Gunman Omar Mateen was armed with an AR-15-type assault rifle, which has become the signature weapon of mass killers in the US. Known as "America's gun," it is owned by around 3.7 million American households, making it the country's most popular rifle. The military-grade weapon was used by Adam Lanza in December 2012 to shoot dead 26 people at Sandy Hook in Newtown, while the San Bernardino terrorists used AR-15s when they killed 14 people in December 2015. It was... (GunPolicy.org)