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.