Ar's can only hold 36 in a clip. A pistol is an extra 16. At most he had 54 bullets. There's video of him shooting it out with police outside of the club. All reports are saying he killed all 50 and injured 53.
FBI already said that there were no bombs or any vest. They didn't say anything about him having a bunch of ammunition either.
It's simply not possible for him to kill 50 people with less than 50 bullets at his disposal.