AR-15s are very common here in the states, as Americans own well over eight million of these semi-automatic assault rifles which continue to be sold by the hundreds of thousands each year as they are legal in states such as Nevada. So I'm guessing the shooter may have used a modified AR-15 with a slide fire or similar modification to simulate automatic fire at a bump fire rate of 900 rounds per minute. Like I mentioned earlier, Nevada has no assault weapons law,  neither do they have any laws restricting magazine capacity.