If you want to be "technical " it is my understanding that California only allows 10 round detachable magazines. To use a magazine of a higher capacity a mag catch requiring a tool to remove (bullet button) is required. There were State limitations on magazine capacity. That was struck down for a brief time. There are specific tools for loading installed magazines. So if the magazine is mounted then the magazine that is attached would be the capacity of the rifle.
This is information from several years ago and could be outdated. Original Poster, as previously posted, I would check with calguns.net for current specifics.
Scott