Scope Height Correction Calculator
Your previous calculation has been restored. You can edit any value below and calculate again.
Why does the measured Scope Height need correcting?
Many ballistic calculators ask for your Scope Height. What they normally need is the Line of Sight height at the precise point where the pellet leaves the barrel.
That point is awkward to measure directly, so most shooters take the measurement farther back along the rifle. Relative to the bore, the Line of Sight converges towards the zero point, so the separation measured farther back is not necessarily the same as the separation at the pellet exit.
This calculator uses the pellet's trajectory, the zero distance and the position of your measurement to calculate the value at the point where the pellet begins its flight.
Why does it matter, if the correction is usually small?
The size of the correction depends on how far back you measured and how the rifle is zeroed. For a scope measured close to the muzzle, it's often well under a millimetre. Measure further back along the tube - long eye-relief mounts, or measuring near the turret rather than the objective bell - and the difference can grow to several millimetres.
Because this calculator solves the correction from your actual trajectory rather than a rule of thumb, there's no reason to guess or leave the field at your raw measurement. It costs nothing to get it exactly right.
GA, G1 and G7
The Airgun Knowledge Base uses the GA drag function because it is designed to represent the flight of diabolo airgun pellets.
Some digital night vision and thermal scopes have ballistic calculators built in but do not offer GA. They may only provide G1 or G7. Use GA whenever the device supports it.
If GA is unavailable, G1 is normally the better fallback for a diabolo airgun pellet. GA and G1 remain reasonably similar over the shorter part of a typical airgun trajectory, up to approximately 35 yards - so for a device that does not support GA, it is better to zero below 35 yards. My own preference is 25 yards. Always use a Ballistic Coefficient that matches the drag function selected in the calculator.
G7 remains available because some devices or supplied ballistic data may require it, but it should not be presented as the preferred model for a normal diabolo pellet.