Although i dont profess to be an expert on the subject, judging from the descriptions it seems they were using
Real Action Markers' (RAM). THese are M4 (M16) repllicas that have a magazine of paintball rounds that are inside small metail tubes, when the gun is fired (small air cylinder in back) the mini paintball is fired and the metal tube in ejected in a realistic manner to simulate spent rounds.
I've seen a vid of one in action, rather silly for playing 'ball, but i kinda see the the point for the context that they are using them for in this show. I'm sure there is some official website out there if you want any more info.