You can shoot the AR faster than normal by tapping the trigger ridiculously fast or having a modded controller(turbo button), also vetoed mapped shoot to two buttons but that didnt work as well.
The reason I say double shot is because he slowed it down and looked at the clip and it looked like two bullets were being shot every frame.
This glitch is very hard to pull off but when vetoed switched to the perspective of the guy doing it in ffa he must have had a modded controller to be able to aim while doing it. he was killing everyone in half the time, wasn't even switching away from his AR ever.
Hope that explanation helps a little