Because as the eyes have certain limitation and there are influences affecting on what it does. Nevertheless good ** can be obtained with consistency of aim.**
Laws Of Aiming :
Focus the target so that a clear picture is formed on the retina of the eye and get the true center of the target and focus to the foresight.
Hold the rifle properly and keep it upright.
Close the left eye and focus the foresight.
See the foresight through the backside ‘U’. The foresight should be seen right in the centre of the ‘U’. The tip of the foresight should be aligned in the centre and in level with the shoulder of the ‘U’.
Elevation is given to counter the force of gravity, while the bullet is in flight. The elevation depends on the range of the target.
