You notice this on free-aim right?
This video shows that it's because while you're free-aiming (not using ADS) the weapon moves faster in a certain small circle area, but then, when the camera starts moving, it slows down. Not to mention the fact that, even though the weapon is pointing in 1 direction at free aim, it'll point in an entirely different position in ADS.
Did you try Settings->Controls->Scope Sensitivity = 1.0
I think if this multiplier is not 1 it messes up the mouse movement.
I'll check that out. I'm not sure if that's what it is though. My look speed suddenly changes to hyperspeed or slows down to where it barely moves. I dont know if its the game or windows 10.