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BEWARE, Problems with Exbal and G7 BC's

 
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  #1  
Old 12-11-2010, 12:03 PM
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BEWARE, Problems with Exbal and G7 BC's

I have been attempting to use G7 BC's with Exbal version 10 and have been having problems. I am wondering if any of you have seen similiar problems or can see something I am doing wrong. I have emailed Mr. Perry many times about these problems but all I am getting from him is that it is something I am doing wrong--like the conditions are somehow incorrect, or I am entering the data incorrectly, or I am using an inaccurate BC, etc. He does not believe that there could be a problem with the software. Here are the specifics--let me know what you guys think:

First of all if you run a set a calclations on Exbal using a G7 BC and then save the file, when the file is reopened at a later date it defaults back to a G1 BC. This is a real pain because it requires you to reselect the bullet and drag curve. If you don't reselect the bullet it will calculate as a G1 and cause huge errors. It is even more of a pain if you enter multiple G7 BC's, then you have to reenter all of them.

More important are the following observations. For those who have read Brian Litz's book, you will know that there are multiple velocity G7 BC's included for many bullets that Mr' Litz measured experimentally. For those who haven't read his book, I suggest you get it---it is excellent!!

For the .223, 77 grain MatchKing he published an average G7 BC of 0.190. I am shooting this bullet at 2750 fps. My conditions are as follows: actual altitude = 3250 ft, Temperature = 10 degrees F, Station Pressure = 26.22"HgA, Relative Humidity = 50%, density altitude of 1140 ft, zero range = 200 yards, sight height above bore = 2.5". All environmental conditions were measured with a Kestrel 4000. Here is what the output from Exbal shows:

Using multiple G1 Sierra BC's and the density altitude of 1140', drop @ 1000 yards of 432.6"
Using multiple G1 Sierra BC's and 3250', 10 degrees, 26.22"HgA, and 50% RH, drop @ 1000 yards of 439.9"
Using single G7 BC of 0.19 and density altitude of 1140', drop @ 1000 yards of 430.4"
Using single G7 BC of 0.19 and 3250', 10 degrees, 26.22"HgA, and 50% RH, drop @ 1000 yards of 350.6".

The first three scenarios all show drops within 10" of one another while the last one shows 80" of variation! I have run these numbers using Berger's ballistic software as well as the JBM software and I get drops at 1000 yards that are also approximately 430" of drop at 1000 yards. Exbal does not seem to calculate accurate drops with the G7 BC if the measured pressure, temperature and humidity from a Kestrel are used. Perhaps there is a problem with the BC but if that was true I would expect the other software packages (Berger and JBM) to show similiar errors.

Any thoughts?
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Old 12-11-2010, 12:49 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

Btt - interesting!
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Old 12-11-2010, 12:57 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

I found this interesting so I ran it into Loadbase 3.0 and laid it out in four tracks to see what I got, my G1 numbers were with in .2 in. @1000yrds and my G7 numbers were with in .1 in. the G1 and G7 tracks were 15in different. Doesn't help with anything really just one more program to compare to.
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Old 12-11-2010, 01:19 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

Quote:
Originally Posted by ericpetritz View Post
Using multiple G1 Sierra BC's and the density altitude of 1140', drop @ 1000 yards of 432.6"
Using multiple G1 Sierra BC's and 3250', 10 degrees, 26.22"HgA, and 50% RH, drop @ 1000 yards of 439.9"
Using single G7 BC of 0.19 and density altitude of 1140', drop @ 1000 yards of 430.4"
Using single G7 BC of 0.19 and 3250', 10 degrees, 26.22"HgA, and 50% RH, drop @ 1000 yards of 350.6".

...

Any thoughts?
I have a thought.

One of the most difficult things to keep straight with ballistics programs, both for the programmers and users, is the distinction between station pressure, corrected pressure, and altitude. I'll skip the full explanation, it can be found elsewhere on this forum. To the point, if the station pressure is entered as (or the program interprets it as) a corrected pressure, and you enter an altitude above sea level, the program will essentially double the effects of altitude. In other words, you give it a station pressure of 26.62". If the program considers that as a corrected-to-sea-level pressure, it will now find out what the station pressure is at 3250' (will be something like 23.?"). Now this false pressure which is too low is used to calculate the air density that the bullet is flying thru. In essence the program is flying the bullet at more than double the altitude you intended.

Your last case (the G7 prediction that is super flat) appears to be a symptom of this problem. My advice is to re-run the scenarios, paying specific attention to the pressure and altitude input. I'm not familiar with exbal so I don't know how the interface works, but with JBM, there's just a checkbox that says corrected pressure (or something like that). Having this checkbox checked can be the subtle difference between a correct trajectory calculation and a bad one. It can be easy to miss.

If you're doing everything correct on the inputs, it's possible that the error is behind the scenes. In other words the program isn't applying the inputs properly. Nothing against Gerald, anyone can make mistakes.

Does exbal display anywhere the air density that's being used for the calculation? I find this is extremely valuable information when troubleshooting ballistics programs. My programs and JBM display the air density which you can use to make sure that the result of your atmospheric inputs are being interpreted as you intended. This allows you to compare ballistics programs in a fair 'apples-to-apples' manner.

I'm sure the problem stems from atmospherics, not the fact that a G7 BC is being used.

Please let us know what you find out.

-Bryan
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  #5  
Old 12-11-2010, 06:59 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

Brian,
Thanks for the response, especially since I am using your G7 BC! I understand the differences between station pressure, corrected pressure, actual altitude, and density altitude. In Exbal you can enter either station pressure or corrected pressure but not both. In other words if you enter station pressure it calculates the corrected pressure for your altitude and vice versa. I never use corrected pressure--only station pressure, temperature, actual altitude, and humidity or I enter density altitude and let the software calculate everything else.

The reason it seems to me like it is related to the G7 BC is because I am using the same atmospherics when I run the case with the G1 BC and I do not get the error. In fact, after I set it up with all of the parameters I mentioned below, I change nothing but the BC---from the G1 Sierra BCs to your G7 BC. Everything else, including the atmospherics, remains constant. It seems like a problem with the software to me but Mr. Perry believes otherwise.

Eric
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  #6  
Old 12-11-2010, 07:02 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

Bryan,
Apologies for misspelling your name......

Eric
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  #7  
Old 12-11-2010, 07:15 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

Bryan,
I do not see anywhere in Exbal where the air density used in the calculation is displayed to the user.

Does anyone have the Shooter ballistic software for Android? I am thinking about purchasing it but I would like to know what it calculates for this same set of conditions before buying it.

Thanks
Eric
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