When entering prone lean the player is able to (regardless of mouse settings) spin in a full 360. This would not be such a problem if the animation reflected it with a pushing hand and a slower turn rate (after all you can crawl in this position)
When you toggle free-look while in optics mode, be it scope or aim point, the gun is only allowed to travel in a certain arc which might be a good idea but on the other hand you also have the issue of needing to stand up to turn around.
Both options have their pro's and cons but ultimately for the best fluidity a slower turn would work best but could be offset by mouse sensitivity, the arc would make it more of a situational stance but wouldn't allow the user to cover nearly as much ground.