A ballistic coefficient is not a property of a bullet on its own. It is a ratio: how well this bullet holds its velocity compared with a standard reference projectile of the same shape family.
Formally, BC = SD / i, where SD is the sectional density and i is the form factor — how much draggier the bullet is than the reference shape. A bullet with i = 1 is exactly as aerodynamically efficient as the reference. A bullet with i = 1.1 has 10 % more drag.
Because the BC is a comparison, you have to say compared to what.
The reference projectiles
The drag models are named after the standard projectiles used to build them, all measured by the US Army’s Ballistic Research Laboratory:
- G1 — a short, blunt, flat-based shape with a 2-caliber ogive. It is essentially a 19th-century artillery projectile, and it is a poor match for anything modern. It is also, for historical reasons, the number almost every catalogue prints.
- G7 — a long boat-tailed shape with a 10-caliber secant ogive and a 10° boat tail. This is what a modern match bullet actually looks like.
- Others exist (G2, G5, G6, G8, GL) for specific shapes, but G1 and G7 cover sporting rifle ammunition.
The drag tables themselves are Mach-indexed drag coefficients measured on those reference shapes. They are US government work and public domain; every open ballistics program uses the same numbers, this one included.
Why a G1 BC drifts with velocity
Here is the practical problem. A boat-tail match bullet’s drag curve has a different shape from G1’s. So the single scaling factor that makes G1 fit at 900 m/s does not fit at 400 m/s.
The manufacturer’s workaround is banded BCs: one G1 value above some velocity, another below. Sierra publishes these routinely. If you take the top band and use it all the way to 1000 m, your drop prediction will be optimistic.
A G7 BC for the same bullet stays close to constant across the whole supersonic range, because the shapes genuinely match. That is the entire argument for using it: one number, valid everywhere the bullet is supersonic.
Reading the numbers
A G7 BC is roughly half the G1 BC of the same bullet. A 175 gr .308 match bullet is around 0.50 in G1 and around 0.24 in G7. Both describe the same bullet. Mixing them up — entering a G1 number while the calculator is set to G7 — roughly doubles the predicted drag and is one of the most common ways to get a nonsense answer out of any ballistics program.
Two more things worth knowing:
- BC is not quality. A heavier bullet of the same shape has a higher BC purely because it has more mass behind the same frontal area. That helps it fight the wind; it does not make it more accurate.
- BC is measured, not declared. Independently measured BCs (Bryan Litz’s Applied Ballistics work being the best-known set) frequently come in a few per cent below the catalogue figure. If your drops at 800 m are consistently short, a slightly optimistic BC is the usual culprit.
What to do in practice
- If a G7 BC exists for your bullet, use it, and set the model to G7.
- If only a G1 BC exists, use it — but expect the last 200 m of a long shot to drift, and prefer the low velocity band if the catalogue gives bands.
- Whatever you use, verify it. Shoot at a known distance, compare with the prediction, and adjust the BC until the calculator matches your rifle. That is what BC “truing” means, and it is more useful than arguing about drag models.