The bullet starts its life below the line of sight, by exactly the distance between the centre of the bore and the centre of the scope tube. Everything the solver computes about the early part of the trajectory depends on that offset, and it is the one number people cheerfully estimate.
What it changes
The offset sets the angle the bore must be tilted to bring the bullet up to the sight line at your zero distance. A taller mount needs a steeper launch angle to reach the same zero, which changes the shape of the whole arc.
| Distance | Sensitivity to a 5 mm scope-height error |
|---|---|
| 25 m | Clearly visible — this is where the offset dominates |
| 100 m | Small, absorbed by the zero itself |
| 300 m | Very small |
| 1,000 m | Negligible in the vertical |
| Any distance, steep angle | Grows again — geometry re-enters |
That table is the crux of it. Scope height matters *close in*, then largely disappears into the zero, then returns when you start shooting at steep angles or ranging with the reticle. People check their solver at 800 m, see agreement, and conclude the input does not matter — then miss a 40 m shot at a boar under a high-mounted scope.
How to measure it properly
You want the distance between the two centrelines: bore axis to scope tube axis. Neither centre is a thing you can touch, so you measure to surfaces and correct.
- Measure the outside diameter of the scope tubeA 30 mm tube is 30 mm, a 34 mm tube is 34 mm. Half of that is the distance from the tube's bottom surface to its axis.
- Measure the outside diameter of the barrel or receiver where you can reach itFor a barrel, the bore axis sits half the barrel diameter below the top surface at that point.
- Measure the gap between the two surfacesCaliper from the top of the barrel to the bottom of the scope tube, at the same station along the rifle.
- Add the two half-diameters to the gapHalf the barrel diameter, plus the gap, plus half the tube diameter. That is your scope height.
On a rifle where the barrel is not accessible next to the scope — a bull barrel under a handguard, for instance — measure from the receiver's flat top and add half the bore-to-receiver-top distance from the manufacturer's spec, or measure at the muzzle and account for taper. Both beat guessing.
Where the error shows up
A wrong scope height does not usually announce itself. Since you confirm the zero by shooting, the solver's zeroing routine simply computes a slightly different launch angle to hit the same point at 100 m. The trajectory then sits marginally wrong everywhere else, in a way too small to see at mid range.
Three situations expose it:
- Very close shots. Under 50 m the offset is a large fraction of the total geometry. This is where hunters and 3-gun shooters find the mistake.
- Steep angles. Uphill and downhill shots re-introduce the offset into the geometry. See angled shots.
- Reticle ranging. Your reticle measures angle from the scope, not from the bore. A wrong offset skews a ranged distance, and that error then propagates into the whole solution.
Getting it into the app
Scope height lives in the rifle profile, in millimetres or inches depending on your unit setting. It is stored per rifle, so a scope swapped between two rifles needs the number checked on each — the same rings on a different action give a different height.
If you only ever shoot past 300 m, an error of a few millimetres will not cost you a plate. Measure it anyway: it is the cheapest accuracy available, and the day you take a 30 m shot you will be glad the profile was honest.
Frequently asked
Can I just use the ring height the manufacturer quotes? +
No. Ring height is usually quoted from the base to the bottom of the tube, and it ignores your rail height, your action's bore-to-rail distance and your tube diameter. It is a component dimension, not a rifle dimension.
Does scope height change my zero? +
Not the zero itself, because you confirm that by shooting. It changes the launch angle needed to achieve that zero, and therefore the predicted trajectory everywhere else — most visibly very close in.
What about a canted rail? +
A 20 MOA rail does not change scope height. It changes how much elevation adjustment you have available above zero. The physical offset between bore and tube is what the solver needs.