"America has grown accustomed to military-style semi-automatic weapons such as the AR-15. It's not hard to see why: These firearms have been heavily marketed to gun owners. But at the same time, they're often the weapons of choice for mass murderers. In an all-too familiar pattern, Omar Mateen, the gunman in the Orlando nightclub massacre, used one of these weapons, Sig Sauer's MCX (a version of the AR-15 rifle), in his deadly attack on Sunday. The AR-15 has grown so popular that the pro-gun lobby National Rifle Association has termed it "America's rifle." The AR-15 and its variants have been used in a number of mass shootings, ranging from the Newtown, Connecticut, elementary school tragedy to the San Bernardino, California, attack in December."