I've found a little bug with the weapon switching and I'm not sure if this is the right place to put it but at least someone else will see it here.
Essentially, I was using the .338 Marksman DLC LMG. When the ammo ran out I pressed reload, and the reload animation started. Whilst the animation was ongoing, I scrolled and clicked to switch to my pistol, but the LMG reload continued. The problem is that at the end of the animation my character pulled out his pistol before closing the cover on the belt of ammo, leaving the LMG on my back with the cover open. I then switched back to the LMG, and the cover remained open, and I was able to ADS with the cover blocking the sights, as well as being able to shoot normally. Further experimentation showed that the cover opens to seemingly random angles.
To summarize, here are the steps to replicate the bug:
1. Shoot a bit with LMG.
2. Begin reload.
3. Whilst reload animation is in action, switch to pistol.
4. Once animation has finished, LMG will be on your back with cover open. Switch it back into your hands.
5. The LMG will now be in your hands, usable, but with the cover open.