Tracking the number of guns sold in the United States is no easy task. Federal law requires firearm dealers to keep records of gun transactions, but -- here’s the catch -- it also prohibits that data from being stored in a central repository. Some states have more stringent laws to encourage record keeping, but their requirements often only apply to certain types of guns or dealers.

So instead of using gun sale data, most researchers use total background checks as a proxy. Since 1998, Federal Firearms Licensees have been required to query the FBI’s National Instant Criminal Background Check System (NICS) at the checkout counter to verify a buyer is eligible to purchase a gun. The FBI tracks and releases data on the number of background checks submitted by month, state, and gun type.