There are a couple of noticeable holes in the Mk200 LMG's ironsights model when the sights are folded down (hasOptics).
The first is the base of the front sight post. There was definitely a solid face here in previous versions of the game but a hole seems to have appeared in recent patches - I've checked that it's missing on but the current Stable and Dev branches.
It could just be that part of the mesh has been split from the topology and isn't part of the ironsight selection/bone that folds down when the weapon is fitted with an optics attachment.
The second is in a similar position on the rear folding sight. Here it does look like part of the mesh is there in front of the hole, but is not animating/rotating as part of the main rear ironsight selection.
I've included pictures of both the holes, picked out in Magenta using photoshop to show exactly what part of the mesh appears to be missing.