When a rash of gun murders takes place, it makes sense for the police to do one of two things: renew tactics that have been effective in the past at curbing homicides, or embrace ideas that have not been tried before. But those options don't appeal to Chicago Police Supt. Jody Weis. What he proposes is a crackdown on assault weapons.

I'm tempted to say this is the moral equivalent of a placebo -- a sugar pill that is irrelevant to the malady at hand. But that would be unfair. Placebos, after all, sometimes have a positive effect. Assault weapons bans, not so much.

If there are too many guns in Chicago, it's not because of any statutory oversight. The city has long outlawed the sale and possession of handguns. It also forbids assault weapons. If prohibition were the answer, no one would be asking the question.

The recent spate of killings gives a misleading impression. Since the peak years of the early 1990s, the number of murders in Chicago has fallen by more than half. In the first three months of this year, homicides were down by 1.1 percent. No one would describe the current murder rate as acceptable, but the city has made huge progress.

It has done so despite the alleged problem cited by Weis, which is the availability of guns, and particularly one type of gun. "There are just too many weapons here," he declared at a Sunday news conference. "Why in the world do we allow citizens to own assault rifles?"

Actually, in Chicago "we" don't allow citizens to own assault rifles. Elsewhere they are allowed for the same reason other firearms are permitted. The gun Weis villainized is a type of semiautomatic that has a fearsome military appearance but is functionally identical to many legal sporting arms.

And its bark is worse than its bite. As of March 31, there had been 87 homicides in the city. When I asked the Chicago Police Department how many of the murders are known to have involved assault rifles, the answer came back: one.

As it happens, we already have ample experience with laws against these guns. From 1994 to 2004, their manufacture and sale were banned under federal law. Yet the number of murders committed with rifles and shotguns began falling in 1991, three years before the law was enacted. Continued...