I was taking myself around in circles on this one yesterday. The trouble with Second-Amendment protection for non-states is simple: the Bill of Rights was — originally — a curb solely upon the power of the new Federal government. Only through judicial construction was the Bill of Rights imputed to apply to state governments as well as to Washington. By “Washington,” of course, I mean the government, not the District of Columbia; it took another court case, if I remember right, to impute the Bill of Rights to the District. But it’s hard for any originalist to hold nowadays that imputation is not settled law. So saving DC from gun bans comes about in suitably torturous fashion as a legal outcome, but that’s often the way we do things in the USA I suppose.