Coefficient Conundrum: G1 vs. G7, Which BC Should You Use
The better, up-to-date ballistics programs let you select either G1 or G7 Ballistic Coefficient (BC) values when calculating a trajectory. The ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. You’ve probably seen that G7 values are numerically lower than G1 values for the same bullet (typically). But that doesn’t mean you should select a G1 value simply because it is higher.
Some readers are not quite sure about the difference between G1 and G7 models. One forum member wrote us: “I went on the JBM Ballistics website to use the web-based Trajectory Calculator and when I got to the part that gives you a choice to choose between G1 and G7 BC, I was stumped. What determines how, or which one to use?”
The simple answer to that is the G1 value normally works better for shorter flat-based bullets, while the G7 value should work better for longer, boat-tailed bullets.
G1 vs. G7 Ballistic Coefficients — Which Is Right for You?
G1 and G7 refer both refer to aerodynamic drag models based on particular “standard projectile” shapes. The G1 shape looks like a flat-based bullet. The G7 shape is quite different, and better approximates the geometry of a modern long-range bullet. So, when choosing your drag model, G1 is preferrable for flat-based bullets, while G7 is ordinarily a “better fit” for longer, boat-tailed bullets.
Drag Models — G7 is better than G1 for Long-Range Bullets
Many ballistics programs still offer only the default G1 drag model. Bryan Litz, author of Applied Ballistics for Long Range Shooting, believes the G7 standard is preferrable for long-range, low-drag bullets: “Part of the reason there is so much ‘slop’ in advertised BCs is because they’re referenced to the G1 standard which is very speed sensitive. The G7 standard is more appropriate for long range bullets. Here’s the results of my testing on two low-drag, long-range boat-tail bullets, so you can see how the G1 and G7 Ballistic coefficients compare:
G1 BCs, averaged between 1500 fps and 3000 fps:
Berger 180 VLD: 0.659 lb/in²
JLK 180: 0.645 lb/in²
The reason the BC for the JLK is less is mostly because the meplat was significantly larger on the particular lot that I tested (0.075″ vs 0.059″; see attached drawings).
For bullets like these, it’s much better to use the G7 standard. The following BCs are referenced to the G7 standard, and are constant for all speeds.
G7 BCs:
Berger 180 VLD: 0.337 lb/in²
JLK 180: 0.330 lb/in²
Many modern ballistics programs, including the free online JBM Ballistics Program, are able to use BCs referenced to G7 standards. When available, these BCs are more appropriate for long range bullets, according to Bryan.
[Editor’s NOTE: BCs are normally reported simply as an 0.XXX number. The lb/in² tag applies to all BCs, but is commonly left off for simplicity.]
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Tags: Ballistic Coefficient, BC, Bryan Litz, G1, G7
I hope that many shooters will follow Bryan Litz’s recommendation.
I think one of the reasons G1 is still used is that it makes bullet’s ballistics look better. Especially when you base BC on short range velocity measurements. Due to its curve shape, the G1 model then shows drag values at longer ranges that are way too optimistic. This is good for factory catalogues, but on the range only the real aerodynamics count. For slender bullets G7 is always closer to reality.
I hope many shooters will follow Bryan Litz’s advice.
A reason for the frequent use of G1 (which originates from French naval projectiles of the 19th century) in my view is that it lets projectile ballistics look better. If you base your BC on short range (high velocity) measurements, the G1 curve shape will show a lower drag at the longer ranges. This may look attractive in catalogues, but is too optimistic compared to actual measured range data. For modern slender bullets G7 always results in more realistic data.
It should be noted that while the G7 profile will in general fit boat tail bullets better when using a simplistic ballistics system which only fits bullets with a single coefficient, systems like Sierra Bullets Infinity which profile bullets with multiple coefficients at different speeds will results in a better fit than a single coefficients regardless of the drag model. Drag coefficients are all speed sensitive, even G7, (hence why the drag curves even exist in the first place).
The G1 through G7 drag functions are simply a curve that fits one particular bullet, and the coefficients are an adjustment factor to try and make the curve fit a specific different bullet. Systems like Infinity allow the curve to be fit in segments to increase the accuracy of the fit at varying velocities.