Range Report G1 v G7 - why such a large variance

[Onehunglow]

I have a question. I am sure it has been asked, but I couldn't find it in search

I was building some wind charts for 223 (80 grain noslers - 2650 fps - 20" WOA upper) and ran the program with a g7 model. Last year I ran the same calcs using a G1 model, and even shot a match with the data which proved accurate for windage.

The difference between the G1 model and G7 at 600 yards in 15 MPH is almost a factor of two.

G1 model - 15 mph cross wind 600 yards = 58.91"
G7 model - 15 mpg cross wind 600 yards = 25.22"

Which should I use? I have you not back checked the data and the matches are Saturday (iron sights) and Sunday (scopes)

I am not good enough of a shooter with irons to know for sure, although I am sure to note if I am off by 25" as that will put me off the frame! I am sure after Sunday I will know, but I'd rather know Friday and keep my scores higher.

I could swear I dialed in 58 inches on a 15 mph crosswind day and shot a 184, which for me is good.

I'm confused as the G7 models worked great at 1000 yards with my Sierra 284 175 grain bullets...perfectly in fact.

 

[Fred @ Bison Tactical]

You should read this book:
AB for Long Range Shooting

But in a nutshell - your bullet (whatever you happen to choose) has a shape. You ballistics calculator makes a prediction about a trajectory based on a standard bullet shape (the G1 or G7 standard). These two standard bullets are very different in shape. The BC you supply is a reference on how similar your bullet is to the standard you choose. If the BC you supply to the ballistics calculator is not as close to the correct shape you will get diverging predictions.

However, I would suggest you check your inputs into the ballistics calculator and make sure your entering correct data. Likely that is the source of the issue.

 

[*Nine]

Are you using the same BC value when you switch between G1 and G7? That would account for the data you're getting out. A bullet like you're using with a ~.415 G1 BC will not give you valid numbers if you just tell the program to use a G7 profile and keep the G1 number in there. You need to find the correct G7 BC for that bullet if you want to have the program use a G7 profile when it does the calculations.

 

[Onehunglow]

I understand all that (above two posts)

I should have been clearer. I have a ballistics calculator Ballistics V4.2.3 by Zdziarski. The prograam supposedly computes both the G1 and G7 native drag models by choosing which one you want to use. I went back and looked at the BC's the calcs were done with and they are in fact identical per velocity and value.

The input data is correct. Lord know why the BC's are identical

My question should ahve been has anyone back tested this kind fo data, and if so what did they verify?

 

[Fred @ Bison Tactical]

could be a programming mistake in the app? It sure doesn't sound right. Maybe send the question into support for the application. I am not familiar with the software, but you should (as Nine) mentioned have different BC's for G1 and G7 drag curves.

 

[Cossack]

Same bullet can not have a equal coefficient number in G1 and G7. For example BC of 308 168gr Berger target VLD, G1 is .473 and G7 is .242. The difference is not only what bullet they are compared to but also how the drag is measured. Reason you getting a whiled discrepancies is because software thinking you using two different bullets.
Find out the G7 for that bullet and use it. G7 number is a bit more accurate because it does not require a velocity to calculate the coefficient.

 

[Onehunglow]

Thanks Cossack, I went hunting on the internet and found the G7 BC for a 80 grain Nosler. Its 0.165 versus the 0.415 the program was using even in the G7 mode.

I plugged it in, ran the tragectory program, and the windage looks a lot more like the G1 models, and match the empirical windage I have in my notes from some older matches.

 

[sharac]

G1 model - 15 mph cross wind 600 yards = 58.91"
G7 model - 15 mpg cross wind 600 yards = 25.22"

I think the unit of measure mpg has something to do with it as it is commonly used for measuring efficiency of a cartridge (miles per gallon of gunpowder) instead of wind speed .

Glad you've sorted it out though...

 

[Ring]

223 (80 grain noslers - 2650 fps - 20" WOA upper).

u did something wrong...

g1 on that = .415
g7= 0.2068

 

[Cossack]

Thanks Cossack, I went hunting on the internet and found the G7 BC for a 80 grain Nosler. Its 0.165 versus the 0.415 the program was using even in the G7 mode.

I plugged it in, ran the tragectory program, and the windage looks a lot more like the G1 models, and match the empirical windage I have in my notes from some older matches.

I'm glad that worked out. I think something is wrong with that software.

 

[onethousandmeters]

Sometimes when a software program has data for BOTH G1 AND G7 values, you can change between the two as the OP stated. However, when you change BACK from one to the other, the values can be left in the field from the previous calculations. I discovered this with iSnipe. It will often default to G7 values for many of the Hornady bullets. However, I had also pulled the G1 BC values from Hornady's website and ran them as well. The problem I discovered was when I accidentally entered G1 BC values when the program was still set on G7. That made the bullet like 500% MORE effective, and gave me erroneous readings and showing something like only a 50" drop from 100 yards to 1700 yards! At first, I thought I'd found the Holy Grail of bullets, only to quickly discover my error! Hope this helps!