The gunman is described as wearing black and wielding an "assault rifle". I just looked up CA law, as far as I can tell assault rifles sales are banned in that state. Though a lot a good that stupid law did. Apparently this gunman didn't get that memo.
Here is a partial description of firearms banned in CA. With the possible/debatable exception of a launcher NONE of these features makes a weapon more lethal or dangerous to the public. (1) A semiautomatic, centerfire rifle that has the capacity to accept a detachable magazine and any one of the following: (A) A pistol grip that protrudes conspicuously beneath the action of the weapon. (B) A thumbhole stock. (C) A folding or telescoping stock. (D) A grenade launcher or flare launcher. (E) A flash suppressor. (F) A forward pistol grip.