How does a gun not “look” legal to you?
In California and some other more backward states gun control laws have a lot more to do with cosmetics than with functionality.
In this case, the fact that the shotgun accepts a drum OR a clip would make it illegal under California law. folding stocks are also a no-no, bayonet lugs, flash hiders and some other features can also be illegal (I believe this may be only the case if they are present in certain combinations but I don’t feel compelled to look up the law).
None of these things make a weapon any more dangerous or likely to be used in a crime, they just look threatening to certain reactionary fascistic politicians.
Ergo, the USAS-12 LOOKS illegal.