I was watching one of the "Making Of" videos for Season 1, specifically for the episode Wildfire and I noticed this really strange rifle. I then checked our wikia and realized we didn't have a page for it, and not even the IMFDb has identified or featured this gun. If you know anything about firearms, your opinion would be greatly appreciated :D
In my opinion, it looks similar to a Stoner 63A, although I am no expert in firearms.
Also on a less significant note, doesn't this "deceased' soldier look alot like Mike Senior? (the actor that played Gaines in a deleted scene)