Which one is closer to actuality?

[Tesoro]

Right now I have a pile of Hornady 105 hpbt to shoot at 600 and 1000 in my 6br. Hornady only lists G1 at .530 and Litz says the G7 BCs are 0.240 over 3000fps, 0.242 for 3000-2500fps, 0.250 for 2000-1500fps and 0.258 below 1500fps, with an average G7 BC of 0.246 from 3000 to 1500fps.

So I ran my velocity chart based on my mv of 2750 and wound up at 1000 with 1356fps for the G1 and 1233 for the G7 (average value). I was expecting the reverse as I have always thought that using G7 with a vld type bullet accounts for the increased drag efficiency at longer distance/reduced speeds. Or is the G1 just over estimating and the G7 is closer to reality?

 

[jdh47]

The G1 is based on a different profile. The G7 is based on the profile that most matches your bullet.

 

[CharlieNC]

If you examine a wide range of calibers and bullets you will find a perfect correlation between G1 and G7, meaning they both tell you the same information without differentation. The flight curves of the standards which they represent differ, and are nearly identical out to 1000yd. Significant for longer distances it matters, and the custom drag curves can help.

 

[Tesoro]

The G1 is based on a different profile. The G7 is based on the profile that most matches your bullet.

Exactly. And the g7 has a profile with less drag. So how can the G1 have a flatter trajectory?! That is my Question

 

[CharlieNC]

Exactly. And the g7 has a profile with less drag. So how can the G1 have a flatter trajectory?! That is my Question

If you calibrate your ballistic app using actual dope at extended distances, both G1 and G7 will give essentially the same trajectories. Very slight differences for your distances.

 

[Tesoro]

If you calibrate your ballistic app using actual dope at extended distances, both G1 and G7 will give essentially the same trajectories. Very slight differences for your distances.

I am obviously new at using the shooter app for long distance. But what I dont get is when using G1 BC the G1 profile says spire point and when using G7 BC the G7 profile says vld. BUT the G1 calculation shows a flatter trajectory with all other inputs the same for both. Logic tells me it should be the reverse, even if just slightly.

 

[Eric Leonard]

I shot some of those in 243 and it seemed the G1 was closer to .480 ,, it’s nowhere near as streamlined as a Berger vld

 

[CharlieNC]

For the same bullet, try using G1 and G7 values from the same source. Do not mix sources like Horn vs Litz. As mentioned do little more research to find multiple sources to examine consistency and overstating claims from manufacturers.

 

[Tesoro]

I shot some of those in 243 and it seemed the G1 was closer to .480 ,, it’s nowhere near as streamlined as a Berger vld

Well humm, I had my rifle dialed in at 600 with 103 eld-x shooting poa and no more than moa. And then I moved to 1000 and used some 105 loaded with the same charge. I dialed up based on the 103 trajectory and added a moa and 50fps less for luck (my crono was acting up) . I shot 3 sighters and they all landed in the dirt around 30 inches below my poa! I just went back to the app and plugged in your .480 G1 compared to hornady's .530 and at 1000 from my 600 zero and it indicates a 20 inch drop below the .530 trajectory! Explains the dirt hits! But I did put 5 consecutive shots in a 6 in group so the bullets cant be too wonky. ( ps; I have 600 Berger 105's on the way!. Those remaining 105's be reserved for plinking.

 

[Ned Ludd]

The G7 profile does not necessarily have "less drag" as far as the ballistic calculator is concerned. That totally depends on the relative G1 and G7 ballistic coefficient values that are actually input into the program. In his "Ballistic Performance of Rifle Bullets, 3rd Ed.", Litz gives respective values for the average G1 and G7 BCs or 0.457 and 0.234, respectively, between 3000 and 1500 fps. The values you used (0.530 per Hornady and 0.246 avg. per Litz) were determined by independent groups and are much farther apart, indicating the relationship between the two different BC values may not be correct.

If you compare G1 and G7 BCs determined from consistent velocity data sets as Litz has done in his book, what you will find is that multiplying the G1 BC for almost any bullet listed in the book by a value very close to 0.51 will generate the G7 value. I believe this relationship is related to the ratio of the form factors of the G1 and G7 standards. Regardless, the ratio of the two values you used is much lower, suggesating one of the two BC values was off relative two the other (0.246/0.530 = ~0.46, instead of ~0.51). I would guess that the Hornady value of 0.530 is inflated relative to its true value, which therefore "tricks" the ballistic calculator into thinking it has less drag than it really does, and therefore greater retained velocity than the same calculation made with the G7 BC value.

Nonetheless, if Litz' measured G1 and G7 Ballistic coefficients of 0.457 and 0.234 are used at JBM Ballistics, the program still predicts the bullet to have greater retained velocity at 1000 yd with the G1 coefficient. The values may be a bit closer than when using Hornday's G1 BC, but they're still not exactly the same, differing by ~42 fps.





However, if you run a similar comparison using Litz' G1 and G7 values for the Berger 90 VLD, the outputs are very close, as would be expected. I did the same thing with a .30 cal bullet, and the results were also very close, as they should be.





I cannot explain the discrepancy between G1 and G7 outputs with the Hornady 105 bullet. However, one would think it almost has to be rooted in a greater difference between the two G1/G7 BC values measured by Litz than there actually is. In other words, one of the two values must somehow be off. Otherwise, it would be expected that velocity decreases predicted over distance using the two different BCs would be very close. Not identical, perhaps, but very close.

 

[Tesoro]

Thanks for taking the time for this! Things are much clearer now. Drag is drag and dosen't change to any great degree by using a different way to calculate it because the shape dosent change! And as I posted above it does appear on the range that the stated Hornady bc is bloated due to the ballistic calc not coming close to the actual trajectory. If I had known then and used the .46 bc then my sighters would have probably been on the board and not in the dirt!