Several bullets I use only list a G1 bc and I prefer to use G7 when shooting longer distances. If you will take your known G1 bc and divide it by 1.950 it will give you your G7 BC. I'm sure allot of you know this but I often see questions asking about G7 bc's of different bullets. This is taking for granted that the factory G1 listed is correct, if not it will get you real close and you can adjust it based on actual drops when testing.
Figuring G7 based off G1
- Thread starter jsthntn247
- Start date
BoydAllen
Gold $$ Contributor
I disagree. If you look at both BCs for several bullets, you will see that simple multiplication will not get you there. The bullets that the G1 and G7 are based on have different shapes, with the G7 model corresponding more closely to modern long range designs, which makes it a better predictor of long range behavior. I had not thought about the relation between the two types of BCs much until I looked as several bullets G1 and G7 BCs as they were displayed on a page of Nosler's web site. http://www.nosler.com/accubond-long-range-bullet/
Modern ballistic coefficient calculations are done by measuring actual velocities (using chronographs) at two different distances, near the muzzle, and at some greater distance, or by the use of doppler radar. The actual behavior of projectiles is compared to that of the standard model for each type of BC. As you can see from the Nosler data, bullets that have quite similar G1 BCs can have quite different G7 BCs. The reason that actual velocities are used for their calculations is that experience has shown that BCs that were calculated by shape alone were not as accurate.
Modern ballistic coefficient calculations are done by measuring actual velocities (using chronographs) at two different distances, near the muzzle, and at some greater distance, or by the use of doppler radar. The actual behavior of projectiles is compared to that of the standard model for each type of BC. As you can see from the Nosler data, bullets that have quite similar G1 BCs can have quite different G7 BCs. The reason that actual velocities are used for their calculations is that experience has shown that BCs that were calculated by shape alone were not as accurate.
BoydAllen
Gold $$ Contributor
If the G7s were just the G1s times a constant, there would be no point in having or using them. If you are doing that, just compare G1s. The differences will be proportionate. BCs are fractions of the performance of a specific bullet. The bullets used for each type of BC are quite different. one is a very blunt spitzer FB the other a much more streamlined BT. As far as published values go, you may find this interesting.
http://arxiv.org/ftp/arxiv/papers/0705/0705.0389.pdf
http://arxiv.org/ftp/arxiv/papers/0705/0705.0389.pdf
BoydAllen
Gold $$ Contributor
I stand corrected. Apparently all of the ballyhoo about the G7 is exactly that. I compared calculated trajectories of a new Nosler long range bullet, using the JBM calculator, using both G1 and G7 BCs and the difference at 1000 yards turned out to be .1moa, or about an inch. My misconception about the superior results using G7 was because I had simply believed the publicity that had accompanied its introduction into general use. Evidently for the kind of uses that most of us would put them to the difference is insignificant. The G7 might as well be ignored because for first shot trajectory calculations, in the field the difference would disappear within the noise at any reasonable range.
Dusty Stevens
Shiner
So i guess the g7 is a publicity ploy just re-engineering known formulas to get a number thats different? It does draw attention and publicity but hasnt slipped by the math types.
I disagree. For G1 to be as useful as a G7, one needs a G1 value that is averaged over the same (or nearly the same) velocity range as which you are shooting. To my knowledge, only one reliable source provides G1 values averaged over approximately 600 yards, and no reliable sources provide G1 values averaged over 1000 yards.
A lot of companies are still publishing G1 values obtained at 100-300 yards. A G7 value obtained over 300 yards or less will be significantly more accurate for predicting retained energy, wind drift, and drop over 1000 yards than a G1 value obtained over the same range.
dmoran
Donovan Moran
^^^^ Nice chart.... Thanks Charlie !.!.!
Personally been using Pejsa based G1 ballistic's for many years, which continually prove very accurate solutions for me. At ELR distances, often G7 predictions I find to gradually get a little higher of what I actually need. With Pejsa G1 ballistics, I'm able to get more precise predictions across distances when I have the "retarded coefficient" adjusted to match the bullets field tested results.
Donovan
Personally been using Pejsa based G1 ballistic's for many years, which continually prove very accurate solutions for me. At ELR distances, often G7 predictions I find to gradually get a little higher of what I actually need. With Pejsa G1 ballistics, I'm able to get more precise predictions across distances when I have the "retarded coefficient" adjusted to match the bullets field tested results.
Donovan
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Of course, you can take Bryan Litz's figures and compute a correlation between average G1 and G7 values. Bryan calculates both values based on exactly the same recorded velocity changes throughout the flight at the same fixed waypoints over the same overall distance, and both values are averaged between 3,000 and 1,500 fps. However, look at his figures for the variances within the average.
Taking the .308 185gn Berger LRBT Juggernaut as an example, the G1 values by speed band run from 0.605 down to 0.505, a reduction of 16.5%, whilst the G7 values cover 0.280 to 0.287, a mere 2.5% variation between lowest and highest. Moreover, that small variation isn't directly related to speed (as MV reduces, they run: 0.287, 0.285, 0.280, 0,281, 0.284) whereas the G1's values are. Since in the case of a bullet speed is not only related to muzzle velocity, but to distance, any direct mathematical correlation may range from being roughly correct to very inaccurate for any single segment of the flight, even if the average works out a x% of the other form's average.
So, if we knew at exactly which distance Bryan measures his bullet speeds over, yes it wouldn't matter if you used G1 or G7 form as long as we shoot over the same distance and also employ exactly the same MV as Bryan used when testing that bullet. If you fired the Juggernaut from a .30 BR at say 2,000 fps and then from a RUM or Lazzeroni super-magnum at 3,500 fps, averaged one pair of readings over 300 yards, and the other over 1,300 yards, you'd find the G1 to G7 ratio was quite different.
The other factor that makes so many bullet manufacturers' G1 values suspect is that we simply don't know what the MV was and what the distance was. I'm not so sure about now, but certainly a few years ago many manufacturers' G1 values weren't based on any testing at all. A computer model was used - feed the bullet shape and dimensions in and it produced a 'calculated' (ie estimated) BC, or more likely a range of BCs based on bullet speed of which no doubt the highest was quoted.
Taking the .308 185gn Berger LRBT Juggernaut as an example, the G1 values by speed band run from 0.605 down to 0.505, a reduction of 16.5%, whilst the G7 values cover 0.280 to 0.287, a mere 2.5% variation between lowest and highest. Moreover, that small variation isn't directly related to speed (as MV reduces, they run: 0.287, 0.285, 0.280, 0,281, 0.284) whereas the G1's values are. Since in the case of a bullet speed is not only related to muzzle velocity, but to distance, any direct mathematical correlation may range from being roughly correct to very inaccurate for any single segment of the flight, even if the average works out a x% of the other form's average.
So, if we knew at exactly which distance Bryan measures his bullet speeds over, yes it wouldn't matter if you used G1 or G7 form as long as we shoot over the same distance and also employ exactly the same MV as Bryan used when testing that bullet. If you fired the Juggernaut from a .30 BR at say 2,000 fps and then from a RUM or Lazzeroni super-magnum at 3,500 fps, averaged one pair of readings over 300 yards, and the other over 1,300 yards, you'd find the G1 to G7 ratio was quite different.
The other factor that makes so many bullet manufacturers' G1 values suspect is that we simply don't know what the MV was and what the distance was. I'm not so sure about now, but certainly a few years ago many manufacturers' G1 values weren't based on any testing at all. A computer model was used - feed the bullet shape and dimensions in and it produced a 'calculated' (ie estimated) BC, or more likely a range of BCs based on bullet speed of which no doubt the highest was quoted.
CharlieNC
Gold $$ Contributor
Laurie the ballistics program I use on my phone app only take the average G1 as input and it agrees well with JBM where the bullet is selected from a menu. The AB program that came with the book also just uses the average BC as input. Which programs actually use velocity bands for enhanced accuracy?
Sierra's Infinity does - that why Sierra quotes velocity / BC bands.
CharlieNC
Gold $$ Contributor
The drag vs velocity curves fit well for both G1 and G7 bullet standards above 1500fps. Slower, the G1 "stubby" standard diverges significantly from the real world performance exhibited by the sleek bullets we use these days. While I don't have the actual equations, this suggests using the G7 vs G1 correlation is ok and the subsequent math based on G7 will render the correction at the slower velocities. Of course DOPE is necessary to true, and it always bugs me that the truing involves fudging the velocity which is probably the best known input.
You guys all know that Bryan Litz has a ton of bullets listed with their G1 , G7 , form factors ETC in his book BALLISTIC PERFORMANCE OF RIFLE BULLETS. Right?
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