If you've got some clothes you don't need anymore, you can give them to Goodwill or the Salvation Army. If you have an old car, you can call various organizations to take it away. And if you're in Chicago and have a gun that's burning a hole in your pocket, you can get rid of it on Saturday, no questions asked.

The city government has a great fondness for gun turn-in events. It's done six of them in the past six years, collecting more than 23,000 weapons. This one will be held at 23 churches, and anyone handing over a firearm will get a $100 gift card. The guns will then be destroyed.

The motive behind these efforts is not hard to understand in a place that had 433 murders last year and has seen a spike this year. Dozens of shootings take place in Chicago every week.

Two years ago, explaining the effort, then-Mayor Richard Daley said, "We have just too many guns in our society. When someone has access to a gun, they use it." The gun buyback is a way "we can reduce the number of guns on our streets," says Mayor Rahm Emanuel.

But don't put too much stock in those pronouncements. The number of privately owned guns in America keeps rising, and at last count it totaled 270 million, or about one for every adult. But nationally, the homicide rate has fallen by more than half over the past two decades.

Contrary to Daley, most people who own guns never use them for anything but legal purposes (hunting, target shooting, self-defense). Contrary to Emanuel, the weapons this sort of venture yields are probably not the ones carried in the streets or the ones used in crimes. The reduction also represents a minuscule share of the firearms in the city, which may number over a million.

Think about it: Who is most likely to turn in a firearm for a $100 reward? Someone with 1) a cheap gun and 2) no criminal propensity—say, Aunt Millie disposing of a rusty revolver her late husband left in the nightstand.

Florida State University criminologist Gary Kleck noted in a 1996 book that when St. Louis did a gun buyback, those participating "were commonly middle-aged and 80 percent white, while those involved in gun violence in that city were mostly young and black."

Criminals will have trouble finding any appeal in this offer. In the first place, their weapons may have cost far more than $100, as handguns and long guns of good quality usually do.

In the second place, thugs practice a trade in which a weapon is essential for doing business. A pistol used in the course of armed robberies will pay for itself many times over. A $100 gift card won't.

The experience elsewhere offers little hope that the program will make a noticeable difference. After a successful 1974 buyback in Baltimore, the firearm homicide rate jumped by 50 percent. A study of a Seattle effort found it "failed to reduce significantly the frequency of firearms injuries, deaths or crimes."

This is the pattern wherever turn-ins take place. A 2004 study by the National Academy of Sciences concluded that "the theory underlying gun buybacks is badly flawed and the empirical evidence demonstrates the ineffectiveness of these programs."

The people who participate are generally those who are least dangerous. Those who are most dangerous have no motive to participate. So when the buyback is done, the number of armed criminals will most likely be unchanged.

Advocates may think that getting rid of weapons will at least prevent accidents and suicides. But some people who hand over a gun will hang on to other guns, which they are just as likely to handle carelessly or leave where a child can find it.

As for suicide, the odd thing about people intent on killing themselves is that if a firearm is not available, they can find plenty of other methods that will serve their purpose. The National Academy study said that "gun control policies may reduce the number of gun suicides, but they have not yet been shown to reduce the overall risk of suicide in any population" (my emphasis).

This year's turn-in will no doubt garner a decent haul of weapons. But for anyone anticipating a drop in gun violence, it will mostly yield disappointment.