All residents of the District of Columbia have to do to purchase guns with far greater ease is simply cross the Potomac River into Virginia. We've long been one of the most liberal states in America when it comes to the freedom to purchase, own and carry weapons and all somebody from Washington needs to do to quench his or her thirst for firearms is hop in their car and make a quick trip to the Old Dominion where the pickings are plentiful.
Along the lines of what
Squiggy said, D.C. isn't isolated in the middle of nowhere with a colossal moat full of genetically engineered super crocs and heavily armed Mad Max wasteland survivalists patrolling the territory on the opposite side.