His shots didn't have to be aimed in a packed house and he was shooting as fast as he could pull the trigger. If he had practiced changing mags then he only needed a of second for reloading.
This is a NY Times account of the shooting:
http://www.nytimes.com/interactive/2016/06/12/us/what-happened-at-the-orlando-nightclub-shooting.html?_r=0
“Adam Gruler was off duty when he tried to stop Mateen outside club Pulse”