I've just noticed an issue on my commando that while my backpack lists 0/15 when in my inventory it is listed as -1/15 while in a store menu. Is this an extra slot for skins/heads?
Also that when I am playing multiplayer I can pick up ammo even when I am full. Now at first I thought it meant I was sharing the ammo and they weren't full but it continued even after I had picked up excessive amounts of ammo without them firing. In fact while in multiplayer I have never been unable to pick up ammo regardless of my stock being full.
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Well, for the ammo it only happens in multiplayer and it is just implemented poorly. But it happens for everyone. I prefer it because it's less loot lying around. I don't know what to say about your backpack though.