If you have a gun equipped in you 1st weapon slot (or any of the 4 slots for that matter) and then equip a loadout with a different weapon type than a gun (i.e. a sword) in the SAME slot as the gun was in with your previous loadout, the animation/idle for the gun stays the same but the weapon changes, this makes swords look like they're being held partially over the shoulder.

I don't think this works for bombs, but I could be wrong..