Mayor Bloomberg’s recently announced national effort against illegal guns appears to involve encouraging the rest of the nation to adopt gun control laws like the very strict ones prevailing in New York. Given that virtually no one can get a pistol permit under the city’s licensing laws, this amounts to asking the rest of the country to adopt a de facto ban on the private possession of handguns. National public opinion polls have consistently indicated that most Americans, at least outside of New York, oppose a general ban on handgun possession, so this part of the Mayor’s initiative seems politically dead on arrival.

But the premise underlying this focus on other states is worth exploring because it is responsible for a more general misdirection of efforts to reduce gun violence. Many might be mystified why one city’s mayor is taking it upon himself to encourage gun laws in states other than his own. The explanation lies in a long-standing – but faulty – theory about how guns get into the hands of the city’s criminals.

New York’s mayors and leaders of its police department have long believed that organized gun traffickers, who deal in large numbers of guns and make a living from gun trafficking, buy large numbers of guns in states with weak gun laws, especially in the South, and “sell them on the streets of New York.” According to this theory, these purchases are made by either the traffickers or “straw purchasers” working for them – people with clean records pretending to buy guns for themselves. They buy weapons from gun dealers who either knowingly make unlawful sales or who are too careless or greedy to prevent them.

The political attractions of this theory are strong. First, it places the blame on outsiders – traffickers, bad gun dealers, and the legislators of other states who refuse to pass laws that would prevent this activity. Second, it makes the problem of unlawful guns appear far more manageable – the more the flow of illegal guns goes through the hands of a few high volume gun smugglers, the easier it should be to reduce the flow by arresting a few such criminals or blocking them from buying large numbers of guns.

It is true enough that nearly all of New York’s crime handguns come from outside the state, meaning they were first sold in another state when they were new. Indeed, given that there are virtually no legal sales of handguns in New York, it could scarcely be otherwise. But very little of the movement of these weapons can be credibly attributed to professional gun traffickers. Although police occasionally catch large volume traffickers, these don’t account for more than a minute share of illegal guns. The city’s own data concerning the guns with which police catch criminals indicate that traffickers have little to do with the guns.

According to the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, the agency charged with enforcing the nation’s gun laws, the hallmark of trafficking activity is rapid turnover. Because traffickers cannot make money from unsold inventory, they sell their guns, often to criminals, soon after purchasing them. When criminals then commit a crime with the gun and are caught by the police, a trace of records pertaining to that crime gun will show it to have a short “time to crime,” a short time from its first sale as a new gun (when the trafficker supposedly bought it) to the time police recovered it in connection with a crime. Indeed, the average time to crime for trafficked guns is under a year. Therefore, one can see if traffickers account for a big share of crime guns in a given city by checking to see if the city’s crime guns have a low average time to crime.

But trace data on New York’s crime guns indicate that their average time to crime in 2000 was a whopping 7.4 years. Only 11% had a time to crime of less than a year, and many of these were merely guns stolen by criminals from their owners rather than trafficked guns. Not only is New York’s average inconsistent with the idea that a big share of its crime guns were trafficked, but the average is actually longer than that of the nation as whole (6.1 years). Instead of traffickers bringing handguns to New York, the bulk were probably brought in by individual owners, one or two at a time. Unless the city is prepared to inspect every moving van, U-Haul trailer, and private vehicle containing the belongings of newcomers, the supply of handguns will not be substantially affected by a focus on interstate movement of guns.

An alternative to this sort of “supply-side” focus is a “demand-side” focus: discouraging criminal gun possession by making it legally risky. The National Rifle Association’s focus on raising penalty severity is as ill conceived as Mayor Bloomberg’s initiative, given that more severe penalties do not deter crime. But raising the probability of punishment does have crime-control impact. In the present context, this means making it more likely that police will catch criminals in possession of guns, especially in public places. At present, most police officers are not very good at identifying people carrying concealed weapons on city streets, but some appear to be very good. In principle, there is nothing to prevent police departments from figuring out what makes these few officers successful, and modifying training programs to pass those skills on to others. This sort of nuts-and-bolts approach is not as politically sexy as calling for new legislation, but in this case it is considerably more practical.

Mr. Kleck is a professor in the College of Criminology and Criminal Justice at Florida State University.