It takes some time to change magazines. From photos, it looks like he used 60 rnd. surefire mags. There seems to be a lot of time between bursts, so apparently he was not making quick magazine changes or didn’t practice doing it much. Probably just switching to another weapon that was already loaded ahead of time.
I’ll respectfully disagree that it takes time to change magazines. I remember changing them quite quickly during night tactical training. If that were the case for this guy then I’ll go back to a concept that focuses on the fact he was familiar only enough with them for other agendas. I would think 60 rounds on pseudo-auto would create a heating issue with the bolt/receiver and barrel. This may have been something that is covered in the bump-stock literature. I don’t know.
http://www.freerepublic.com/focus/chat/3593097/posts?page=31#31