I think DC is an anomaly because of its small size and proximity to Virginia, which has one very permissive gun sales laws. State-by-State reporting only really

I think DC is an anomaly because of its small size and proximity to Virginia, which has one very permissive gun sales laws. State-by-State reporting only really works when states are large enough that the populations you are studying live almost entirely within the border of such state and the actions that take place in the state are done by the same population. DC as a very small city (500K or so actual residents) is not going to be accurately captured by such a study because many of the activities that take place in DC are undertaken by residents or Maryland or Virginia. For a more mundane example than gun deaths, one could look at traffic accidents. If one took the number of traffic accidents that occur in DC and dvided by the number of DC residents, I would expect DC residents would look like bad drivers (high rate of accidents per resident), but that is a false conclusion because many of the cars on the road on any given day come in from neighboring states (I have been told that daytime population of the city is three times the number of residents, but I don't have a citation for that, so it may be wrong but is certainly directionally correct). So your denominator is wrong, you should look at traffic accidents in DC/cars on the road in DC or some similar metric.