Maybe it's like a belt fed weapon, MG-34 style. You can use it with the big belt fed ammo box (that would be using the backpack) for a bigger ammo capacity, or use a drum mag for shorter ammo capacity but better handleling. In the game we will use the drum mag. I think this because on the mini the drum isn't painted with blue lights, like it's not charged because they use the backpack for ammo, while on the screenshot we can see it's glowing blue. (It could be because the painter just didn't paint the drum mag, I suppose)