You enter a catalogue muzzle velocity and a published ballistic coefficient, and the solver produces a trajectory that is close to reality but not equal to it. Truing closes the gap by adjusting inputs until the prediction matches impacts you have actually recorded.
Done properly it turns a good approximation into data you can trust for first-round hits. Done carelessly it hides one error inside another and produces a solution that is right at exactly one distance.
The core insight: velocity and BC fail differently
Both a wrong muzzle velocity and a wrong ballistic coefficient make the solver predict the wrong drop. What separates them is *how the error grows with distance*.
| Wrong input | Error at mid range | Error at long range | Shape |
|---|---|---|---|
| Muzzle velocity | Already visible | Grows steadily | Roughly proportional throughout |
| Ballistic coefficient | Barely visible | Grows sharply | Accelerates as velocity bleeds off |
That difference is the whole method. Correct the input whose error signature matches what you observe, and you fix the cause. Correct the other one, and you have compensated a velocity error with a fictional bullet shape — which will be wrong again the moment you change distance, altitude or temperature.
The order that works
- Confirm the zero firstA zero error is a constant offset at every distance and it will contaminate everything else. Five rounds at 100, centred, before you go further.
- Check the boring inputsScope height measured, not guessed. Zero distance correct in metres or yards. Click value correct. Drag model matching the coefficient. Most 'BC errors' are one of these.
- True velocity at mid rangeSomewhere the bullet is still comfortably supersonic and the drop is large enough to measure — commonly 400–600 m. Adjust muzzle velocity until predicted and actual drop agree.
- True BC at long rangeNow go out to the far end of your supersonic envelope. If the prediction is still off, adjust the coefficient. Velocity is already anchored, so what remains is drag.
- Go back and re-check mid rangeThe BC change moves the mid-range prediction slightly. One iteration is normally enough; if it is not, something else is wrong.
Getting impact data worth fitting to
Truing is curve fitting, and fitting to noise produces a confident wrong answer. The quality of the data decides everything.
- Five rounds minimum per distance, and measure to the group centre. Truing to a single shot is fitting to your own dispersion.
- Calm conditions, or a pure headwind. Wind moves impacts horizontally, but a strong head or tail component also changes drop slightly.
- Record the conditions. Temperature, pressure and altitude at the moment you shot. Data without conditions cannot be reused.
- One variable at a time. Do not change ammunition, scope or zero mid-session and expect the numbers to mean anything.
- Stay supersonic. Data taken in the transonic region reflects transonic behaviour, and truing to it distorts the entire trajectory.
When truing is the wrong tool
If your predicted and actual drop differ by a large amount at *every* distance including short ones, you do not have a drag problem. Look for a wrong zero distance, a mixed-up unit, a scope height taken from a catalogue, a click value that does not match the turret, or a G1 coefficient running against the G7 table.
Truing can absorb any of those into a plausible-looking velocity or BC. That is the danger: the numbers will match at the distances you fitted, and diverge everywhere else. A trued solution that required an implausible input — a velocity 200 fps from measured, a BC thirty per cent from published — is telling you that something else is broken.
How the app handles it
The truing screen takes distances and the corrections that actually worked, and solves for the adjustment that reconciles them with the trajectory. Because it works from your recorded impacts rather than from a slider, you can see which input it wants to move and by how much — and judge whether that number is believable.
The step-by-step version, screen by screen, is in the truing guide.
How long a trued solution lasts
A trued velocity is valid for that ammunition lot, that barrel and roughly that temperature. Barrels erode, lots change and seasons turn. A trued coefficient is more durable, because bullet shape does not change — but it is still tied to the drag model you fitted it with.
Re-confirm at the start of a season, after a new lot of ammunition, and after any significant round count through the barrel. It costs ten rounds and it is the difference between data and folklore.
Frequently asked
Should I true velocity or BC first? +
Velocity, at mid range, then BC at long range. Velocity errors show up earlier and more uniformly; doing it in the other order lets a velocity error hide inside a distorted coefficient.
My trued velocity is 150 fps off the chronograph. Is that normal? +
No. That gap usually means another input is wrong — scope height, zero distance, click value or drag model. Find it rather than accepting a velocity you know is false.
Do I need to re-true if I move to a different altitude? +
No, if you enter the new conditions. A properly trued solution corrects the inputs; the solver then handles atmospherics on its own. Needing to re-true for a new altitude is a sign the truing absorbed an error it should not have.