The gun looks like M6 or "M6A1" as taiso pointed out. It is probably shooting 6.8 SPC. The M6A1 is kinda of an obscure gun - very few are out compared, they are expensive, and the majority of shooters have not seen one. They also look a lot like an AR-15 because it is modeled on an AR-15...hence the confusion.
Whether it is a AR-15, M4, M16, or M6A1, it doesn't matter: it ripped through that case like butter.