Data from the Bureau of Alcohol, Tobacco and Firearms underscores this point: In 2014, ATF traced the source of over 170,000 guns used in crimes in the U.S. And well over a quarter of them -- 28 percent -- were used to commit crimes in a state other than the one they were purchased in. The map below shows which states these border-crossing crime guns came from.

In 2014, over 3,200 firearms originally purchased in Georgia where used to commit crimes in other states, making it the biggest exporter of crime guns in the U.S. Texas, Virginia and Florida weren't far behind with over 2,500 crime guns each. Arizona and Pennsylvania were each the source of over 2,000 guns used in crimes elsewhere.

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Part of this is simply a function of population -- states with more people are going to have more gun shops, and hence more guns that will eventually make their way elsewhere. That's why a state like California, with the country's biggest population, also has a high number of crime guns.

But looking at the map it's also clear that the prevalence of crime guns isn't just due to population. New York, for instance, has the fourth-highest population in the country, but it's not even close to the biggest crime gun exporter. Georgia is the 8th-biggest state but is number 1 when it comes to crime guns.

We can correct for some of this by adjusting for population. The map below shows the number of crime guns exported per 100,000 state residents.

The picture is somewhat different here. West Virginia becomes the biggest exporter of crime guns at the per-capita level, with 52 crime guns per every 100,000 residents. Alaska and Mississippi also rank highly, as do a number of states in the south and mountain west.

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States with stricter gun laws, like New York, Massachusetts, New Jersey and California, on the other hand, export relatively few crime guns on a per-capita basis.

It bears repeating that the majority of crime guns in ATF's data -- 72 percent of them -- were purchased in-state. So most bad guys are getting their guns closer to home. But the relatively high numbers of firearms moving across state borders represent a challenge for lawmakers.

One important thing to note is that the average time between when a gun is purchased and when it's used in a crime is over 10 years. This means that people aren't typically buying in one place and then immediately driving across state lines to commit a crime.

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But that 10-year window does make something of an argument for better ability to track what happens to firearms after they're sold by a licensed dealer. How does a gun lawfully purchased in Mississippi end up being used in a homicide in Chicago a decade later? That's the type of question better firearms research could answer.

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But Congress has explicitly made it difficult, if not impossible, for researchers to answer these questions. A budget rider known as the Tiahrt Amendment, passed in 2004, makes it unlawful for ATF's detailed gun trace data -- which contains information on individual purchasers, guns and retailers -- from being used for any purpose other than crime investigation. That means it's off-limits for researchers who could use that information to learn more about where crime guns come from.

The ATF data is not perfect, of course. A 2009 Congressional Research Service report notes that there is variation in how different jurisdictions recover and report on crime guns, that many crime guns can't be traced, and that guns recovered by police may be not be representative of the total universe of guns used by criminals.