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.
It is conceivable that a magazine size ban will help limit the deadliness of some rare 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 do next to nothing (or perhaps outright nothing) to save lives.
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 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). For one of the apparently very rare situations in which civilians acting in self-defense shot many rounds, and might well have found higher-capacity guns especially useful, see Massad Ayoob’s account of the Beverly Hills Jewelry Store (Richmond, Va.) robbery. But even there the defenders fortunately had enough people and enough guns to succeed even without the large magazines.
And indeed this analysis — focused on the degree to which the law burdens a constitutional right — 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.
Similarly, while the Court has concluded that the Constitution protects abortion rights, it has said that restrictions on such rights are unconstitutional only when they create a “substantial burden” on women who seek abortions. While there’s the obvious risk that restrictions that aren’t very burdensome will be followed by broader and broader one, courts solve this by saying that they’ll step in when the music volume restrictions or abortion restrictions become too burdensome, not by saying that all such restrictions are categorically unconstitutional.
More broadly, even if bans on magazines with more than 10 rounds are unwise, not all unwise restrictions are unconstitutional. That’s true for speech restrictions. It’s true for abortion restrictions. And I think it’s true for gun restrictions as well.
UPDATE: For more on “substantial burden” inquiries in constitutional law, and why they’re likely to make sense in right-to-bear-arms cases — as well as how they’ve been used in such cases at the state constitutional level in the past — see pp. 1454-1461 of this article (“Implementing the Right to Keep and Bear Arms for Self-Defense”).