Height over bore, and why every zeroing number starts with it

Height over bore — mechanical offset, optic height, same thing — is the vertical distance between the centre of the bore and the centre of your optic’s line of sight. On most rifles it lands between 1.5″ and 3″, depending on rings, mount and optic body. It sounds like a detail. It is the reason your first shot at 10 yards lands low enough to miss a steel plate entirely.

Why it matters

Because the scope sits above the barrel, your line of sight and the bullet’s path are two different lines in space. Zeroing tilts the line of sight down slightly so the two cross at a distance you choose. They are never parallel, which means there is no distance at which the offset simply goes away.

The double zero

For most rifle setups the bullet crosses the line of sight twice:

• Near zero — the bullet is still rising and crosses the line of sight for the first time, typically 25–35 yards.

• Far zero — the bullet is falling and crosses back, at the distance you actually zeroed for.

Between those two points the bullet travels above your line of sight. That is why a 100 yard zero hits high at 50. The full mechanics get their own article — understanding double zeroes — at the end of this path.

Measuring yours

1. 1

   Clear the rifle. Chamber open, magazine out, checked twice.

2. 2

   Rest it on a flat surface with the barrel parallel to that surface.

3. 3

   Measure bore centre to optic centre. Easiest route: outside bottom of the barrel plus half the barrel diameter, up to the centre of the objective.

4. 4

   Record it in inches or millimetres — it is an input to every calculation that follows.

Or skip the tape: most mount makers publish their mount height, and half your tube diameter is 0.5″ for a 1″ tube, 0.59″ for a 30mm.

Typical values