When a rifle is fired at a distant target, the barrel is not lined up exactly on the target. Why ?

Explanation

As soon as a bullet is fired from a gun whose barrel is lined up exactly on the target, it starts falling downwards on acconunt of acceleration due to gravity. Due to it, the bullet will hit below the target just to avoid it, the barrel of the gun is lined up little above the target, so that the bullet, after travelling in parabolic path hits the distant target.