The Vertical Offset Between Sight and Bore
Sight height over bore is the vertical distance between the center of the aiming device and the centerline of the bore. Though it is often only a few centimeters, this small offset shapes where a projectile strikes at every distance. It is a required input for any honest trajectory prediction.
Line of Sight Versus Line of Departure
The line of sight is the straight path from the eye through the sights to the target; the line of departure is the direction the bore actually points. Because the sight sits above the bore, these two lines are not the same. The barrel must be angled slightly upward relative to the sight so the two paths cross downrange.
Near Zero and Far Zero
Since the projectile launches below the line of sight and arcs upward, its path can cross the line of sight twice: once on the way up, the near zero, and again on the way down, the far zero. A single "zero distance" usually refers to one of these crossings. Between and beyond them, the projectile sits above or below the aim by a predictable amount.
The Starting Offset
At the muzzle, the projectile is below the line of sight by exactly the sight height over bore. A taller mount increases this initial gap, so the projectile must rise farther to reach the aiming line, which changes the whole near-range portion of the trajectory. This is why the number belongs in the assumptions of any drop chart.
Why Precision Matters
An error in the assumed sight height feeds directly into calculated holdovers, especially at close range where the offset dominates. Getting it wrong by even a centimeter shifts the predicted impact enough to matter for precise work. That is why it is measured rather than estimated.
Worked Example
Suppose a rifle has a sight height over bore of 4.0 cm and is zeroed at 100 m. At the muzzle the projectile starts 4.0 cm below the aiming line. At a very close 10 m, it has barely begun to climb, so it still strikes roughly 3.5 cm low relative to point of aim. A shooter who forgets this offset and aims dead-on at a close target will hit noticeably low, purely because the sight sits above the bore.
A Common Misconception
Many assume that once a rifle is zeroed, the sight and bore are effectively aligned and impacts land where you aim at all distances. In truth the two lines only meet at the zero distance; everywhere else the height over bore produces an offset. This is exactly why close targets often strike low even on a perfectly zeroed rifle.