"When Omar Mateen embarked on his deadly rampage at a gay nightclub in Orlando early Sunday, he carried an assault-type rifle called a Sig Sauer MCX, law enforcement sources tell NPR. The MCX is described by its manufacturer as "the ultimate modern hunting rifle" that is "engineered from the ground up to be short, light and silenced." It is similar in appearance and lethality to the AR-15, the name given to a very popular family of rifles made and sold by nearly two dozen companies. Law enforcement sources call the MCX an "AR-15 equivalent made by a different manufacturer." ... Despite its similarities, the MCX is also fundamentally different from an AR-15. Its internal operating system — its method of propelling bullets from its chamber and cycling to the next round — works differently, and several gun experts contacted by NPR say they do not consider it to be part of the large family of AR-15 rifles."