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.]
Similar Posts:
- Coefficient Conundrum: G1 vs. G7, Which BC Should You Use
- G1 vs. G7 Ballistic Coefficient Models — What You Need to Know
- G1 vs. G7 Ballistic Coefficient Models — What You Need to Know
- G1 vs. G7 Ballistic Coefficients — What You Need to Know
- Applied Ballistics Expands Bullet Library to 533 Bullets
Share the post "G1 vs. G7 Ballistic Coefficients — What You Need to Know"
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.
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.
Using Litz’s Applied Ballistics Program that comes with his book “Applied Ballistics for Long Range Shooting”, there are some discernible differences in bullet drop at 1000 yards using the G1 vs. G7 models.
As an example, for a .308 caliber, 175g. bullet, he shows a BC of .233 using the G7 model, and a BC of .500 using the G1 model. While all other input variables remain the same, and using a MV of 2720 (which is the average MV I shoot using 45.6g. of CFE-223 out of a Rem. 700), the program shows a drop at 1000 yards (using a 100 yard zero)at 394.22″ using the G7 model, and 366.75″ using the G1 model.
I would presume that using a Sierra MK 175g. BTHP would probably be considered a long range bullet, but still it’s a considerable difference in bullet drop at 1000 yards of 27.47″ between the 2 models.
Point being that choosing the wrong model to base your calculations on could be a serious miss….
So, is there a correct answer….??
THE ONLY WAY TO GET A TRUE BALLISTIC
COEFFICIENT TO ENTER INTO THE G7 PROGRAM IS TO CHRONOGRAPH THAT LOAD AT
TWO DIFFERENT DISTANCES (I USE 10FEET FOR THE FIRST CHRONOGRAPH AND 100 YARDS FOR THE SECOND). USE THE JBM BC CALCULATOR AND YOU MAY GET A SERIOUS SHOCK AS I DID. THEY DO NOT ALL WORK OUT
TO HALF THE G1.
I’m an engineer. Bryan Litz is an engineer. I wonder if some of the “expert opinions” on this forum come from engineers or technicians? I know the difference, I started at the bottom and worked my way up. It is amazing how many mechanics and technicians I had to work with who like to tell “dumb engineer” stories that explain how smart techs are and how dumb engineers are because techs have to live and work in the “real world…” Sorry fellas, I’ve lived at both addresses and it is amazing how difficult life can be when you have a blank piece of paper put in front of you rather than a finished drawing or completed program to criticize. Go to school, take the classes, learn how to do the math, start a design from SCRATCH then get back with me and tell me how dumb engineers are. I’ll wait…
I just tring to learn how to do the math for long distance so I won’t try to sound like I know what everyone is saying. So where did someone say engineers are dumb? That’s the only questino I have.
I think Lester just wanted to brag a little!
Hey all,
I’m a retired Electrical Engineer working at Keystone as an Industrial Electrician. I know math, some things need real measured verification as each firearm set up is different, Barrel harmonics, load developed to match your set up & all. Math gives us a large range numerically to begin our quest for mapping our set up. Stay cool and learn well this feels good when accomplished!
7mmmag 150 in Remington gun is g what ¿
I landed here just because I am trying to figure out if federal used G1 or G7 when they calculated their BC on their .308 168-grain sierra matchking BTHP rounds. anyone know? thanks!
@Sambo- It,s G1 @ .462/.464 depending on the website.
I double checked a few sites, but the ammo companies publish the G1 number because it,s so much higher (read impressive)!
G7s are more in the .100 to .200 range.
Halving a G1 to estimate G7 is rather crude. Most G7s seem to be about 55% of G1
I agree with Lester. I am licensed PE in mech engineering and civil engineering. I can’t cite the number of times that I have been confronted by non-engineer technicians or technologists and their disdain for engineers. It is a no-win situation if management is technically clueless. One should read the Standard Occupation Classifications published by the government. Hands-on vs analytical and build tools vs inspection tools matter. I am glad lots of testing happens. It is sometimes better than pure analysis, but every circumstance must be tested rather predicted by scientific equations.
https://www.bls.gov/soc/2018/major_groups.htm#17-0000
My Comment: Mac Lorry’s comment in 2013 is interesting. He cites that trivial small difference between G1 and G7 BCs. I tried an on-line ballistic calculator with both G1 and G7 BCs at http://gundata.org/ballistic-calculator/.
I input 300 Win Mag 180 Grain Winchester Ballistic Silvertip bullets (recommended by rifle manufacturer Savage 111) and ran the calculator with both G1 and G7 BCs. The differences were profound. For the G1 calculation (BC=.508) the drop is 46″ and 283″ for 500 yds and 1000 yds, respectively. For the G7 calculation (BC assumed ~ G1/2 = 0.2535) the drop is 46″ and 283″ for 500 yds and 1000 yds, respectively. By this calculator and this cartridge & bullet, the G1 and G7 are equal. So, the G1 vs G7 BC gets more confusing. For any equation of fluids, velocity applicability limits have to be involved. I would like to see the actual equations and the limits or parameters like in engineering or any other technical field.
My understanding of ballistics coefficient is that it is a coefficient of the ratio between a standard bullet and usually a smaller bullet of interest and it should stay close to constant, the standard bullet which is has the two-caliber radius ogival head and flat base is used to produce the G1 drag function, this bullet also has a specific form factor and is related specifically to the G1 drag function. You can multiply this function by about one half .497 and use G7 ballistics coefficient which will give you about the same results as just using the G1 with G1 ballistics coefficients. So, by this approach you’re not using the G7 ballistics function you’re just getting G1 results with G7 Coefficients.
In Hatchers note book there are tables on page 566 there is a table with different drag functions with their retardation at certain velocity increments, the Aberdeen J drag function is one were they use a boa tailed spitzer bullets for the standard Bullet which will give a closer approximation to the G7 drag function. Happy calculations.
Sorry guys. I am a new shooter, just learning the ropes but I can tell you what I have found is that there are lots of shooters out there who know very little, even less than me.
I dont understand. Who cares, g1 or g7. All I want is a model that matches my rifle and my load so when I dial the elevation in to my scope, the bullet lands where I expect.
Can anyone help?
Hey John,
I’m new too. I have usually hunted pigs with dogs and a knife. Getting older now…
I agree with the shooting where my rangefinder and ballistics calculator tell me a bit is. I’ve had a lot of success with G7 Hornady 7mm ELD-X rounds using the App, Streylock Pro as the Ballistics calculator.
You need your barrel length and twist rate for accurate calculations. A good bi-pod makes easy 500yard shots.
Hope that’s useful.
Jamie
There are not places I can shoot to even 300 yards. So none of this really matters-lol.
Long range ballistics knowledge takes rifle shooting to a whole different level. Though I do find it all interesting . Now all I have to do is go buy expensive precision rifles ,scopes, rangefinders, etc.; and spend even more money to travel to places to shoot.