into CA unless the were previously possessed within the state of CA prior to 1/1/00 (see the wiki above) Disassembled magazines are considered simply parts and may be imported. See the rifle flowchart AR/FAL pattern rifle questions.
2) A Mossberg 500 is not an AW by name - but be certain to check the SG flowchart. As of 1/1/13 unloaded open long gun carry in CA is generally illegal. The safest bet is to transport it unloaded & concealed in a 'known' gun case (unlocked) or in any non-gun case (locked). This law is brand new and the boundaries are not particularly clear yet; hence my recommendation of unloaded, locked in a non-gun case. "Known" or "Designed to Carry" long-gun cases might now be a minefield in CA for getting 'inspected for the loaded state of the firearm' Keep the 4th amendment on your side... See the wiki for citations
3) As of 1/1/12 unloaded open carry of a handgun is generally illegal. Carry of a concealed handgun on your person or in a vehicle is generally illegal. Generally speaking pistols must be transported unloaded in a locked container. Your trunk can be a locked container, but that too can have pitfalls (e.g. open or unlock the trunk in a Gun Free School Zone and you just committed at least 2 felonies; 1 federal and 1 state) See the wiki for citations
4) I think its all been covered above...
Additionally, in CA it is legal have a loaded firearm for the purposes of self defense at you place of residence. This includes temporary residences such as hotel rooms and campsites. (See the wiki)
CA is a great state to visit, I hope you enjoy your time here. Most of California (by Geography) is not the land of fruits and nuts...