In the aftermath of the massacre of 26 people in a small-town Texas church, you might have seen that the killer used a gun called an AR-15. It’s a popular weapon—relatively easy to use, endlessly customizable, military in appearance. How popular? It’s the same gun that a killer used in the massacre of 58 people at a Las Vegas concert last month, and by the killer who murdered 49 people in a nightclub in Orlando, and the one at an elementary school in Newtown, Connecticut.