The Supreme Court recently heard a challenge to the DC handgun ban in a case that has the potential to definitively clarify the Second Amendment , which states:

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

There has been a long-standing debate between gun rights activists and gun control advocates as to whether this describes an individual right or whether it applies only in the context of a regulated militia. The ruling is not expected until June, but based on questioning before the Court, it appears likely that a majority of Justices will rule that the DC ban is unconstitutional.

From the LA Times :

Five of the justices, a bare majority, signaled that they thought the amendment gave individuals a right to have a gun for self-defense. It was not limited to arms for "a well-regulated militia," they said.

By adopting that view, the justices are likely to strike down the nation's strictest gun control law, a ban on handguns in the District of Columbia.

The only real judicial precedent is United States v. Miller , a case in which the Court ruled that it was acceptable to require registration of certain weapons. The registration in question involved a tax that was ostensibly aimed at generating revenue but was probably actually geared towards decreasing ownership of such weapons. The Court reasoned that since a short-barreled shotgun did not have some reasonable relationship to the preservation or efficiency of a well regulated militia, we cannot say that the Second Amendment guarantees the right to keep and bear such an instrument. On the other hand, they also clarified what was meant by "Militia" to be all males physically capable of acting in concert for the common defense... these men were expected to appear bearing arms supplied by themselves and of the kind in common use at the time.

Two points on which I believe we can say that a consensus is developing in this debate: (1) The reference to "arms" should be interpreted in the context of ordinary weaponry and does not extend to specialized military equipment such as could not have been envisioned at the time the amendment was drafted. Thus, granting the right to possess guns does not mean the state may not regulate or prohibit, for example, the private ownership of surface-to-air missiles (to take a deliberately extreme example). Since there is no overriding general principle on which to ground a differentiation between what is covered by the Second and what is not (the language of Miller is somewhat vague), we are reduced to quarreling over specifics -- but at least we can agree that some regulation of private weaponry is constitutional. (2) The reference to "Militia" should be interpreted as applying to anyone who could participate in military defense -- ie, pretty much everyone (but not criminals, or the underage, etc; even the NRA agrees with some limitations). Even ignoring the expansive definition, it is possible to read the amendment as an introduction (describing Militias) and then a statement of an individual RKBA. As the likely swing-vote in the current case, Kennedy, put it: In my view . . . there's a general right to bear arms quite without reference to the militia. There will still be debate around the margins -- for example, what sort of background checks are acceptable? However, after the expected ruling the legal interpretation will be squarely in line with an individual right.

The main point of disagreement is what sort of response is "proportional" and acceptable. Here the liberal Justices argue that the DC ban is a necessary response to gun violence; Breyer asked In the District, I guess the number is somewhere around 200 to 300 dead and maybe 1,500 to 2,000 wounded. Now, in light of that, why isn't a ban on handguns, while allowing the use of rifles and muskets, a reasonable or proportionate response on behalf of the District of Columbia? Of course the implicit assumption here is that the ban would reduce the level of gun violence, and that's actually not at all clear either way. Because there's no way to compare DC as it is to DC as it would have been absent the ban, you need to look at "similar" cities in terms of crime rates, poverty levels, drug trafficking, and so on, which is not an easy thing to do even when unburdened by an ideological perspective and which lends itself nicely to (sometimes deliberate) cherry-picking in service of a political agenda. Here is one assessment of results in DC:

the gun culture on the city's mean streets during the crack epidemic has not abated, police statistics show. Even as the homicide toll declined in D.C. after 1991, the percentage of killings committed with firearms remained far higher than it was when the ban was passed. Guns were used in 63 percent of the city's 188 slayings in 1976. Last year, out of 169 homicides, 81 percent were shootings.

Meanwhile, periodic ATF reports have documented that firearms, flowing in from elsewhere in the country, remain available on D.C. streets - exactly what the ban was designed to prevent.

To be fair, as the article notes, at the time the ban was passed it was hoped that neighboring regions would follow suit, increasing the effectiveness of the policy by making it more difficult for an underground traffic in guns to develop.

While the Justices have no real legal reason to consider the success (or lack thereof) of the DC ban in making their ruling, one suspects that at least some of them would have been responsive to significant results either way and would have at least tailored the scope of their decision to match what seemed to work on the ground. Lacking conclusive evidence of the effectiveness of the ban, a majority of the Justices seem prepared to rule that DC has gone too far. As Chief Justice Roberts put it:What is reasonable about a total ban on possession?

An interesting side-note as regards the Democratic Presidential primary: one of the (admittedly few) Senate votes with Clinton and Obama on opposing sides was the Vitter Amendment which proposed To prohibit the confiscation of a firearm during an emergency or major disaster if the possession of such firearm is not prohibited under Federal or State law. Obama voted Yea with the majority, Clinton voted Nay with 15 other Democrats.