What is BC truing?

Why a box BC is only a starting point

A catalogue or “box” ballistic coefficient is a single number, measured once, under conditions you did not control. Two things make it a starting point rather than an answer.

First, BC is not constant — it changes with velocity. The drag a projectile feels depends on its speed relative to the speed of sound, and it shifts sharply through the transonic region (roughly the band around Mach 1). A coefficient that fits at the muzzle can be wrong by the time the projectile is slowing downrange. For diabolo airgun pellets and for .22LR — both of which spend much of their useful flight at modest, changing speeds — that drift is large enough to matter at the target.

Second, the published number was measured on a different system. Your barrel, crown, twist rate and how a given pellet actually fits your bore all nudge the real coefficient away from the catalogue figure. Across airgun pellets and rimfire ammunition the shot-to-shot and tin-to-tin scatter is wide, so a single quoted value is at best an educated guess for your setup.

None of this means box BCs are useless — they are a sensible place to begin. It means the honest next step is to check that number against reality. For a deeper look at where the figure comes from, see our explainer on the airgun pellet ballistic coefficient.

What BC truing actually does

Truing is a calibration loop. You confirm your zero, then observe the actual drop at one or more longer ranges — the distance, and how far below the line of sight the projectile struck. You hand those observations to the calculator, and it works the problem backwards: it searches for the ballistic coefficient that, run through the same trajectory model, would reproduce exactly the drop you measured.

How much you can recover depends on how many observations you give it:

• One observation calibrates a single, scalar BC. The calculator finds the one number that makes the model pass through your measured point. This already removes most of the error between a box value and your real setup.
• Two or three observations fit a velocity-dependent BC curve. With drops measured at different ranges — and therefore at different velocities — the model can describe how the coefficient changes as the projectile slows, rather than pinning it to a single value. This is what keeps a solution honest through the transonic region.

The output is not a prettier guess; it is a coefficient anchored to evidence you collected. The model now agrees with reality at the points you checked, and interpolates sensibly between them.

How PelletArc does it

PelletArc is built around this loop, because for airgun and rimfire shooting it is the strongest answer to BC scatter. There are three pieces:

• Single-shot scalar BC (free). Give it one observed drop past your zero and it calibrates a single BC for your setup — at full solving accuracy, the same as everything else in the free tier.
• Multi-distance velocity curve (Pro). Add two or three observations and PelletArc fits a velocity-dependent BC curve, so the solution tracks the projectile as it slows.
• A built-in DOPE log that feeds truing in one tap. The drops you record in the field are exactly the input truing needs, so you do not re-type anything — your DOPE becomes your calibration.

It all runs on-device and fully offline, against PelletArc's airgun-specific drag laws and a .22LR rimfire model — so the projectile is solved as what it is before you even start truing. You can read more in the guides or jump to the airgun ballistics calculator overview.

Truing vs chronograph vs doping

These three are often confused, but they measure different things and complement one another. Used together they pin down a trajectory from muzzle to target.

• A chronograph measures muzzle velocity — how fast the projectile leaves the barrel. It tells you the starting speed but nothing about how drag behaves downrange.
• Doping (keeping DOPE — “Data On Previous Engagements”) is the record of the holds and drops you have actually seen at given ranges. It is observation: what happened, not why.
• Truing fits the ballistic coefficient so the model reproduces that observed drop between your zero and your target. It turns your dope into a calibrated input the calculator can extrapolate from.

A clean muzzle velocity from the chrono, drops recorded as dope, and a BC trued to match them is the whole loop: measured start, measured reality, and a model calibrated to connect them. If your reticle is the part that confuses you, the mil vs MOA guide explains how holds and drops are expressed.

When truing matters most

You will not always need it — at short range with a well-known pellet a box BC may be close enough. Reach for truing when the stakes for an accurate BC rise:

• Longer shots, where small BC errors grow into real misses.
• Transonic ammunition, where the coefficient changes fastest and a single number is least trustworthy.
• A new pellet or a fresh lot, whose real behaviour in your barrel is unknown until you check it.
• FT and HFT, where range estimation and holdover have to be exact to score.

The principle behind all of it is simple: measure, then calibrate. Log the drop you actually see, true the BC to it, and let the calculator carry the rest. See it in action via truing in PelletArc, or check what is free versus Pro on pricing.