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BEWARE, Problems with Exbal and G7 BC's

 
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  #8  
Old 12-11-2010, 07:15 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

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
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And: Accuracy and Precision for Long Range Shooting

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  #9  
Old 12-11-2010, 08:13 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

Quote:
Originally Posted by ericpetritz View Post
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
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!
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  #10  
Old 12-11-2010, 08:19 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

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
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  #11  
Old 12-11-2010, 08:30 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

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"
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Last edited by bsl135; 12-11-2010 at 08:35 PM.
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  #12  
Old 12-11-2010, 08:32 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

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
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  #13  
Old 12-11-2010, 08:38 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

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
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  #14  
Old 12-11-2010, 09:28 PM
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Re: BEWARE, Problems with Exbal and G7 BC's

Quote:
Originally Posted by ericpetritz View Post
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

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!
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