ORLANDO, FLORIDA — Omar Mateen, the shooter who killed 49 people at a Florida nightclub on Sunday morning, did so with the help of a SIG Sauer MCX rifle and not an AR-15.

While both rifles look similar, they both operate differently, the Washington Post reported.

According to the Washington Post, the MCX uses gas piston technology when firing bullets. Once a shot is fired, the resulting gas is pushed into a tube connected to a piston, which then moves the bolt and another bullet is ready to fire. Guns using this system require further care and cleaning as their internal cylinders become dirty faster.


AR-15 rifles use a direct impingement system. Here, the bullet is fired, and the resulting gas flows back up the barrel, ejecting the casing, with the bolt then loading another bullet. According to weapons technology company Adams Arms, gas piston systems are easier to maintain as the gun runs cooler and cleaner.