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Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by barnesuser28, Apr 9, 2012.
can someone explain to me the difference between a G7 B.C and a G1 B.C
They're merely different drag models used for predicting ballistic performance of a given bullet. The G1 is the "standard" bullet used throughout the industry, despite the fact that it's a relatively poor fit for most of our more modern projectiles. It's relatively short (about three calibers in length), flat based, and has a rather blunt ogive. In the absence of anything denoting some other drag model, you can usually correctly assume that a manufacturer's listed BC is based on the G1 model. Berger (or more specifically, Bryan Litz) has lead the charge to use the G7 drag model for the more modern designs. The G7 model is significant;y more streamlined, has a boat tail and more accurately represents the flight characteristics of modern VLD or OTM Match bullets we use today. The result of this is that a ballistic program using the G1 model for a trajectory calculation will see some significant variation downrange, from what is calculated. The farther downrange, the more that discrepancy will grow. For a bullet more similarly shaped to the G7 model, the calculated trajectories will be far more accurate when compared to the actual firing results.
There's several other drag models in this same series (E.D. Lowry's, developed in the late 1940's or early 1950's) that may be more appropriate for some other given bullet, say, a flat-nose .30-30 design or a LSWC pistol bullet, but they see very little use outside some professional circles. Hope that helps a bit, but believe me, this is a very (VERY) abbreviated explanation of a rather complex topic.
thank you that helps, i understand it better now
In addition to Kevins response, you can refer to this article:
Berger Bulletin » A Better Ballistic Coefficient
And if you want the full story:
The book explains G1 vs G7 in detail, as well as lists the measured G1 and G7 BC's for 225+ common long range bullets.
My 'short summary' is: G7 BC's have less variation over long range. A G1 BC may vary over 20% from muzzle to a long range target which makes it very difficult to predict a trajectory that's accurate all the way. A G7 BC typically has less than 5% variation over long range. The reason is because the G7 standard is a better fit for modern bullets. So it's naturally more predictive over long range than the less representative G1 standard.
i went to bergers website and read bryan litz's explanation on it and that helped a lot
Thanks Bryan i didnt see your post when i posted. You posted it as i was reading it
To put it simply, it is the air-drag-only model, that factors the sectional density of a bullet OUT of it's BC. At least that's the way I understand it.
No Max, G1 & G7 are calculated the same as [sect density / Form factor], and form factor is [your bullet's drag Coef / a standard bullet's drag Coef] throughout the velocity range.
The different results here(with G7) are due to different 'standard bullet drag Coef' applied.
The closer you get to [apples / standard apples], the better the results.
I think for shooters that are only shooting out to 700-800 yards, it doesn't make a whole lot of difference as long as they are using accurate G1orG7 data. I have used the G7 and G1 inputs with my 6.5x284 using 140 VLD's. My drops are essentially identical out to 800 yards in both actual drop data in the field, as well as with the output generated from my ballistic calculator. Until I get out to 800 yards or so, the G7 formula then begins to show less than a one click difference in elevation to at least 1000. This is less than 3" difference at 1000 yards. Beyond this range the difference increases. This is also visually apparent when looking at the ballistic curves in Bryan's book. Once the velocities fall much below 1800 FPS, the difference in drop data occurs with the G7 form factor. I'm working on a 338 Lapua that I will use at +1000 yards. Using the G7 data will make a more substantial difference in developing accurate drops at those extended ranges. Bryan's book is very useful since he has developed good G1 and G7 data for most of the bullets available.
I'm not sure what I was thinking. With SD factored out, it would be the FF. It is the "standard" compared to that is different. Got it.
Also, don't forget that the G1 BC is the correct BC to use for shorter, flat based bullets (at all ranges), like many varmint bullets.