Hours after last Friday's massacre in Aurora, Colorado, New York Mayor Michael Bloomberg demanded that the two major parties' presidential candidates explain how they plan to prevent such senseless outbursts of violence. "No matter where you stand on the Second Amendment, no matter where you stand on guns, we have a right to hear from both of them concretely," Bloomberg said in a radio interview. "What are they going to do about guns?"

Whether you accept the premise that something must be done about guns, of course, might be influenced by where you stand on the Second Amendment and where you stand on guns. But according to Bloomberg, even people who object to gun control on practical or constitutional grounds are morally obliged to support it. Such arrogant illogic may help explain why public support for new gun restrictions has been falling for two decades.

Consider how the Brady Campaign to Prevent Gun Violence reacted to news that a man had shot 70 people, 12 of them fatally, at a midnight showing of The Dark Knight Rises. "This tragedy is another grim reminder that guns are the enablers of mass killers and that our nation pays an unacceptable price for our failure to keep guns out of the hands of dangerous people," said the group's president, Dan Gross. "We are outraged."

But outrage is no substitute for rational argument, and the response urged by the Brady Campaign—a petition demanding that Congress keep guns away from "convicted felons," "convicted domestic abusers," "terrorists," and "people found to be dangerously mentally ill"—had nothing to do with what happened in Aurora. As far as we know, James Holmes, the 24-year-old former neuroscience graduate student arrested for the murders, has no criminal record, no links to terrorist groups, and no psychiatric history that would have disqualified him from owning guns.

Similarly, a New York Times story regretted that Holmes was "unhindered by federal background checks" when he bought ammunition online. Since he passed background checks to buy his pistols, shotgun, and rifle, why would a background check for ammunition have stopped him?

Other gun control advocates focused on the AR-15 rifle used by Holmes, a civilian, semi-automatic version of the M-16. Depending on the details of its design, it might have been covered by the federal "assault weapon" ban that expired in 2004. But such legislation targets guns based mainly on their military appearance, as opposed to features that make a practical difference in the commission of crimes (a purpose for which they are rarely used). It is hard to see how the presence or absence of a bayonet mount, a threaded barrel, or a collapsible stock, for instance, matters much for a man shooting unarmed moviegoers in a darkened theater.  

Holmes also had large-capacity magazines: one holding 100 rounds for the rifle (which reportedly jammed) and one holding 40 rounds for his .40-caliber Glock pistol. But reinstating the federal ban on magazines holding more than 10 rounds, as recommended by Sen. Frank Lautenberg (D-N.J.), would have no impact on a determined killer, since millions of larger magazines are already in circulation. Even if all of them disappeared tomorrow, switching magazines (or weapons) takes just a few seconds—probably not a crucial consideration when no one is shooting back.

Instead of restricting guns, magazines, or ammunition for everyone, why not focus on the tiny percentage of buyers who will use them to commit mass murder? Because there is no reliable way to identify those people before the fact. As Vasilis Pozios, a Detroit psychiatrist who specializes in risk assessment, recently conceded to USA Today, "We're just not good at predicting who does this."

Peter Ahearn, a former FBI agent, made the same point in an interview with the Associated Press. "There's nothing you can do to predict that type of crime," he said. "There's no way you can prevent it."

That message is not reassuring, popular, or politically useful. It just happens to be true.

Jacob Sullum is a senior editor at Reason and a nationally syndicated columnist. Follow him on Twitter.