Ballistic coefficient is the most quoted number on a box of match bullets and the least understood. The piece almost nobody explains is that it is not a single property of the bullet at all, but a comparison to a reference shape, which is why the very same bullet wears two different numbers, a G1 and a G7. Pick the wrong one for your solver and its predictions miss by more and more as the bullet flies farther out. This guide explains ballistic coefficient without the marketing spin, so you understand what the number means, why there are two of them, and how to feed your solver the one that stays accurate.
We will build it up in order, starting with what ballistic coefficient actually measures and why it is a comparison rather than a constant. Then we will cover the two ingredients behind it, sectional density and form factor, how the G1 and G7 reference shapes differ, why one bullet has a number for each, and how to pick the model that matches your bullet. By the end the number on the box will be a tool you control instead of a marketing figure you trust on faith.
What ballistic coefficient actually measures
A ballistic-coefficient is a single number that describes how well a bullet resists air drag as it flies. A higher BC means the bullet sheds velocity more slowly, which gives it a flatter trajectory, less wind drift, and more retained energy downrange.1 A lower BC means the air slows it faster, so it drops and drifts more. In that sense BC is a shorthand for aerodynamic efficiency, the bullet's ability to hold its speed against the air.
The catch, and the source of all the confusion, is that BC is not an absolute measurement like weight or diameter. It is always a comparison between your bullet and a standard reference projectile, a number that says how your bullet's drag stacks up against that reference. Change the reference and the number changes, even though the bullet has not moved.
This is the single idea that makes everything else click. When someone quotes a BC, they are really saying that the bullet sheds speed a certain fraction as fast, or as slow, as a particular standard shape. The number is meaningless until you know which standard shape it was measured against, which is exactly what G1 and G7 specify.
BC is a comparison, not a constant
Because BC is a comparison, it is built from two physical things about your bullet, and seeing them makes the whole concept concrete. The first is how much mass the bullet packs behind its frontal area, and the second is how cleanly its shape slips through the air compared to the reference.
The mass part is sectional-density, a bullet's weight divided by the square of its diameter. A heavier bullet of the same caliber has more mass behind the same frontal area, so it carries more momentum to push through the air and resists deceleration better. A 175-grain .308 bullet has higher sectional density than a 110-grain bullet of the same diameter, which is part of why the heavier bullet bucks the wind better.
The shape part is the form-factor, the ratio of your bullet's drag to the drag of the reference projectile. A form factor of 1.0 means your bullet sheds speed exactly like the standard shape, while a lower number means it is sleeker and more efficient than the reference. Ballistic coefficient is essentially sectional density divided by form factor,2 which is why a long, pointed, boat-tailed bullet, sleek in shape and dense for its caliber, posts a high BC.
The two reference shapes, G1 and G7
Everything turns on the reference projectile, and in practice two of them matter. The drag-model named G1 uses a short, flat-based reference shape with a blunt nose, a profile that looks like an old unpointed bullet from over a century ago. The G7 model uses a long, sleek, boat-tailed reference shape that looks very much like a modern match bullet.
The reason both exist is history layered with progress. The G1 standard came first and became the default for the whole industry, so nearly every bullet has a G1 number and most reloading manuals are built around it. The G7 standard arrived as long-range shooting matured, offering a reference shape that actually resembles the streamlined bullets precision shooters use today.
The shape of the reference is not a technicality, it is the whole point. A drag model is really a curve of how drag changes with speed for that reference shape, and your bullet's BC scales that curve to fit your bullet. The closer the reference shape is to your actual bullet, the better that scaled curve matches your bullet's true flight, which is the heart of why the choice between G1 and G7 matters so much.
Why one bullet has two different numbers
Here is the fact that trips up so many shooters. The same bullet has both a G1 and a G7 ballistic coefficient, and the two numbers are very different, even though they describe one unchanging piece of metal. For a typical modern match bullet, a G7 BC of around 0.300 corresponds to a G1 BC of around 0.585 for the same projectile.3
The numbers differ because they are measured against different reference shapes. The G1 reference is draggier than your sleek bullet, so compared to that blunt standard your bullet looks very efficient and gets a high number. The G7 reference is already sleek and similar to your bullet, so compared to that standard your bullet looks ordinary and gets a lower number. Neither number is wrong, they are answers to two different questions about two different references.
The critical consequence is that the two numbers are not interchangeable. You cannot take a G1 BC and use it in a field expecting a G7 BC, because the solver will scale the wrong drag curve and produce a wrong trajectory. A BC means nothing without its model attached, and treating a bare number as universal is the root of most BC mistakes.
Why G1 reads inaccurately for modern bullets
The deeper problem with G1 for long-range bullets is not just that the number is bigger, it is that the number does not hold steady. The true measure of drag, the drag-coefficient, changes with velocity, rising sharply as the bullet slows toward the speed of sound and behaving differently across the speed range. A BC tries to summarize that whole varying curve in one number, and how well it succeeds depends on how closely the reference shape matches your bullet.
When you fit a sleek modern bullet to the draggy G1 reference, the mismatch in shape means a single G1 number cannot describe the bullet across its whole flight. The bullet's real drag curve and the scaled G1 curve diverge as the speed drops, so a G1 BC that is accurate up close reads too high farther out, and the solver predicts less drop than you actually get. Many programs work around this by using several G1 numbers banded by velocity, a clear sign that one G1 figure does not fit.4
A G7 BC for the same bullet behaves far better, because the G7 reference shape closely resembles the bullet. The scaled G7 curve tracks the bullet's real drag across the velocity range, so a single G7 number stays stable and predicts drop and drift reliably from the muzzle to long range. This is why precision shooters working past several hundred yards lean on G7 numbers, and why a G1 figure can leave a solver reading inaccurately right where it counts.
Picking the model so your solver tells the truth
The practical rule falls right out of the physics. Match the drag model to the shape of your bullet, and feed your solver a BC and a model that agree. For a modern boat-tail match bullet, that means using the G7 BC with the G7 model set, which gives the solver a reference close to your actual bullet and a number that holds across the range.5
For a blunt, flat-based, or round-nose bullet, the G1 model is actually the better fit, because that bullet resembles the G1 reference more than the G7 one. The lesson is not that G7 is always right and G1 always wrong, it is that the model should match the bullet, and most long-range bullets are G7-shaped while many traditional hunting bullets are closer to G1. Use the number that goes with the shape.
The mistake to avoid above all is mixing the two. You never enter a G1 BC into a solver set to G7, or the reverse, because the number and the model have to come from the same standard. When a manufacturer publishes both numbers, you pick the one matching your model setting and ignore the other. Get that pairing right and your solver works from a reference that fits your bullet, which is the whole game.
Ballistic coefficient changes with velocity
It helps to understand why even the right BC is still an approximation, because it keeps your expectations realistic about the number's limits. A bullet's drag is genuinely different at 3,000 feet per second than at 1,200, since the drag-coefficient climbs steeply through the transonic region near the speed of sound. No single number can perfectly capture a curve that bends that much.
A published BC is therefore an average across a velocity band, a best single value for the speeds the bullet typically flies. A well-matched G7 number averages cleanly because the reference curve has the same shape as the bullet's, so the average holds across the range. A poorly matched G1 number averages badly, which is why it needs velocity banding to stay useful, and why it drifts at the extremes.
This is also why your solver can still miss at extreme range even with a good BC, and why velocity-truing and drag adjustments exist. The BC gets you very close, but the last refinement at distance comes from confirming on target and nudging the inputs. Knowing the BC is an averaged approximation tells you to verify rather than trust it blindly past a thousand yards.
Custom drag models, beyond a single number
The endpoint of all this is that a single BC, even a good G7 one, is a compression of a richer truth, and the industry has begun moving past it. A custom-drag-model describes one specific bullet's drag across the full range of velocities it will fly through, point by point, rather than scaling a standard curve to a single number. It is the bullet's own measured drag, not a comparison to a reference shape.
These curves come from real measurement. Manufacturers track bullets with doppler-radar, recording true velocity decay over distance, and build a drag curve specific to that projectile.6 A solver running a custom drag model does not need a G1 or G7 BC at all, because it has the bullet's actual aerodynamics instead of an approximation scaled from a reference. When such a model is available for your bullet, it sidesteps the whole G1 versus G7 question.
Custom drag models do not make ballistic coefficient obsolete, though, because most loads and quick references still use BC, and it remains the common language of the field. The practical takeaway is a layered one. Use a matched G7 BC for most long-range work, reach for a custom drag model when your bullet has one and your solver supports it, and remember that both are tools for the same job, predicting how the air will slow your bullet.
How I would use ballistic coefficient
If I were setting up a solver for a modern long-range bullet, I would use the published G7 BC with the G7 model and not give the G1 number a second look. I prefer the G7 figure because its reference shape matches my bullet, so a single number stays accurate across the range instead of needing velocity bands to patch it. Matching the model to the bullet is the one habit that prevents most BC errors.
My approach when only a G1 number is published is to use it with the G1 model and stay alert for drift at distance, since a G1 BC on a sleek bullet tends to read high far out. I would confirm my drops on target and true the solver if the predictions and the impacts disagree, treating the BC as a strong starting point rather than gospel. A BC gets me close, and confirmation closes the gap.
When a custom drag model exists for my bullet and my solver supports it, I would use that instead and skip the BC question entirely, because the measured curve beats any single-number approximation. Short of that, a matched G7 BC, verified at distance, is what I would trust. The whole skill is just making sure the number and the model agree and that both match the bullet in my barrel.
FAQ
What does a ballistic coefficient actually tell you?
A ballistic coefficient tells you how well a bullet resists air drag compared to a standard reference projectile. A higher BC means the bullet holds its velocity better, giving a flatter trajectory, less wind drift, and more retained energy at distance. It is not an absolute property of the bullet but a comparison, which is why the number only means something once you know which reference model, G1 or G7, it was measured against.
Why does the same bullet have a G1 and a G7 ballistic coefficient?
The same bullet has two ballistic coefficients because BC is measured against a reference shape, and there are two common references. The G1 model uses a blunt, flat-based reference, while G7 uses a sleek, boat-tailed one. Compared to the draggy G1 shape a bullet looks very efficient and gets a high number, while compared to the sleek G7 shape it gets a lower one. The two numbers are not interchangeable.
Should I use the G1 or G7 ballistic coefficient?
You should match the model to your bullet's shape. For a modern boat-tail match bullet, use the G7 BC with the G7 model, because the G7 reference closely resembles the bullet and one number stays accurate across the range. For a blunt or round-nose bullet, the G1 model fits better. Never enter a G1 number into a solver set to G7 or the reverse, since the number and model must come from the same standard.
Why does a G1 ballistic coefficient drift at long range?
A G1 ballistic coefficient drifts at long range because the sleek modern bullet does not match the draggy G1 reference shape, so a single G1 number cannot describe the bullet across its whole flight. The bullet's real drag curve and the scaled G1 curve diverge as velocity drops, so a G1 BC that is accurate up close reads too high far out, and the solver predicts less drop than you actually see. A matched G7 number avoids this.
Citations
- (2020). BC Effect on Accuracy. Berger Bullets.
- Travis Pike. (2018). Ballistic Coefficient: Everything You Ever Wanted to Know. Pew Pew Tactical.
- (2022). G1 & G7 Ballistic Coefficients: What's the Difference?. Kestrel Meters.
- Doc Beech. (2021). Ballistic Calibration, a Misunderstood Process. Applied Ballistics.
- Cal Zant. (2019). G1 BC vs G7 BC vs Bullet-Specific Drag Models. PrecisionRifleBlog.com.
- Jayden Quinlan. (2019). 4DOF Knows Your Bullet and What It Will Do. Hornady Manufacturing.