I checked into the long gun issue. Like I mentioned, even though it is legal to buy a long gun out of state, the common experience of residents from DC is that stores will not even sell them long guns.
Google it and you'll find a couple of stories along those lines. Of course if one is successful in buying a long gun they are unlikely to write about a problem buying the gun. But it seems to be at least somewhat common that stores will not sell even a long gun to someone with a DC license, much less a handgun.
...maybe I should just find a local gun show!