It requires a new camera mode: basically it is locked on xhair, i.e. xhair is always in screen center, and mouse moves the camera, not the xhair. The base distance of the camera from the player is controlled by the mouse scroll, i.e. zooming function of the sniper is actually changing the camera's base distance from the player, while mouse movement changes the relative position around that distance.
It requires a new camera mode: basically it is locked on xhair, i.e. xhair is always in screen center, and mouse moves the camera, not the xhair. The base distance of the camera from the player is controlled by the mouse scroll, i.e. zooming function of the sniper is actually changing the camera's base distance from the player, while mouse movement changes the relative position around that distance.
A full-screen scope mask should be also applied.