The generation that fought the Revolution and wrote the Constitution understood that law-abiding, armed citizens are the best domestic defense. The Ninth Circuit appears to intentionally overlook this point. They cite George Mason at the Constitutional Convention saying: "I wish that, in the case the general government should neglect to arm and discipline the militia, there should be an express declaration that the state governments might arm and discipline them."
But taking this quote out of context misrepresents Mason's argument. Reading the whole paragraph from which it is lifted reveals that Mason was actually arguing for the individual right to own firearms. Further up in the paragraph he said: "The British Parliament was advised by an artful man, who was governor of Pennsylvania, to disarm the people; that it was the best and most effectual way to enslave them; but that they should not do it openly, but weaken them, and let them sink gradually, by totally disusing and neglecting the militia."