I believe that's an M249 SAW. They came along after I was out of the big green machine, so I never handled one. I only heard a couple incoming rounds. Seems like they were going through a lot of ammo. I'm sure they know better than me what needed shooting, though.
And I believe there's 200 rounds to a can, but don't quote me on that.
That is an M249 PIP...this is the latest version of the SAW. Above is correct, polymers and soft bags (Nut sacks) as we called them, beveled edges, the older ones would cut the shit out of you. I carried the MK 46 version which fired the 7.62x51 mm NATO round in Somalia and various other locations. Excellent weapon platform, but the earlier variants suffered a little as the bipods would easily break and it had way to many large openings for dirt and what not to enter the weapon. It has been around since 84 and has had many different configurations added and taken from it. All in all a good platform.
Yep 100 & 200 round bags, belts or also in emergency situations a typical M4 or M16 magazine can be used as well (on the 5,56 versions), but this seemed to cause malfunctions as the magazine could not keep up with the cyclic rate of the weapon so emergency use only. We typically carried 5 200 round belts and an extra 500 rounds in prepacked nut sacks. Cleared many a beach with this weapon, most notably was in Port Au Prince, Haiti in the mid 90's.