At Reason, Jacob Sullum writes:
[C]hanging magazines takes one to three seconds, which will rarely make a difference in assaults on unarmed people.
Magazine size is more likely to matter for people defending against aggressors, which is why it is dangerously presumptuous for the government to declare that no one needs to fire more than X number of rounds. As self-defense experts such as firearms instructor Massad Ayoob point out, there are various scenarios, including riots, home invasions, and public attacks by multiple aggressors, in which a so-called large-capacity magazine can make a crucial difference, especially when you recognize that people firing weapons under pressure do not always hit their targets and that assailants are not always stopped by a single round. …
If magazines holding more than 10 rounds are not useful for self-defense and defense of others, shouldn’t the same limit be imposed on police officers and bodyguards (including the Secret Service agents who protect the president)? And if the additional rounds do provide more protection against armed assailants, it hardly makes sense to cite the threat of such attacks as a reason to deny law-abiding citizens that extra measure of safety.