A drag model is a measured curve: how much drag a particular reference projectile experiences at every speed, expressed against Mach number. Your bullet's ballistic coefficient scales that curve up or down. The solver then integrates the result to produce a trajectory. Everything depends on the curve being roughly the right shape for your bullet — and G1 and G7 are very different shapes.
Two reference bullets
| G1 | G7 | |
|---|---|---|
| Reference shape | Flat base, short blunt ogive | Boat tail, long 7-calibre ogive |
| Vintage | Late 19th century artillery standard | Modern long-range standard |
| Matches | Round nose, flat base, most pistol and hunting bullets | Modern match and long-range hunting bullets |
| BC magnitude | Larger number (e.g. 0.485) | Smaller number (e.g. 0.243) |
| Stability across velocity | Drifts noticeably | Nearly flat |
The larger G1 number is not a better bullet. It is the same bullet measured against a worse-performing reference, so the ratio comes out higher. Comparing a G1 figure with a G7 figure tells you nothing except that someone quoted two different standards.
Why G7 is steadier
A ballistic coefficient is only constant if your bullet's drag curve has the same *shape* as the reference bullet's. A modern boat-tail match bullet looks almost nothing like the stubby G1 standard, so its G1 coefficient has to change with velocity to keep the maths honest — which is why manufacturers publish banded G1 values.
Against the G7 standard, that same bullet is nearly a scale model of the reference. Its G7 coefficient stays close to constant from the muzzle down into the transonic region, so a single number describes the whole flight. That is the practical argument for G7: fewer moving parts, less error you cannot see.
The mistake that costs plates
The error is mixing them: entering a G1 coefficient while the solver runs the G7 table, or the reverse. Because the G1 number is roughly twice the G7 number for the same bullet, the solver ends up modelling a projectile with either half or double the real drag.
What makes this dangerous is that it hides at short range. Confirm a 100 m zero with a mismatched model and everything looks fine — the bullet has barely slowed and drag has barely acted. The error compounds with time of flight, so it emerges as an increasingly large miss somewhere past 500 m, exactly where you stop being able to diagnose it by eye.
In the app, the drag model is stored with the load, not with the rifle. When you pick a load from the database the model comes with it, and the value shown is the one that matches. If you type a coefficient by hand, check the model selector in the same breath.
When the choice barely matters
Inside about 400 m with a supersonic centrefire load, a correctly entered G1 and a correctly entered G7 will agree to within a few centimetres. Both models describe the supersonic regime well and the bullet has not had time to diverge from either curve.
The models separate as the bullet decelerates. Around and below Mach 1.2 the drag curves differ sharply in shape, and a G1 fit to a modern bullet starts predicting the wrong deceleration. This is the same region where groups open up for other reasons — see the transonic zone.
What to do with a load that publishes both
- Prefer the G7 valueFor any boat-tail bullet, enter the G7 coefficient and select the G7 model. It will hold across the whole trajectory.
- Keep the G1 value as a cross-checkRun the same shot both ways. Agreement within a few centimetres at your typical distance means neither number is a typo.
- Never average themThey are measurements against different standards. An average of the two is a number that describes nothing.
- True the model you actually useOnce you have picked a model, adjust its coefficient against real impacts. Truing a G1 setup does not transfer to a G7 setup.
The ammunition reference lists both coefficients side by side for every load that publishes them, along with the manufacturer's preferred model, so you can see which bullets have a genuine G7 figure and which only carry a G1.
Frequently asked
My bullet only lists a G1 BC. Should I convert it to G7? +
No. Use the G1 coefficient with the G1 model. A converted value inherits the error of the original figure plus the error of a shape-dependent conversion, and you lose the ability to true against a known starting point.
Are there better models than G1 and G7? +
Yes — custom drag curves measured for one specific bullet remove the reference-projectile approximation entirely. They are not published for most ammunition, and for the great majority of shooting a correctly chosen G7 coefficient is not the limiting factor. Your muzzle velocity and your wind call are.
Which model do pistol bullets use? +
G1. Pistol bullets are blunt and flat-based, which is close to the G1 reference shape, and they operate at distances where the choice makes almost no difference anyway.