BEWARE, Problems with Exbal and G7 BC's

[ericpetritz]

Bigngreen,
I shoot this rifle all the time but 1000 yards is quite a ways for a 223 AR15! On a side note, it is a Les Baer super varmint and it is amazingly accurate! I will shoot it in the 600-700 yard range quite a bit. As I said before, the 1000 yard drop number was more for comparison. The actual range isn't overly important from a comparison point of view. Or said another way, there were significant errors at shorter distances as well.

Bryan,
I have to laugh at the comment your collegue made. Just the other day while troubleshooting a difficult problem at work, a very experienced senior engineering collegue of mine made the same comment---"It's either that....or something else"--good stuff! Engineers are all very similar creatures!

 

[ericpetritz]

It is very interesting to me that Bigngreen is getting very similiar results with Loadbase as I am getting with the "upgraded" version of Exbal. I do not have Loadbase but it seems like some of the programs are calculating drops of around 430" and some of the programs are calculating drops of around 447" at 1000 yards. The questions is: Which one is correct? I am not sure at this point. Perhaps it has to do with the atmospheric standards as Bryan mentioned (ASM vs ICAO)?

 

[bigngreen]

Thanks, my only interest is because you gave such good complete data compared to most and since I don't do all the engineering thing I have to hack a good correction out of trial and error so the more trajectories a guy can run the better you get with the tools at your disposal, not trying to crash out you thread.

What is of interest to me is the last few hundred feet per second before going transonic, LB is within a few inches all the way with Berger and JBM, didn't check my Exbal cause all I use is the PPC version, till you get into the farthest end of the trajectory. So far in my shooting near the transonic Exbal, Berger, JBM will all be about 2 MOA low, LB will be really close. My interest is not specifically any is better than the other but how to make each one function correctly out to max range. I will be adding Shooter at my next phone up grade, I lack the engineering but love muddling my way through it.

Nice thread, picked up a few things. Brian if Exbal is using ASM should I convert the BC from ICAO to ASM for a more accurate correction in Exbal?

 

[bigngreen]

It is very interesting to me that Bigngreen is getting very similiar results with Loadbase as I am getting with the "upgraded" version of Exbal. I do not have Loadbase but it seems like some of the programs are calculating drops of around 430" and some of the programs are calculating drops of around 447" at 1000 yards. The questions is: Which one is correct? I am not sure at this point. Perhaps it has to do with the atmospheric standards as Bryan mentioned (ASM vs ICAO)?

I was also intrigued when I saw that, LB is ICAO but I can use it to convert to ASM to input a value to Exbal in ASM, I'll charge my PPC up and see what that changes.
I enjoy shooting the solution, sometimes it's back to the drawing board but like I said earlier I can not apply the engineering to a problem, I'm wired for hands on problem solving!

I converted the ICAO BC of .19 to a ASM BC of .187 and it made an 8in difference at 1000yrds in LB, definitely something to watch and makes me think I need to do some work with Exbal and may explain some of the error that I've been tweaking out with velocity changes that should be a BC change.

 

[Mikecr]

I believe ICAO + 78%RH = standard artillery atmosphere (SAA)
Pejsa was stuck on this oddity for quite a while, and Gerald may be borrowing from Pejsa's math for his solutions.
Personally, I convert all to ICAO. I could post a spreadsheet to do this for you if needed.
One thing I notice with this discussion though,, you keep referring to altitude. You already measure & hold station pressure, so there is never any reason to consider altitude.
DO NOT ENTER ALTITUDE

 

[BryanLitz]

You already measure & hold station pressure, so there is never any reason to consider altitude.
DO NOT ENTER ALTITUDE

Good point Mike.

Regarding Loadbase which uses Pejsa equations...

As I recall, the math used in the Pejsa equations defines drag as a function of velocity, not Mach number. This practice fails to capture the effects of temperature on Mach number and drag, which is real. The error would have the effect of predicting too much drop in low temperature.

The extra high drop predicted by Gerald's latest exbal suggest that he locked down the definition of speed of sound to the standard value, and removed the temperature dependence. 447" is excessive.

Nice thread, picked up a few things. Brian if Exbal is using ASM should I convert the BC from ICAO to ASM for a more accurate correction in Exbal?

You can, by multiplying the ICAO BC by 1.018. Again, I'm assuming that exbal is using ASM. Careful with assumptions.

Interesting that exbal uses 29.92", 59 deg and 78% hum. Seems like a mixture of ASM and ICAO? I never heard of SAA, but it's a possibility that may explain why 1.8% doesn't exactly make up the difference.

-Bryan

 

[bigngreen]

Thanks for the reply Bryan! I try to dig into the underlying stuff but it can be messy!

So we have three standards out there ICAO, ASM and SAA?

 

[BryanLitz]

So we have three standards out there ICAO, ASM and SAA?

Unfortunately, yes.

Good news is that ICAO is used most of the time.

Also, the consequence of mis-matching the BC with the program is less than 2% error in BC, which only shows up at very long range.

-Bryan

 

[ericpetritz]

I sent Mr. Perry from Exbal an email asking him what atmospheric standard Exbal uses. Here is his response:

"Eric;

ICAO has a sea level pressure 29.92 and by definition of ICAO it is bone dry air (ie no humidity). ASM has a sea level pressure of 29.53 and 78% humidity by definition. ASM is the standard set by US Army Aberdeen proving grounds long ago. Half of the vendors use ASM and half use ICAO. The statement that a BC value calculated for ICAO is different than a BC calculated for ASM is misleading. BC is the ratio of how far a given bullet travels in one second to how far the standard bullet travels in one second. This ratio will not change with ASM or ICAO. Trajectory calculations are a different matter. you can play with humidity and pressure at sea level and see the difference.



Exbal use default values of 29.92 pressure and 78% humidity at sea level so it is a hybrid solution.

Gerald L. Perry "

 

[Buffalobob]

If you are going to quote him on line you should be courteous enough to tell him before hand that you are going to do so.

What you have just done is dishonest, cowardly and disgusting if you do not have his prior approval to post his reply.

 

[BryanLitz]

BC is the ratio of how far a given bullet travels in one second to how far the standard bullet travels in one second.

This is absolutely incorrect.

BC is defined as the Sectional Density (SD) of a bullet, divided by it's form factor referenced to a standard projectile. BC has units of lb/in^2.

In equation form:

Here is the rest of the Wiki article:
Ballistic coefficient - Wikipedia, the free encyclopedia

Other sources:
Modern Exterior Ballistics by Robert McCoy, page 101
Exterior Ballistics by George Klimi, page 46
Rifle Accuracy Facts by Harold Vaughn, page 203

I don't wish to engage Mr Perry, just wanting to share my information with others who are interested.

Interesting that he's using a 'hybrid' atmosphere. Given that fact, I have no idea how to properly adjust BC's for exbal.

 

[BryanLitz]

If you are going to quote him on line you should be courteous enough to tell him before hand that you are going to do so.

What you have just done is dishonest, cowardly and disgusting if you do not have his prior approval to post his reply.

I disagree. Mr Perry was simply answering a customers question. Why would he answer any different with the knowledge that his reply would be 'peer reviewed'? Suggesting he might answer differently is suggesting... what?

I've seen my emails to individual customers quoted on forums many times. It's always done in places where others can benefit from the information and I've never had a problem with it. I'm actually grateful because it's a way to get the information out to many people so the question doesn't have to be answered as many times.

If he'd reveled any secrets, I would agree. But he just provided information about how his program works. That can only help his customers, right?

 

[Mikecr]

My older version of Exbal(7.1) defaults at standard metro(29.53, 59, 78%).
Doesn't make any sense that they/he would drop this for standard artillery, instead of ICAO.
I think he's made a mistake.

 

[ericpetritz]

I am not sure how posting Mr Perry's response was is any way distasteful? People on this thread are interested in what model his software is using. His response to me answered that question and provides benefit to all who are using his software. I have been using Exbal for a long time and will continue to use it. Had he divulged information that was private or personal in any way I would not have posted it.