In the hours after Sunday’s mass shooting at an Orlando night club, Chief John Mina of the Orlando Police Department said the gunman’s weapons included a pistol and an “AR-15-type assault rifle.” On Monday night, officials clarified that the rifle Omar Mateen used in the shooting was not an AR-15, but a Sig Sauer MCX rifle. While aesthetically similar to and just as lethal as an AR-15, the MCX is internally a different beast, thus all but removing it from the AR-15 family of rifles. Yet while the weapon is different, the MCX and the AR-15 share the same design...