Just because D.C. is a federal enclave doesn't mean the bill of rights doesn't apply to the citizens living within it. If anything, those rights should directly apply to those people, since people who have their rights infringed by the state they live in have to find out if the particular right being infringed upon was incorporated into their state's own constitution under the 14th amendment, given that the bill of rights only originally applied to the FEDERAL government. Since DC is a FEDERAL enclave, the second amendment should apply.