Well it's in multiplayer. It's used in one of the campaign videos they showed at E3, I would go find the video but I found this picture first and it pretty much shows you everything you need to know:
Basically it's like a more devastating Mulcher that you lug around on your shoulder, but it has an ammo box that a 2nd player needs to hold to feed ammo into it.
Now I'm not too sure but by the looks of one video I saw it seems 1 person can use it, they just can't move anywhere with it because the ammo box wont budge, and I think they have to slowly reload everytime the clip finishes when they don't have someone else there to feed ammo into it.
It apparently shoots dozens of bullets a second so that gave it the illusion of being 1 or 2 bullet kill. It just tears people apart in less than a second. Luckily it looks like you can't run or get in cover with it, meaning snipers will love it, and it has massive recoil.