This presents an interesting problem for the high court...which will almost certainly have to address this if it is brought to them.
The trouble, as I see it is that there are really two constitutional issues here. The first, clearly enough, is the second amendment question. Rephrased, is the right guaranteed in the second amendment, "The right of the people to keep and bear arms...", a right which is construed on the individual citizen. The court could uphold the ruling of the lower court, that it is...or not.
Second, if the court decides that it is a right guaranteed to the individual, then you have to deal with the enfranchisement issue...Are the residents of the District of Columbia citizens of the United States? The court could easily rule that they are not and strike down the Circuit's decision without even ruling on the 2nd Amendment issue. The court has a long history of doing this...Dred Scott v. Sanford and many others. It sounds crazy that they may not be, but there is a long legal precedent for this sort of thing. For more than half of their existance for example, the Bill of Rights were thought not to apply to the states and correspondingly, not their citizens.
Don't get me wrong here guys...I would love to see this go down...but this case gives the Justices many ways out of addressing the 2nd Amendment question we all want to see...Not as cut and dry as it looks.