The Truing screen takes distances and the corrections that actually worked, and solves for the input adjustment that reconciles them with the predicted trajectory. This guide covers how to feed it data worth fitting to, and how to read what it gives back.
Before you open the screen
Truing can absorb almost any error into a plausible-looking velocity or coefficient. That is precisely why the checks come first — otherwise you are fitting a drag model to compensate for a mistyped scope height.
| Check | Consequence if wrong |
|---|---|
| Zero confirmed with five rounds | A constant offset at every distance |
| Zero distance matches what you shot, in the right units | The classic 100 m vs 100 yd error |
| Scope height measured with a caliper | Skews the whole trajectory geometry |
| Drag model matches the coefficient | Roughly double or half the real drag |
| Click value verified | The right angle expressed as the wrong turret count |
| Level fitted and used | Cant error masquerading as windage |
Collecting the data
- Pick two distances, not oneA mid-range distance where the bullet is comfortably supersonic — commonly 400–600 m — and a long one near the edge of your supersonic envelope.
- Shoot five rounds at eachThree is a rough centre with a wide error bar. Five is the minimum for a number worth fitting to.
- Record what you dialled and where it landedMeasured from the group centre to the aiming point, in your angular unit.
- Record the conditionsTemperature, station pressure, altitude. Data without conditions cannot be reused or compared.
- Do it in calm airWind moves impacts horizontally, and a strong head or tail component moves vertical too.
Working the screen
Enter the rifle, the load, the conditions and then each distance with the correction that actually worked. The screen compares those against the predicted trajectory and proposes an adjustment.
The order to work in is velocity first, at mid range, then coefficient at long range. The reason is that the two errors have different shapes:
| Symptom | Cause | Adjust |
|---|---|---|
| Error roughly proportional at all distances | Muzzle velocity | Velocity, using the mid-range point |
| Small until 600 m then growing sharply | Ballistic coefficient | Coefficient, using the long point |
| The same offset at every distance | Zero or click value | Neither — go back and fix the profile |
| No pattern at all | Data quality | Neither — reshoot on a calmer day |
After adjusting the coefficient, re-check the mid-range point: the change moves it slightly. One iteration is normally enough. If it takes three, something outside the truing screen is wrong.
Judging the proposed number
This is the part that separates truing from curve-fitting. Ask whether the number it wants is physically believable.
- A velocity within 30–60 fps of the catalogue figure — normal. Test barrels differ from yours.
- A velocity 150 fps or more from a chronographed measurement — not normal. Something else is wrong; find it.
- A coefficient 5–10 per cent below the published value — common and unsurprising. Published figures run optimistic.
- A coefficient 30 per cent off — you are compensating for a different error, most often a model mismatch.
If you own a chronograph, measure velocity rather than truing it, and true only the coefficient. A measured input stays valid when conditions change; a fitted one carries whatever it absorbed.
After truing
- Re-run the DOPE ladder across all your distances and confirm the whole curve, not just the two points you fitted.
- Regenerate your DOPE card and label it as confirmed rather than calculated.
- Note the ammunition lot. The trued velocity belongs to that lot and that barrel.
- Diary it. Re-confirm each season, after a new lot, and after significant round count.
How long it lasts
A trued velocity is valid for that lot, that barrel and roughly that temperature. A trued coefficient is more durable — bullet shape does not change — but it is tied to the drag model you fitted it with, so switching from G1 to G7 means starting again.
Frequently asked
Can I true with only one distance? +
You can adjust one input against one distance, but you cannot tell whether the error was velocity or drag, and the two behave differently everywhere else. Two distances is the minimum for a diagnosis.
The screen wants a velocity far from my chronograph reading. What now? +
Do not accept it. That gap is telling you another input is wrong — usually scope height, zero distance, click value or the drag model. Find the real error.
Do I need to re-true when I travel to a different altitude? +
No, provided you enter the new conditions. If a solution only works at one altitude, the truing absorbed an error it should not have.