A limit on magazine capacity is emerging as a leading contender for the something that supposedly must be done in response to last month's massacre at Sandy Hook Elementary School in Newtown, Connecticut. A ban on "large-capacity ammunition feeding devices" is one of the new gun restrictions approved by the New York legislature this week and one of the measures President Obama wants Congress to enact.

The rationale for such limits is that mass murderers need "large-capacity" magazines, while law-abiding citizens don't. Both premises are questionable, and so is the notion that politicians should be the arbiters of necessity under the Second Amendment.

The problem with letting legislators decide what gun owners need is immediately apparent when we ask what qualifies as a "large-capacity" magazine. Under current New York law and under the federal limit that expired in 2004 (which Obama wants Congress to reinstate), more than 10 rounds is "large." This week the New York legislature redefined large as more than seven rounds.

Why? Because seven is less than 10. Duh. Or as Gov. Andrew Cuomo put it last week, "Nobody needs 10 bullets to kill a deer."

That might count as an argument if the right to keep and bear arms were all about killing deer. But as the Supreme Court has recognized, the Second Amendment is also about defense against individual aggressors, foreign invaders, and tyrannical government.

Toward those ends, the Court said, the Second Amendment guarantees the right to own weapons "in common use for lawful purposes," which clearly include guns capable of firing more than 10 rounds (and certainly more than seven) without reloading. The Glock 17, one of the most popular handguns in America, comes with a 17-round magazine. One of the most popular rifles, the AR-15 (a style made by several manufacturers), comes with a 30-round magazine.

Measured by what people actually buy and use, magazines that hold more than 10 rounds are hardly outliers. In fact, there are tens (if not hundreds) of millions already in circulation, which is one reason new limits cannot reasonably be expected to have much of an impact on people determined to commit mass murder.

Another reason is that changing magazines takes one to three seconds, which will rarely make a difference in assaults on unarmed people. The gunman in Connecticut, for example, reportedly fired about 150 rounds, so he must have switched his 30-round magazines at least four times; he stopped only because police were closing in, which prompted him to kill himself.

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.

Living in Los Angeles during the 1992 riots, I was glad that shopkeepers in Koreatown had "large-capacity" magazines to defend themselves and their property against rampaging mobs. I bet they were too. In fact, argues gun historian Clayton Cramer, those magazines may have saved rioters' lives as well, since they allowed business owners to fire warning shots instead of shooting to injure or kill.

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.