It should come as no surprise that 2015 was a strong year for gun sales.
The FBI processed more than 23 million background checks in 2015 through the National Instant Background Check System (NICS), shattering the previous record set in 2013.
The total number of background checks conducted is considered the most accurate measurement of gun sales, though it's not always perfectly accurate: Checks aren't required at gun shows (the gun show loophole), and a single check can be used for a person purchasing multiple guns at once.
Despite the grey areas, a key component of the Bureau of Alcohol, Tobacco, and Firearms' (ATF) mission is to track where — and how — all these guns are moving across the country.
The ATF releases a state-by-state report of firearms traces each year. The report includes firearms that are both legally and illegally bought but have been recovered from crime scenes (known as "crime guns") or suspected of use in crimes. The 2015 report, based on data gathered throughout 2014, was released in August 2015.
To conduct a trace, law enforcement officials send all the information they have about the gun to the ATF's National Tracing Center in West Virginia. The ATF then goes through a complicated process of unraveling the gun's sales history, calling manufacturers, and requesting information from gun shops across the country.
Traces aim to identify the purchaser, original legal retailer, and possessor of the crime gun in question, according to the ATF's tracing guide.
These traces help the ATF understand firearm supply chains and pinpoint crucial sources of guns to the black market. For every traced firearm, the ATF reports the type, caliber, criminal category (i.e. "dangerous drugs" or "firearm under investigation"), and the time-to-crime (how much time has elapsed between the original sale and the crime).
Even guns purchased legally can end up in the hands of criminals. For example, the four weapons used in the San Bernardino shooting in December, though modified, were originally purchased legally from gun stores, the Wall Street Journal reported.
On top of that, guns are routinely trafficked from states with weak regulation, like Georgia, subverting strong gun-control regulation in states like New York, according to Mayors Against Illegal Guns.
In Georgia, for example, no state law requires permits and registration for gun owners, and an average handgun may sell legally at a store for $100, reports the New York Times. That same handgun can sell for upwards of $500 in the black market in New York. All the seller needs to do is take a 12-hour drive up the I-95 for a 400% profit.
From 2012 to 2014, 29% of guns recovered from crime scenes were first purchased from legitimate retailers in other states, according to the Center for American Progress.
These maps below show how guns travel in-and-out of states.
The ATF traced 5,891 firearms in Alabama in 2014. The vast majority, 3,579, were pistols and linked to possession of weapon and firearm under investigation charges.
Alabama has lax firearm regulations. 3,561 guns eventually linked to crime originated from within the state, while neighboring Georgia and Mississippi were the largest outside sources.
The ATF traced 714 firearms in Alaska in 2014. A little over half (434) of the firearms traced in Alaska came from within the state, though a sizable proportion came from Texas, California, and Washington.
Alaska has the most gun-related deaths of any state, with nearly 20 gun-related deaths per 100,000 people— more than twice the national rate. Most, however, were suicides.
The ATF traced 7,610 firearms in Arizona in 2014. Of those, 2,686 firearms were recovered from Phoenix alone.
The majority were pistols and came from within the state, though a sizeable portion came from California, Texas, and neighboring New Mexico.
912 traced guns were linked to drug crimes, the second highest category, owing to Arizona's proximity to the cartel lands of northern Mexico.
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