California has some additional "requirements" beyond the typical 10 round mag limit, no flash hiders, etc. I don't know what the actual approval process is, but I do know that some guns have been banned simply because of their name.
The SKS, for example, has a 10 round fixed magazine, can be had without a flashhider or bayonet, does not have a collapsable stock or pistol grip, etc...yet it is banned because it looks scary and shares the Kalashnikov gas system with the AK-47.
*edit: I think guns are approved on an individual basis, likely so that the DOJ can ban whatever it chooses without having to go to the trouble of writing legislation or doing any research beforehand.