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Old June 4, 2013, 07:33 PM   #1
taylorce1
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Ballistic Coefficient G1 vs. G7

We seem to be getting a lot of questions about ballistics on a regular basis on various cartridges. Luckily today we have a lot of tools between Apps and online ballistics calculators that anyone with a decent chronograph can pretty reasonably calculate the flight path of the bullet they are using. However, there is a lot of miss representation of the ballistic coefficient (BC) with bullet manufacturers these days with regards to the G1 BC listed for most bullets.

G1 BC is the most rudimentary of ballistic coefficients, and is actually only accurate for flat base bullet designs. The G1 is also the most misquoted BC used out there, as most bullet manufacturers give a G1 BC to their bullets which results in some pretty amazing BC numbers. However, if you are using a G1 BC with a boat tail bullet you are using the incorrect figure.

G7 BC is the ballistic coefficient for a boat tail bullet with a tangent ogive. If using a ballistics calculator instead of using the manufacturer's BC look for the G7 or Litz BC for your calculations. I'm really only focusing on these two BC types because they cover the majority of bullets being used and discussed in the forums.

Explanation of different Ballistic Coefficients.
Bullet Design-Secant vs. Tangent
Comparing Advertised Ballistic Coefficients with Independent Measurements

For example I shoot a 6X47 Rem (6mm-.222 Remington Magnum) I use a 70 grain Nosler Ballistic Tip with an average speed on 3066 at the muzzle. I don't shoot this rifle at 1000 yards, but it is a good example of the difference between G1 and G7 BC differences as seen in the data below. I think manufacturers like to use a G1 BC because most shooters will never see the differences between the two. I know before I started learning to shoot long range I didn't, but as you can see the BC difference really start to magnify as you approach 1000 yards.

G1


G7


So when you plot your ballistics use G7 (Litz) when you can for boat tail bullets. It will help you to dial in your ranges faster, and maybe save you a few rounds at the rage. Especially when used with a chronograph, it makes your life much simpler.
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Old June 4, 2013, 08:23 PM   #2
Jim Watson
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I think they publish G1 because it lets them advertise with larger numbers.
At least Sierra is honest enough to scale down the G1 by velocity range, which I have been assured is a law of nature, not a kluge to offset a mathematical mismatch.

But looking at your chart, the difference in BC only amounts to an inch on the target somewhere past 400 yards and a MOA on the sight beyond 700.
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Old June 4, 2013, 09:04 PM   #3
kraigwy
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BC is over rated, either G1, G7, or G8

Take the OP's example. Set aside the drop for a minute and look at the energy. You'll see the energy for the first bullet is higher then the second, G7 Bullet.

Isn't the reason for shooting is for the bullet to do its job at a given range? That means Energy on Target.

You can use the knobs on your sights to make up for the windage/elevation changes.

But back to the BC differences. The bullet used, (70 gr 243) is limited to about 400 yards for all practical purposes.

The difference in drop is .1 MOA or .1047 inches. The 70 gr 243 would be limited to small deer or antelope (if we're realistic). The vital area of such animal is 8.5 - 9 inches.

If you want to get further into the different BC factors, read McCoy's "Modern Exterior Ballistics". McCoy also covers G8. But from his readings, you see that altitude makes a greater difference.

But again, this can all be compensated for by experience in shooting your given rifle, with your given bullet in varying conditions.

Learn your sights and form factor doesn't matter. Or you can take you computer, do the math and find the antelope you're after over the next ridge when you get your numbers plugged in and are ready to shoot.
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Old June 4, 2013, 10:50 PM   #4
taylorce1
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Quote:
I think they publish G1 because it lets them advertise with larger numbers.
At least Sierra is honest enough to scale down the G1 by velocity range, which I have been assured is a law of nature, not a kluge to offset a mathematical mismatch.

But looking at your chart, the difference in BC only amounts to an inch on the target somewhere past 400 yards and a MOA on the sight beyond 700.
In my long range rifle I use the Sierra 107 grain, DTAC 115 grain, and the Berger VLD 105 and 115 grain bullets. I have to agree that the ballistics data on the Sierra's is pretty spot on as well as the other bullets I mentioned.

Quote:
BC is over rated, either G1, G7, or G8
You might think so but I see it as another tool in the tool box. I agree it isn't the only tool, but one that gets used and misused a lot. To me using the correct BC is less bullets used at the range, and more bullets used for the other things I like to shoot. My 6X47 that I mentioned is pretty much a varmint rifle and steel ringer out to 500 yards. I shoot targets much smaller with it than the kill zone on a deer or pronghorn that you mentioned. So the variance in BC is magnified on smaller targets.

Quote:
Learn your sights and form factor doesn't matter. Or you can take you computer, do the math and find the antelope you're after over the next ridge when you get your numbers plugged in and are ready to shoot.
I agree with what you say and you're going further than what I was even discussing. I used that particular load because that is the rifle I've been working with the most over the last few years. It isn't a long range round by any means but I've made a few long range shots with it.

I wasn't suggesting you take a computer out hunting, but using resources available taking advantage of them while at the range. When I started using this bullet I was using the G1 and the adjustments on the scope were never spot on. When I switched to the G7/Litz BC numbers my adjustments matched up more closely.

All that work at the range allowed me to work up better range data for my cards that I attach to the butt stock or scope. So that I don't need my phone or computer while in the field shooting prairie dogs, coyotes or ringing steel at various distances. It is what you would call "knowing your sights."
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Old June 5, 2013, 07:55 AM   #5
Bart B.
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The only real comparison I know of between bullets A vs. B as far as their BC's are concerned that actually measured velocity loss over a long range was made back in the mid 1990's.

One .308 Win. match rifles had two lots of ammo shot in it. One lot had Sierra 190 HPMK's and the other had Berger 185 VLD's. First shot with a Berger, second with a Sierra, then back and forth between them. 10 shots were fired with each bullet that left close to 2600 fps.

Two chronographs were set up; one centered 15 feet from the muzzle, the other 990 yards from the muzzle. Standard screens were used at the first one. The one at 990 yards had aluminum foil covered 1/4" thick 3-foot square foam boards connected to the Oehler chronograph's start-stop. connectors.

The 185's left the barrel about 10 fps faster than the 190's.

The 185's lost 40 to 50 fps more velocity downrange than the 190's.

So, with the Berger 185 VLD's higher claimed G7 BC of .281 than the Sierra 190 G7 BC of .270, they didn't maintain velocity as well as the 190's.

Both did shoot with equal good accuracy.
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Old June 6, 2013, 10:00 AM   #6
jmr40
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I've been experimenting a bit with Berger bullets which lists both on their box. Running the numbers with both G-1 and G-7 BC's just doesn't show enough difference for me to worry about with the calibers and ranges I shoot. It is true that the G-1 is a bit more optomisic. But as long as the manufactureers publish accurate BC numbers of either type it is close enough for me.

The way I see it running those numbers through a ballistics program is only good enough to get you in the ballpark at anticipated ranges. I'd still shoot at those ranges to verify bullet drop rather than trust printed ballistics charts.
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