DC residents may purchase/own rifles or shotguns with a barrel 20" or greater & no means of holding more than 12 round [no removable magazine]. They must be registered with DC police through the "Firearms Registration Section" at Police HQ prior to transport into the District. There are several FFL's in MD & VA that commonly do this & know the 'routine'. There are also a handful of stipulations about home storage [unloaded, disassembled, etc.], but in spite of what most people think, legally registered/owned long-guns aren't that uncommon.
-dc2wheel