Semi-auto shotguns are a bit more relaxed in CA than they were under the fed ban. A semi-auto shotgun MUST have 2 of either a detachable magazine, pistol grip and telescopic/collapsible/folding stock. There is no limit on fixed (tube) magazine capacity. You can have a Benelli M1 Super 90 with a pistol grip and a 30-round tube magazine if you want. It would be a ridiculous gun, but it would not be an AW.
The only time you need worry about magazine capacity in CA if if you're talking about the capacity of a detachable magazine. Anything over 10 rounds cannot be sold, bought, transferred or imported into CA. There is no such thing as "magazine capacity" when it comes to a firearm, legally speaking. AW rifles and shotguns are defined by their ability to accept a detachable magazine of any capacity.