You can’t purchase from a licensed dealer and leave with the gun if you’re from out state, and a DC resident would be considered out state. You could arrange to have the weapon shipped to a licensed dealer in your state and pick it up from him, but you would undergo a background check when you did. You could purchase a gun from a regular citizen wandering around the gun show(not a dealer). There are individuals that go to gunshows to sell their personal guns.
If we learned anything from the Boston Bombing:
1) Every house should have at least one gun in it to protect yourselves from terrorists and or nuts on the loose.
Imagine yourself in Watertown knowing a kook is on the loose and you need to lock yourself in your house and you had NO protection?