G1 vs. G7 Ballistic Coefficients — What You Need to Know
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, ballistics, BC, Berger Bullets, Bryan Litz, bullets, G1, G7, JBM Ballistics
While Bryan Litz published real-world measurements for 236 bullets, 165 of these cover a speed range of at least Mach 0.5 (approx. 560 fps between fastest and slowest measured velocity).
Of these, 147 bullets give a better fit to G7, while G1 is better for only 18 bullets.
I think that is clear proof that G7 is typically a better drag model for modern bullets than G1.
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In my opinion the difference is an academic one for the target shooter. It may bring you better on target for the first time, but only if you know all the other variables in the equation for sure. E.G. strength and direction of wind is not the same from muzzle to target at longer distances. It even changes with the height above ground.
It matters to varmint hunters
Whenever possible, I’d use a G7 BC for a boattail. However, if the G1 number is all you have and the range is under 600 yards the difference is neglible (provided, of course, that the BC number is any good…I’ve seen some notoriously inflated numbers from some manufacturers other than Berger or Sierra).
Other than scale there’s no difference between the G1 and G7 drag models between 3,000 fps down to 1,800 fps.
Prove it to yourself. On any ballistics program that works with G1 and G7, select some set of atmospheric conditions and enter a load with a MV of 3,000 fps with a G1 BC of 0.500 and compare it to using a G7 BC of 0.250. You’ll find nearly an exact match in drop, velocity and time of flight out to 650 yards. Even at 1,000 yards the difference in drop is less than half an inch.
Think it’s a trick? Try it again with any atmospheric conditions you like and any practical BC. Just remember to set the G7 BC to half of the G1 BC value and keep the MV at 3,000 fps and all other conditions the same. You’ll find the same close match out to a range where velocity drops to 1,800 fps. Even if you use a MV higher than 3,000 fps you’ll find the drop matches within any practical consideration out to ranges where velocity drops to 1,500 fps.
Now you know why most manufactures don’t publish G7 values.