oi vey.. I thought this was already answered...
Semi auto shotguns with removable magazines are specifically called out as illegal in CA. They don't follow the same guidlines as CA rifles.
For Rifles it states you cannot have a "A semiautomatic, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds."
For Shotguns you cannot have, "A semiautomatic shotgun that has the ability to accept a detachable magazine."
Note where it states "the ability to accept".
http://ag.ca.gov/firearms/regs/genchar2.htm
You can however own a detachable mag shotgun, but it would have to be pump, lever, or bolt action.
BUT BUT BUT... If you walk into a CA gun stor and they have an item for sale that you thought was listed... JUST BUY IT! LOL!