Two bullets leave two barrels at the same 2,700 fps. At 1,000 yards one has lost half its speed and the other has lost more. The difference is not powder, barrel or luck — it is drag, and the ballistic coefficient is the single number that summarises how much drag a bullet suffers relative to a reference projectile.
The formula behind the number
BC is built from two pieces. The first is sectional density: bullet mass divided by the square of its diameter. Heavy for its calibre means more momentum pushing through the same hole in the air.
The second is the form factor: how the bullet's shape compares with the reference projectile's shape. A long, sharp ogive with a boat tail slips through the air better than a blunt flat-base of identical weight, and gets a form factor below 1.
Because sectional density carries units of mass over area, BC is conventionally quoted in pounds per square inch — which is why the number looks dimensionless but is not. You will see 0.243 and 0.485; both are lb/in².
What a higher BC buys you
A higher BC does not make a bullet leave the muzzle faster. It makes it keep more of the speed it started with, and every downrange consequence follows from that one fact.
| Effect | What a higher BC does | Where you notice it |
|---|---|---|
| Drop | Less, because time of flight is shorter | Past 400 m, growing fast |
| Wind drift | Much less, because lag time is smaller | Everywhere — this is the big one |
| Retained energy | More at every distance | Hunting range limits |
| Transonic distance | Pushed further out | Past 800–1,000 m |
| Muzzle velocity | Nothing at all | — |
Wind drift deserves the emphasis. Drift is driven by lag time — the difference between the bullet's actual time of flight and the time a bullet in a vacuum would take. A bullet that sheds velocity slowly accumulates less lag, so the same 10 mph crosswind pushes it less. This is why competitive shooters chase BC rather than raw speed: velocity you can buy with powder, and it costs barrel life; BC you buy once, in the bullet you choose.
BC is not a constant
This is the part the box does not mention. A bullet's drag relative to the reference projectile changes with speed, so its BC changes as it flies. A match bullet may show one BC at 3,000 fps, a slightly different one at 2,000 fps, and a noticeably different one as it approaches the speed of sound.
Manufacturers deal with this in two ways. Some publish banded BCs — one value for each velocity range. Others publish a single averaged value and let you live with the error. A solver that integrates step by step, as this app does, uses the drag curve directly and applies your BC as a scaling factor at each step, which handles the variation far better than a closed-form approximation ever could.
Published numbers run optimistic
Independently measured BCs are frequently below the marketed figure, sometimes by five to ten per cent. The reasons are mundane rather than sinister: different test velocities, different atmospheric corrections, a different reference standard, or a measurement taken from a barrel with a faster twist than yours.
Practically, this means a printed BC is a starting point, not a verified property of the ammunition in your bag. Your barrel length, your twist rate and your particular lot all move the real figure. The fix is not to argue with the catalogue but to shoot at distance and adjust — see truing.
Where BC stops mattering
Inside 300 m with a centrefire rifle, BC barely earns its keep. Drop is dominated by velocity and gravity, drift is small, and a 0.05 error in BC moves the impact less than your group size does. Chasing high-BC bullets for a 200 m deer rifle is spending money on a problem you do not have.
Past 600 m the ranking inverts. A 10 per cent BC error that was invisible at 200 m becomes a miss on a plate, and it grows non-linearly, because the bullet that decelerates faster spends longer in the wind, which makes it decelerate over a longer time still.
Getting it right in practice
- Take the G7 value if one existsFor any modern boat-tail match bullet, the G7 figure is the more honest one. Use it with the G7 drag model, never mixed with G1.
- Enter it exactly as publishedDo not pre-adjust the number because you have read that manufacturers are optimistic. Start from the catalogue figure so you know what you changed later.
- Confirm at two distancesZero as normal, then shoot at a distance where the predicted drop is large — 600 m or beyond. A BC error is invisible up close and obvious out there.
- Adjust the BC, not the zeroIf the near distance is correct and the far one is not, the drag model is what disagrees with reality. Nudge BC until the far impact matches, then re-check the near one.
The app carries both G1 and G7 coefficients for the loads that publish both, so you can switch models and see the difference immediately rather than taking it on faith. Browse the numbers in the ammunition reference.
Frequently asked
Is a higher ballistic coefficient always better? +
For wind and drop at distance, yes. But BC is bought with length and weight, and a longer bullet needs a faster twist to stabilise, eats case capacity, and may not feed in a short magazine. Inside 300 m the advantage is too small to pay for.
Can I convert a G1 BC into a G7 BC? +
Only approximately. For typical long boat-tail match bullets the G7 value is roughly half the G1 value, but the ratio varies by shape, and a converted number carries the error of both the original figure and the conversion. Use a published G7 value if one exists.
Why does my ballistic app disagree with the drop chart on the box? +
Box charts assume standard atmosphere, a specific barrel length and the manufacturer's own BC. Change the altitude, the temperature or the barrel and the chart no longer describes your rifle. That is precisely what a solver is for.