That’s a good point. Originally, the Bill of Rights was designed to limit the Federal Government, but the 14th Amendment drastically changes the relationship that the Bill of Rights has with state laws. But, as you say, D.C. is not a state.
Ironically, what gun rights advocates want is a liberal interpretation of the 14th together with a conservative interpretation of the 2nd.
I dont know that there is any judge on the court that thinks like that, except for maybe Kennedy. Its possible Alito thinks like that too, but its too soon to tell.