BEWARE, Problems with Exbal and G7 BC's

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by ericpetritz, Dec 11, 2010.

  1. ericpetritz

    ericpetritz Well-Known Member

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    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?
     
  2. royinidaho

    royinidaho Writers Guild

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    Btt - interesting!
     

  3. bigngreen

    bigngreen Well-Known Member

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    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.
     
  4. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    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
     
  5. ericpetritz

    ericpetritz Well-Known Member

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    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
     
  6. ericpetritz

    ericpetritz Well-Known Member

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

    Eric
     
  7. ericpetritz

    ericpetritz Well-Known Member

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    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
     
  8. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    Eric,

    It sounds like you know what you're doing. Without running the software first hand, I would be inclined to agree with you that the problem is 'behind the scenes'.

    If others see this and report the same problem to Gerald it may prompt some action. This was probably the intent of your post all along.:)

    Good luck.

    -Bryan
     
  9. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    Eric,

    I consulted with the developer on the Shooter application. The solver is a point mass model and it calculates trajectories that match JBM and my programs. In my opinion it's the best program out there for a portable device, but maybe I'm biased :)

    I ran some cases for comparison.

    Using single G7 BC of 0.19 and 3250', 10 degrees, 26.22"HgA, and 50% RH

    Shooter calculates an air density of 0.07397 lb/ft^3, and a drop of 428.2"
    JBM calculates an air density of 0.07397 lb/ft^3, and a drop of 427.9"
    My PM Solver calculates an air density of 0.07397 lb/ft^3, and a drop of 428.06"

    So all 3 arrive at the same air density from those atmospherics, and predicted drop is within 0.3" at 1000 yards for a subsonic .22 cal bullet.




    Using single G7 BC of 0.19 and density altitude of 1140'

    Shooter calculates an air density of 0.07395 lb/ft^3, and a drop of 428.0"
    JBM calculates an air density of 0.07396 lb/ft^3, and a drop of 428.2"
    My PM Solver doesn't accept density altitude input.


    A couple notes.
    1) In reality, this projectile would probably de-stabilize around 800 yards where it goes transonic, so none of these trajectories are likely to be valid in the real world. However, it is a valid way to compare ballistics programs and their outputs.

    2) Because we're going thru transonic speeds for this trajectory, there are some interesting things to consider dealing with the use of density altitude (DA).

    DA is commonly thought of as a single metric that encompasses everything a ballistics program needs to know about atmosphere. It's one input that saves you from inputing temp, pressure and hum. If used properly, it can produce accurate results.

    However, there is a 'gotcha' if you're dealing with trajectories that approach transonic speed.

    The speed of sound in air determines the Mach number of the bullet, and the speed of sound depends on air temperature. If you're giving a program only DA and no information about air temperature, the program can't know what the speed of sound is, so it assumes 'standard' value. However, if the actual air temperature is much different from the standard value, the program will be using a speed of sound, and a Mach number that are incorrect which leads to inaccurate trajectory prediction.

    In order to be fully correct, a program that accepts DA as an input still needs to know air temp in order to make the speed of sound calculation and get the right Mach number. If you're using DA and not input temp as well, your solution will have error in cases where the air temperature is not 59 degrees F.

    Having said all that, the error only becomes significant when the trajectory gets down to transonic speed.

    This was probably way more than you wanted to know!
     
  10. ericpetritz

    ericpetritz Well-Known Member

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    I just received an email from Mr. Perry at Exbal and he found some problems with the software. I am not sure if he has been reading this thread but he finally looked into it (I mean this in the best possible way--I too am a stubborn professional engineer!). He sent me an updated version of the software which I just finished loading and testing. First of all, he fixed the problem of the software not retaing the G7 BC when it is saved and reopened--this is very nice!

    Second, he made some changes to the G7 calculation. He did not let me know all of the specifics of these changes but I can say that the software is producing vastly different results now. For all the same conditions listed in my first post, I am now finding trajectories of -447.3" at 1000 yards. It does not matter if I use the measured density altitude or the measured temperature, station pressure, relative humidity, and actual altitude--I still get a trajectory of -447.3. While this is much closer than the numbers I was getting before, it is still an approximately 17" variation from the other programs using G7 BC's and the calculation using the Sierra G1 BC's. I am not sure which one is most accurate--I will have to do some more shooting at the range to determine.

    To all the Exbal users: get the newest version of the software if you will be using G7 BC's. After trying it please let us know how accurate it is for you.


    Eric
     
  11. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    One more observation :)

    You might wonder why the outputs I gave above are different from the exbal value you got using 26.22", 10 degF, and 50% RH. Exbal got 439.9" of drop in these conditions, and the other programs got 428" of drop.

    I believe the reason is because exbal assumes that you're giving it BC's that are corrected for the old Army Standard Metro (ASM) atmosphere. In fact, my BC's are corrected for the newer International Civil Aviation Organization (ICAO) atmosphere. My program, JBM, and Shooter all use the ICAO atmosphere.

    In other words, my BC's are corrected for the ICAO atmosphere, and are only 'compatible' with programs that use the ICAO atmosphere.

    The difference between a BC corrected to ASM vs ICAO atmosphere is about 1.8% (air density is .0751265 lb/ft^3 for ASM vs .0764742 lb/ft^3 for ICAO).

    In other words, the effect of using my BC's in a 'non-compatible' program, is that the program applies the BC as if it were 1.8% lower than I intend them to be.

    Reducing the G7 BC of 0.19 by 1.8% gives you .1866. Plugging this BC into an ICAO solver should produce the same output as an ASM program running with .19. I found that this BC produces a drop of 437", which is much closer to the exbal answer of 439.9".

    I don't know for sure that exbal uses ASM, but I would bet that it does based on the above analysis.

    One clue will be the default atmospheric inputs. When you turn on exbal from scratch, what does it populate the atmospheric fields like? If it has 29.53", 59 deg F, and 78% Hum, that's a good sign that it's using ASM because those are the ASM standard sea level conditions. The standard conditions for the ICAO atmosphere model are 29.92", 59 deg F, and 0% RH.

    As one of my old engineering colleagues used to say to sum up a diagnosis: "It's either that... or something else":)
     
    Last edited: Dec 11, 2010
  12. ericpetritz

    ericpetritz Well-Known Member

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    Bryan,
    Actually that was great information! I know that the bullet in this particular case will be subsonic at 1000 yards and I likely won't be shooting this rifle at that range anyway. I simply chose 1000 yards to compare the ballistic programs because it is much less painful to post one number for all the different scenarios than it is to post complete drop charts. The fact that all the other programs as well as the calculations using Sierra's G1 BC's all results in drops of around 430" at 1000 yards while the new Exbal results in drops of 447.3" still causes me some concern as to which one is accurate.
    Eric
     
  13. ericpetritz

    ericpetritz Well-Known Member

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    Bryan,
    You are like the ballistics oracle! Excellent information! When Exbal is started up it populates with: 0 ft, 59 dgrees F, 78% RH, 29.92"Hg.

    It sounds like I need to use different software if I want to use your G7 BC's.

    Eric
     
  14. bigngreen

    bigngreen Well-Known Member

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    Loadbase G7 was 445.7 and 445.6. Are you going to shoot this or are you just tinkering with the programs, it would be interesting to see the real world results!