This is seems to be an animation problem, but the result causes scopes to be unusable in some circumstances.

If you take a scoped weapon, and lie prone with it, depending on the direction you face, the actual scope viewpoint moves forwards and backwards.
This can cause issues as in some cases, the viewpoint moves inside the 3d scope itself, either leading to a massivly expanded scope view, or in some cases a distored view or no view at all.
This typically happens with addon weapons with large scopes, and can make them unusable when prone.
Not certain if this is a specifc problem with addon modelling or with the core animation when prone.