G1 vs G7 profiles. How much does it REALLY matter??

[Mr.Knowit all]

I only have one rifle that I shoot ELR with and it is a custom 300WM on an AICS. I am on my 5th barrel and all of them were 1-9, 1-8.5, or 1-8 twist. I am a believer that the extra spin gives me better, more stable travel through transonic range. JMO. I shoot the 208 ELDM and sometimes the 230 OTM.

I have a Kestrel 4500 with the Horus ballistic software and it only has G1 capability. I understand that the bullets I shoot have a much closer profile to the G7 but I do not have that option. I noticed that as I shot 400, 600, 800 meters that my Kestrel was right on. I use a Magneto Speed and of course do my own reloading with an ES of less than 20 fps. So one day I decided to set up my steel out a little further and set it up at 1734 yds. I entered all the correct data in my Kestrel and it was on. I didn't need to true it. The truth was that it was so close/on that I was unable to make any adjustments to true it. I know that in theory it was "more off" than if I would have been able to enter the G7 and had the software to give me the data needed to make the shot but it worked and has worked on several other rounds/rifle that I have. Granted I don't shoot anywhere close to that distance (over 1500 meters) I shoot with my other rifles but it is on the money. Science and logic tells us we would have a more accurate prediction entering a closer profile for the projectile but it has been my experience that they were close enough it has not mattered, at least in this case. When I enter (in Shooter app) the two different BCs I can see the slight difference in my elevation but what ever is in my Kestrel has been good. I know this goes against reason and before all you guys with your custom profiles start freaking out on me... save it unless you have FIRST HAND experiences please.
This is NOT me promoting anything or putting any other equipment down. I don't want to hear about what you think and what science says but I want to know REAL life, empirical data found first hand. Anyone ever find something similar to this while shooting? Maybe I have just been lucky....? This Spring when my 338 Norma and Creedmore are done I will test at extended ranges too.
Thank you for your time and input.

 

[dmoran]

My own inputs:
- to shape/design of most commonly used bullets today, G7 is a more accurate description
- for ballistic solutions/predictions, either G1 or G7 solutions work well enough
- foresee future ballistics solutions will be dominated by Doppler based trajectory
My 2-cents
Donovan

 

[Ned Ludd]

Due to the fact that the G7 std is shaped much more closely to the bullets we actually shoot, the drag function is closer to reality. For that reason, you will notice that "velocity banded" G7 BCs are really unnecessary, the value simply doesn't change enough with respect to velocity that it becomes critical. The fact that G1 BCs do change enough with respect to projectile velocity that they are given in velocity bands tells you something. As long as you use the appropriate velocity range, the G1 BCs work just fine. However, it is one more thing to think about. IMO - simpler is usually better.

 

[JPeelen]

Ballistic coefficients are often measured relatively close to the muzzle. A G1 BC obtained this way will, at the longer ranges, show a much too optimistic (low) arerodynamic drag. Computed velocity will be too high, crosswind effect too small etc. etc.
If you take, on the other hand, your real 600 yd range data and adjust the G1 BC so it fits your range results, it may be a good fit.

The main problem is taking G1 BC from short range data and use it for long range computations. G7 will give much more realistic trajectories in this case.

G7 was created during WW2 for a projectile shape that is much closer to todays match bullets than G1, which is based on French naval shells as used in the 1880s. After all, the G in G1 comes from the French naval proving ground Gavre. After a short stint with letter J, letter G was kept also for later models based on U.S. and U.K. range firings. You will hate to hear it, but G7 is actually based on British data. G2 through G6 are based on U.S. firings.

 

[Mr.Knowit all]

Thank you Gentle Men. I appreciate all your expertise, opinions and the time you took to respond.

 

[SheepDog]

It is possible to translate from G1 to G7 BCs or the reverse. When you enter the BC in ballistics software it makes the same calculatione either way. The difference id the value the software assigns to the BC. If you assemble the drop tables side by side for the same bullet using both G1 and G7 information the answer will come out the same. The parameters in the equations change but the actual flight of the bullet doesn't change because you use a different BC table. The only reason, that I can see for US manufacturers to continue using the G1 tables is that high BCs sell bullets. If they switched to the G7 tables then they would have lower BC numbers and people buy high numbers. Most people expect high BC to be faster and more accurate and neither is true. High BC bullets hold their speed better and buck the wind better but the don't shoot faster and they have very little to do with the accuracy of the round. It's all based on illusion.

No, I am not a ballistics engineer. I have written 3 exterior ballistics programs and I am working on another. To write software you only have to learn enough to tell the computer how to do the job. I don't need to understand why - but it does help when I do.

 

[CharlieNC]

These days there is more debate about the basis of the solver/algorithm which is used than G1 vs G7. And Applied Ballistics measures actual drag curves for Berger bullets. Due to the nature of the PRS matches there is lengthy discussion about this on Snipers Hide.

 

[Berger.Fan222]

I've worked a lot with the 208 ELDM over the supersonic range using Doppler Radar to measure and independently verify Hornady's results. Both Hornady's custom drag curve and their G7 BC measurements are right on. The G7 BC of this bullet varies little over the supersonic range, so there is little need of a custom drag model in this case.

The G1 BC of the 208 ELDM varies considerably over the supersonic range. The more time and distance your bullet spends below Mach 2.0, the more your actual trajectory, wind drift, and retained velocity will differ from predictions based on the shorter range G1 BC.

I've worked less with the 230 OTM, but I have done enough to observe that it is much more sensitive to twist rate than the 208 ELDM. I've seen the 208 ELDM perform up to factory specs from both 1 in 10" and 1 in 11.25" twist barrels. The 230 OTM really needs a twist faster than a 1 in 10" to perform up to factory specs.

 

[SheepDog]

Of all the ballistic coefficients the G1 has been studied most. The data points for it are twice as numerous as any other set of points. Part of this is the fact that the G1 profile is less exact for small arms ammo than it is for larger diameter military rounds with blunter noses. We need more data points to estimate the path a given bullet will take as it slows from the muzzle velocity to the impact velocity. You can still take the velocity change over the distance and collect either the G1 or the G7 ballistic coefficient for the same bullet. The flight path of the bullet will match either sets of data points within the tolerance used. Not surprisingly the less actual data used in the calculations the less precise the output. In the software there is always some trade off between the amount of data input and the speed of the calculations. The accuracy of the input is also limited by the ability to measure conditions over the range of fire. That also limits the accuracy of the results. What surprises me most is that software is as accurate as it is, especially over the 1000 yard ranges and longer.

 

[dcali]

If you look at a chart of drag coefficient vs Mach number, you will find that at normal rifle velocities (Mach 1.5-Mach3 or so), there is very little difference between G1, G7, and reality for most bullets. They both work very well.

You will see differences in the sub and transonic regions, and sometimes dramatic differences above about Mach 3, where G1 is not good at all. So if you're shooting 4800 fps, or really, really, far, the difference is noticeable. You only *begin* to see it around 600-700 yards for normal shooting of normal bullets.

 

[dcali]

It is possible to translate from G1 to G7 BCs or the reverse. When you enter the BC in ballistics software it makes the same calculatione either way. The difference id the value the software assigns to the BC. If you assemble the drop tables side by side for the same bullet using both G1 and G7 information the answer will come out the same. The parameters in the equations change but the actual flight of the bullet doesn't change because you use a different BC table.

You will get different results from G1 and G7. The computer does not just translate one to the other - they are fundamentally differently shaped curves. If it didn't matter, there would be no point in more than one drag function to exist at all.

 

[Mr.Knowit all]

You will get different results from G1 and G7. The computer does not just translate one to the other - they are fundamentally differently shaped curves. If it didn't matter, there would be no point in more than one drag function to exist at all.

Right. There are many different profiles G1, G2, G3, ...etc... typically we shoot one that resembles a G1 or closely matches a G7 profile. At least that is my understanding. I also thought that when a bullet goes through transonic to subsonic the closest profile match will give you the best predicted trajectory, given if the bullet has the stability to handle the rough transition. Correct????

 

[DocUSMCRetired]

I am glad you asked:

1) http://www.appliedballisticsllc.com/Articles/ABDOC130_CDM.pdf

2) http://www.appliedballisticsllc.com/Articles/ABDOC118_PracticalBallistics.pdf

3) http://www.appliedballisticsllc.com/Articles/ABDOC2.2 - A Better Ballistic Coefficient.pdf

4) http://www.appliedballisticsllc.com/Articles/ABDOC2.3 - Form Factors A Usefull Tool.pdf

 

[dcali]

The best solution will come from the bullet's actual drag function. We only use G1 or G7 because they are convenient, and they allow us to compare two bullets easily, assuming that both are reasonably good matches for the actual drag function. Honestly, a lot more is made of the issue than it deserves. But in order of preference, it's custom, G7, G1 for pretty much any modern long range bullet.

 

[SheepDog]

The G7 BC won't be a good match to a flat based bullet but that bullet could be assigned either or both G1 and G7 ballistic coefficient. Either would give fairly accurate information - to a point. The thing to remember is that if the manufacturer provides a G1 value then it is imperative that the same G1 value is used in calculations. The only way we get to use a different value is if we test the bullets ballistic curve and match it to the curve we want to use, or plug the data into a calculator that produces the different G value BC.
The "best" designation is listed as:
G1 for rifle bullets with blunt nose and most pistol bullets
G5 for Moderate boat tails 7 degree 30' tail taper with 6.19 caliber tangent nose ogive.
G6 for Flat base, spire points 6.09 caliber secant nose ogive
G7 for "VLD" boat tail. Long 7 degree 30' tail taper with 10 caliber tangent nose ogive.
For flat base spitzer designs and for shorter or stepped boat tail designs it is some where betweenG1 and G5. The two most common BC values listed are G1 and G7 with the others left pretty much alone.
It is important to note that one can assign the same bullet different BCs based on each model and be technically correct for solving ballistic trajectories. Especially if you provide multiple BCs based on velocity as Sierra does.

 

[DocUSMCRetired]

I would suggest ditching the use of a BC all together. Use a CDM instead, it is far more accurate.

 

[SheepDog]

Coefficient of drag also changes with velocity.
We do the best we can to estimate trajectory and then we fire at that range to adjust the estimation to real life.

 

[dcali]

CDM = Cofficient of Drag vs Mach function, which is the specific bullet's drag as a function of velocity. It doesn't get any better.

 

[Berger.Fan222]

CDM = Cofficient of Drag vs Mach function, which is the specific bullet's drag as a function of velocity. It doesn't get any better.

Yep, this is the approach in the AB software, a Lapua solver, and also in the Hornady 4 DOF. One hopes that the CDM curves become available in a format where they can be more easily integrated into other ballistic calculators. I think Lapua is the only source of CDM data that releases the curves.

 

[dcali]

Yep, this is the approach in the AB software, a Lapua solver, and also in the Hornady 4 DOF. One hopes that the CDM curves become available in a format where they can be more easily integrated into other ballistic calculators. I think Lapua is the only source of CDM data that releases the curves.

As far as I know, they are, which is disappointing. I get why AB doesn't - their whole calculator product is basically this data, and I don't suppose they want to give that away. But Hornady? What's their reasoning, I wonder? They sell bullets, not software. What do they care if I use their free calculator, Lapua's free calculator, or one I wrote myself? I suppose you could reverse engineer the curves. But don't make me work so hard!