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It's unclear how William Spengler, a convicted felon, got the gun he used to shoot at firefighters he'd lured to the scene of a blaze. But data suggests that an NRA-supported loophole makes it pretty easy for anybody to buy a gun, regardless of his or her background. Think Progress points us to the "gun show loophole," an exception to the Brady Law -- ironically, the same one that bans felons from being able to buy or posses a firearm -- that permits the purchase of firearms at gun shows without a background check. More specifically, it does not require a background check if the gun is bought from an unlicensed firearms dealer. As many as half of the dealers at gun shows are unlicensed.

This is a problem. "Not surprisingly, criminals exploit the private sales loophole," reads a 2009 study from the Johns Hopkins Center for Gun Policy and Research. "Data from a national survey of inmates indicated that nearly 80 percent of those who had used a handgun in a crime had acquired it through a transaction with an individual who was not a licensed gun dealer." It's hard to know how many of those came from gun shows and how many came from the black market, but it's easy to see how closing one little loophole could have a pronounced effect on gun violence. And the horrible Christmas Eve shooting in Webster, New York provides a pretty powerful case study.