By Chris Wilson

Jump to the interactive map of firearm ads in the Chicago region below.

At one point last January, Brad Dixon says he was entertaining so many offers for his collection of assault rifles, shotguns and ammunition that he was getting a new text message every 10 minutes.

Dixon, a 26-year-old volunteer firefighter in Mebane, N.C., is a prolific user of ArmsList.com, the premier website for private sales and trades of firearms. During a recent surge in demand for rifles and high-capacity magazines, Dixon was able to sell his AR-15s for $1,300 or more apiece, garnering profits of hundreds or even thousands of dollars on each sale. (Dixon assembled some of the weapons himself, and sold others unused in the original box.) During the height of a recent run on AR-15s and extended clips, driven by a fear that Washington might ban them, a government issue 30-round magazine for this military-style rifle, normally about $12, could go for $45—bullets not included.

“I’ll be honest, I’ve used the gun scare to sell my gun collection,” Dixon says. He was careful to specify that collecting guns is a hobby, not an occupation, lest he get in trouble for not having a federal license to sell firearms.

The AR-15 is an extremely potent assault weapon similar to the M16, the rifle widely used by the U.S. military. It comes in many varieties and names, including the Bushmaster XM15, one of the weapons that Newtown, Conn., shooter Adam Lanza used, and the Smith & Wesson M&P15, favored by Aurora, Colo., gunman James E. Holmes.

The AR-15 is not merely the preferred firearm of mass murderers. It is by far the most popular gun sold on ArmsList.com as well.

After the New York Times ran a long story on ArmsList.com last month that included an analysis of 170,000 ads on the site posted over a three-month period, I decided to gather my own data on the site in the spirit of independent replication. Over the past three weeks, I was able to download and analyze 235,000 ads posted over the past five months that were still active on the site between mid-April and early May.

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As the Times noted, it can be difficult to know exactly how many of these postings represent unique weapons. While all ads are anonymous—the people who post ads do not even have a public username—I was able to approximate the number of unique people posting to the site using a “Listings by this user” feature that links posts by the same person. Even when counting only one gun per user, I was able to collect data on 54,000 handguns, rifles and shotguns advertised for sale or trade on ArmsList.com this year. Many more ads were posted for ammunition, accessories, and other ballistic accouterments.

That data led both to people like Dixon, the most frequent user of the site in my database, and to a bird's eye view of the online gun market in America.

Of the weapon advertisements that Yahoo News analyzed, 5,700 were advertised as AR-15s. Another 800 were advertised as a brand-name variety of the rifle. (ArmsList.com does not neatly categorize weapons by model and make, so in most cases this information was extracted from the text of the posts using a simple, hand-built pattern recognition program. I have open-sourced all of the code I wrote to gather and analyze the data.)

The AK-47, perhaps the most recognized assault rifle in the world, is the second most-popular gun with at least 1,200 models for sale online in the past year. All told, 19,200 handguns, 27,300 rifles, and 7,462 shotguns were listed for sale or trade on ArmsList.com at some point in the past three weeks.

Armslist.com is the largest of several major online markets for guns, ammunition and accessories. These sites operate like gun shows without the folding tables and $3 admission fees. Like Craigslist, ArmsList.com exists only to connect sellers to buyers, and does not play a role in mediating the transactions. Neither this hands-off treatment of transactions, nor the twelve-point Terms of Use Agreement that all new users must actively agree to, have protected it from legal scrutiny. Nowhere is that scrutiny more intense than in Chicago.

Guns and the Great Lakes

At least two recent murders have been linked to weapons purchased on ArmsList.com: Those committed by Radcliffe Haughton, who shot and killed his wife, two other women, and himself at a spa outside Milwaukee in October 2012, and Demetry Smirnov, a Russian immigrant who stalked and murdered a 36-year-old Chicago woman named Jitka Vesel after she rebuffed his romantic overtures. Neither man was legally entitled to purchase a firearm in the United States at the time; Haughton had a restraining order placed against him and Smirnov was not a citizen.

Smirnov pled guilty to the crime and is serving a life sentence. The man who sold him the .40-caliber handgun he used to shoot Vesel a dozen times, Benedict Ladera, received a one-year prison sentence after pleading guilty to the illegal sale of a firearm to an out-of-state resident.

In fact, the Lake Michigan region, including Chicago and Milwaukee, was among the most active territories for ArmsList.com sellers and buyers during the period Yahoo News studied. In the past four months, at least 1,300 handguns were advertised for sale or trade within 150 miles of Chicago, where there have been more than 100 homicides already in 2013. There were 1,700 rifles advertised as well. Over 400 people in this same radius posted wanted ads for handguns.

The following map charts the number of ArmsList.com postings by both sellers and would-be buyers in the region since mid-April:

It is far from clear that there is any causal chain between ArmsList.com activity and violent crime beyond the two murders linked to the site—or that ArmsList.com is any more culpable for selling weapons linked to murders than any of the other means by which criminals can buy a gun. That question is quite literally on trial in Cook County, Ill., Circuit Court, where the family of Smirnov’s victim is now suing ArmsList.com, alleging that the site facilitates unlawful interstate gun sales because it has "designed its site to not require the input of any verifiable identification by buyers or sellers, such as a driver’s license number, to demonstrate residency in a particular state." The case is ongoing.

At the present time, the odds that Congress will pass any sort of expansion on background checks for online gun purchases, much less a ban on assault weapons, appear very long. Barring such an action, sites like ArmsList.com are free to operate with the freedom of a gun show and the namelessness of the Internet.