Truing BC and muzzle velocity

What is ballistic coefficient?

Ballistic coefficient (BC) is a single number that describes how well a bullet slips through the air — how slowly it sheds velocity to drag. A higher BC means the bullet holds its speed better, so it drops less, drifts less in the wind, and stays supersonic further out. A long, heavy, sharply pointed boat-tail bullet has a high BC; a short, light, blunt one has a low BC.

The number only means something alongside a drag model — a standard reference projectile shape the BC is measured against. The two you will meet are G1 (an older flat-based reference, still quoted for most hunting and lighter bullets) and G7 (a boat-tail reference that fits modern long-range match bullets much better). The same bullet has a different BC number under each model, so always enter the BC and the drag table as a matched pair from the manufacturer.

Manufacturers publish BC from their own testing, often rounded or optimistic, and it also changes slightly with velocity and altitude. That published figure is a starting point — truing, below, corrects it to what your rifle actually does.

What truing does

The calculator predicts drop from muzzle velocity and ballistic coefficient. If those inputs are slightly off, the error is invisible near your zero but grows with distance — you dial the card at 800 m and land half a metre low. Truing adjusts BC, muzzle velocity, or both so the predicted trajectory passes through the impacts you actually recorded.

Roughly speaking: a velocity error shifts the whole curve and shows up from mid-range out; a BC (drag) error compounds and dominates at long range and toward the transonic zone.

Before you start

• A confirmed zero. Every observed drop is measured relative to it.
• Your best muzzle velocity. Ideally a chronograph average from this rifle and lot, not the box figure.
• Environment entered for the day you collected the data.
• Impacts at two or more distances — three or more spread across your usable range gives a much more reliable solve. Include a distance well out (but before the bullet goes transonic, where drop stops behaving predictably).
• Calm conditions, or at least a known, steady wind — vertical stringing from gusts will pollute the drop numbers.

Running the wizard

On the Calculator page, open the Ammunition section and click Tune to open the Zero/DOPE Truing Wizard.

1. Choose a strategy

2. Measure and enter observed drop

At each distance, fire a group aiming at the same point of aim you zeroed with. Measure vertically from the point of aim to the centre of the group:

• Impact below the point of aim → enter a negative value.
• Impact above → positive.
• Use the distance and drop units shown in the wizard (they follow your card).

Add a row per distance with Add Observation, then click Solve Truing.

3. Check the result

The Tuned Values box shows the solved MV and BC, and the chart overlays the tuned and original trajectories with your data points. If the solved number is wildly different from the book value (a BC that halved, a velocity 150 fps off a good chrono reading), your data or units are probably wrong — recheck before trusting it.

4. Apply to card

Apply to Card writes the solved values back into the Ammunition inputs. Save the card, then confirm the new dope on paper at a distance you did not use in the solve.

Caveats

• Garbage in, garbage out. A sloppy zero or mismeasured drop produces a confident, wrong BC.
• It is specific to this rifle, barrel and lot. A new lot of powder or a fresh barrel can shift things; re-true if your confirmed dope drifts.
• Truing hides other errors. If your sight height or drag model is wrong, the solver will distort BC to compensate and the fit will fall apart outside the distances you used.
• Past the transonic zone, all bets are off. Don’t true against data collected there.