In the FPS apart from placing weapon in the down right corner (hip shooting), there is usually the second, alternative setting. Shooting when a player uses a sight / scope.
In such a setting weapon is placed in the middle of the screen and a player targets not with a crosshair (as in case of the hip shooting) but by adequately modelled parts of weapon sight.
If there are any elements of a whitebox object that should be precisely modelled, those are the sights.