DC Gun Ban
Ryan Avent pleads "Can we all agree that however one feels about the merits of gun-control, the District’s tightest-in-the-nation gun laws are unlikely to tell us much about the actual costs and benefits of gun control, seeing as we share a border with Virginia?" Well, yes and no. I don't think you can infer anything about the merits of adopting DC-style rules as a national gun regulation regime from the effects of the DC gun ban in DC. But the vast majority of gun control regulations take place at the city or state level, for which purposes the DC case is an illustrative example. The slogan "when guns are outlawed, only outlaws will have guns" is obviously an oversimplification, but DC is close to a pure instance of the principle. There's little practical impediment to handgun ownership in DC save a self-image as a law-abiding person.
And this is a not-infrequent scenario. Strict gun control regimes are popular in many urban areas, but it's precisely such areas that are in no position to enforce these regulations in a productive and effective manner. That, in turn, tells you something about the psychology and politics of the issue -- that on both sides its a form of identity politics; on the controller side, a means of expressing dislike of gun culture and various aspects of American folkways, a kind of liberal version of sundry ineffective "tough on crime" nostrums from the right.