Converting a ballistic coefficient
Goal of this article: answer the everyday “my bullet is quoted G1 but I want G7” question, and understand why the answer comes with a reference velocity attached.
Tools → Convert Ballistic Coefficient… does one thing: restates a coefficient quoted against one standard
drag table as a coefficient against another.
The dialog
Four fields, and no Convert button — the answer follows the inputs as you type them, because the interesting output is the relationship, not a one-shot calculation.
| Field | What it is |
|---|---|
| Source BC | The coefficient you have, with the table it is quoted against |
| Destination Table | The table you want it against |
| Reference Velocity | The velocity at which the two curves are matched |
| Target BC | The result, read-only |
Set Atmosphere is there because the reference velocity has to be turned into a Mach number, and Mach depends on temperature. Standard air is assumed if you leave it alone; the difference only matters if you are working at the edge of the transonic region.
Underneath, the dialog states the answer as a sentence including the reference — 0.243 G7 at 2,600 ft/s,
say — rather than presenting the number bare. That is deliberate, and the next section is why.
Why there is a reference velocity at all
The same projectile has the same drag whatever curve you describe it with. What differs is the shape of the reference curves: at one Mach number the ratio between the G1 and G7 curves is one value, at another it is a different value. The conversion is exactly that ratio:
BC_target = BC_source × Cd_target(M) ÷ Cd_source(M)
So a converted number is exact only at the velocity it was computed for. There is no such thing as “the G7 equivalent” of a G1 BC — only its G7 equivalent at 2,600 ft/s, which is a slightly different number from its equivalent at 1,800.
How much does it matter in practice? Between roughly Mach 1.8 and 2.5 the conversion lands within about 1 % of manufacturer-published G1/G7 pairs. Near Mach 1.3 it comes out around 9 % low. The dialog warns you when the reference falls below Mach 1.5, where the curves diverge in shape and the number stops deserving three decimal places.
Choosing a reference velocity
Pick the velocity where you care about being right:
- The velocity band you actually shoot in. If your shots land between 700 and 500 yd, use the velocity the bullet has around there, not the muzzle velocity.
- Not the muzzle velocity by default. It is the highest speed in the flight and the shortest part of it; matching the curves there makes the conversion worst where the bullet spends most of its time.
- Not a transonic velocity. If the reference has to be transonic for your purpose, the honest conclusion is that a converted single number is the wrong tool — see below.
When not to use this at all
Converting is a compromise, made necessary by a missing number. Three cases where something better is available:
- The maker publishes both. Use the published G7 rather than converting the published G1. Measured pairs beat computed ones.
- You have BCs at several velocities. That is a curve, not a number — feed it to
From BC Curve and get a
.drg, which is strictly more information than any single converted coefficient. - A measured
.drgexists for your bullet. Then the whole question dissolves: there is no coefficient to convert because there is no reference shape in play. See custom drag tables.
Where converting genuinely earns its place is the ordinary case: one published G1 number, a long shot, and a desire for the drag bookkeeping to extrapolate less badly than G1 does. Convert at a sensible reference, note what that reference was, and treat the result as a good approximation rather than a datum.
Next
Hit probability — the Monte-Carlo tool, and the error budget it needs from you.