He was using the M8A7, the 4-burst mode assault rifle. M8A7 is the last assault rifle that one can unlock. You might wonder why as burst mode assault rifles are general not as easy to use as a normal assault rifle such as ICR-1.
I have not unlocked this yet as it requires you to be at level 55. I am still using the 3-burst mode XR-2. Watching this might change your mind on this type of assault rifle.