As discovered in this thread throughout this year, Insurgency: Sandstorm is using equivalent of the "Boxed freeaim" setting from Rising Storm 2. What we need is the "Smooth rotation freeaim" setting, it is basically the same thing but without the hitch in the middle, causing the mouse movement to feel off and as if the game is having hitches.
Lots of people seem to be put off from the game because of this, me included. Would be awesome to have the option for this freeaim setting.