The Colorado shooting has led to renewed calls to ban or otherwise restrict access to large-capacity magazines (see, for instance, Elliot Spitzer’s proposal, though that one is likely to be ineffectual). I think such bans might well be constitutional, for reasons given in my Implementing the Right to Keep and Bear Arms paper (p. 1489):
A gun with a larger than usual capacity magazine is in theory somewhat more lethal than a gun with a 10-round magazine (a common size for most semiautomatic handguns), but in practice nearly all shootings, including criminal ones, use many fewer rounds than that. And mass shootings, in which more rounds are fired, usually progress over the span of several minutes or more. Given that removing a magazine and inserting a new one takes only a few seconds, a mass murderer — especially one armed with a backup gun — would hardly be stymied by the magazine size limit. It’s thus hard to see large magazines as materially more dangerous than magazines of normal size.
Still, these same reasons probably mean that the magazine size cap would not materially interfere with self-defense, if the cap is set at 10 or so rather than materially lower. First, recall that until recently even police officers would routinely carry revolvers, which tended to hold only six rounds. Those revolvers were generally seen as adequate for officers’ defensive needs, though of course there were times when more rounds are needed. Second, the ability to switch magazines in seconds, which nearly all semiautomatic weapons possess, should suffice for the extremely rare instances when more rounds were needed (though to take advantage of this, the defender would have to make a habit of carrying both the gun and a spare magazine).
And indeed this analysis is similar to what we see in other areas of the law, though subject to the qualifier that all analogies across different constitutional rights are necessarily limited (they are, after all, analogies and not identities). For instance, the government may limit the volume of music or constrain sound amplification generally, even though that would necessarily diminish to some extent the potential audience for such music or political advocacy. Substantial restraints on the ability to reach the public would be unconstitutional, but more minor ones — when they don’t discriminate based on the content of the speech — are generally constitutional.
But, as the block quote above notes, the restrictions are also unlikely to help prevent crime, given how quick and easy it is to change magazines, and given the likelihood that mass shooters will have a backup gun that they could use to protect themselves while they are changing the magazine. It is conceivable that a magazine size ban will help limit the deadliness of some mass attacks, if the murderers comply with the law and don’t get a black-market magazine; the Jared Loughner killings, according to press accounts, were stopped when Loughner stopped to reload and was tackled by several people.
But given that only a tiny fraction of gun homicides involve more than 10 shots fired (see Kleck, Point Blank, p. 79, and Kleck, Targeting Guns, p. 123), that mass shooters who really want large-capacity magazines will likely be able to get them even if they are outlawed, that mass shooters can and generally do carry multiple guns, and that only very rarely will people be able to tackle someone during the second or two that he needs to reload, I suspect that large-capacity magazine bans will have extremely little effect. So while a large-capacity magazine ban would impose only a very small burden on law-abiding citizens who want to defend themselves — which is why I think it would be constitutional — it would also provide, at best, only a very small extra amount of public safety (and might be a net zero or a negative if it interferes with law-abiding people’s self-defense in the very rare situations when more than 10 rounds are needed and the defender doesn’t have an extra magzine).
UPDATE: For an example of a relatively rare situation in which civilians acting in self-defense shot many rounds, and might well have found higher-capacity guns especially useful — though they fortunately had enough people and enough guns to succeed even without that — see Massad Ayoob’s account of the Beverly Hills Jewelry Store (Richmond, Va.) robbery.