WASHINGTON — When Omar Mateen entered an Orlando, Florida, nightclub yesterday to carry out the deadliest mass shooting in US history, he wielded a weapon that has been used in massacres from California to Connecticut: a military-inspired semi-automatic rifle. Though so-called assault rifles account for a small fraction of the United States' 30,000 annual gun deaths, they have been used in at least 10 mass shootings since 2011, according to a database compiled by... (GunPolicy.org)