The U.S. has perhaps 250 million or more guns in private circulation. Guns substantially increase the lethality of crime in America. UC-Berkeley professor Franklin Zimring has noted that the U.S. has overall violent crime rates that are fairly similar to what we see in many other developed nations, but our homicide rate is substantially higher - presumably due in large part to our greater availability of guns. The social costs of gun violence may be on the order of $1 million per gunshot injury. A useful summary of patterns of gun ownership, violence, and regulation in the U.S., as well as a review of the available empirical research literature, is available in a recent essay by Crime Lab members Philip Cook and Jens Ludwig.

With so many guns in America, it is not surprising that many people have come to believe that it is impossible to keep guns out of the hands of youth, criminals, and other high-risk people. But a study of the underground gun market in Chicago by Sudhir Venkatesh together with Crime Lab members Philip Cook, Jens Ludwig, and Anthony Braga suggests that this conventional wisdom may be overly pessimistic. Transaction costs in underground gun markets appear to be substantial: prices are high relative to the legal gun market, wait times are considerable, mistrust is common between buyers and sellers, and many transaction attempts go unfulfilled, even by people who are well-connected in the underground economy. The underground market seems to work far less smoothly for guns than for drugs, perhaps in part because guns, unlike drugs, are durable goods, so the number of market transactions is lower and exchange becomes more difficult to manage. These patterns suggest opportunities for enforcement efforts that disrupt the illicit gun market. Measures such as buy-and-bust operations or efforts to incentivize arrestees to provide information about buyers and sellers in the gun market could potentially prove more effective than those directed at illegal drugs. However to date there is little rigorous evaluation evidence available to determine the effectiveness of such interventions.

Deterring gun carrying may also help reduce homicides, above and beyond efforts to prevent gun access in the first place. In Chicago, 80 percent of homicides involve firearms and nearly three-quarters of all homicide victims were found outdoors. These figures suggest that in a large share of all homicide events the offender must have been carrying a gun in public beforehand. Young people, criminally involved young adults, and even drug-selling street gang members tell Sudhir Venkatesh that they change their gun-carrying behavior in response to police pressure against illegal gun carrying. A "natural experiment" study of a policing program in Pittsburgh targeted at illegal gun carrying seems to provide some support for the potential effectiveness of this approach, although a formal randomized experimental evaluation of this strategy has to date not been conducted.