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Computerized Ballistic Solutions

 
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  #8  
Old 10-24-2003, 05:10 PM
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Location: San Jose, CA
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Re: Computerized Ballistic Solutions

Brent, thanks for your thoughts.

With regard to the G1 function: I agree, it doesn't work well with small caliber bullets. However, the scaling factor associated with the G1 function is universal. All bullet makers publish coefficients using the G1 function as the baseline. If you want to use other functions, that's fine, but you will also have to derive appropriate scaling factors as well. So, realistically the approach that makes the most sense (to me at least) is to develop a drag function that uses G1 scaling factors.

For example, here is a graph showing the G1 (Meyevski-Ingalls) function plot for a .30 caliber, 150 gr. flat-based bullet. The drag function above the G1 is based upon a different function (derived by Prof. Arthur Pejsa) but uses G1 ballistic coefficients to scale the function. Note how this function tracks the actual data points extremely well.



[ 10-24-2003: Message edited by: Blaine Fields ]
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  #9  
Old 10-25-2003, 02:50 PM
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Join Date: Aug 2003
Location: McKinney TX
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Re: Computerized Ballistic Solutions

Blaine,


Didn't mean to come across wrong. Over the last few years I have really gotten into understanding ballistics. One of my biggest pet peeves is all these ballistic programs that use the G1 drag model. Sorry, but they are only accurate out to 350 to 500 yards. If you're really into shooting long distance, beyond 500 yards you might as well do it right and use the right drag model.

I don't care about how they try to massage the G1 drag model. Bottom line any bullet of the same weight but different design ie: spire point flat base, boat tail or VLD is not going to fly the same beyond a given range.

Plugging in the proper drag model can make all the difference in the world. Designing a program with only one drag model is short sighted.

Anyway, sorry if I bent your tail feathers. Have a great weekend.
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  #10  
Old 10-27-2003, 03:33 PM
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Location: San Jose, CA
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Re: Computerized Ballistic Solutions

Jeff,

There is no single drag function that will describe a bullet's velocity from muzzle to zero fps. If you look at the graph above you will see essentially four distinct zones. Supersonic (curvalinear, above 1400 fps); transonic (flat and linear, 1200 to 1400 fps); subsonic (linear, 1200 - 900 fps), and the zero to 900 fps region which is basically flat and linear. Because three of the zones are essentially linear, describing them is not difficult. It is the curvalinear portion at 1400 fps and above that is the problem.

But it turns out that the major practical differences between spire-point, flat based bullets and VLD types has more to do with the rate of deceleration between the two as opposed to the underlying mathematical drag function. So, as Pejsa has shown, it is possible to use a G1 scaling factor together with a uniform drag function as long as the user can also vary the rate of deceleration from bullet type to bullet type. In other words, the shooter must be able to correlate a bullet with a deceleration constant.

I'll tell you what: take your program and input: atm. pressure @ 29.85, temp @ 75, humidity @ 78%, 190 gr. .30 cal. bullet (Berger VLD) with a BC (G1) of 0.583 and muzzle velocity of 2900 fps. My program forcasts an elevation correction of 29.5 MOA for a 1000 yd shot. It is right on in real life. I know because I have shot that bullet under these precise conditions and I am dead-spanking-on.

What does your program forecast using the G1 or G7? If it ain't 29.5 MOA, it ain't right. The Horus program I have (the Atrag2P, ver 2.40) predicts 27.6 MOA. It is not even close to real world.

[ 10-27-2003: Message edited by: Blaine Fields ]
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  #11  
Old 10-27-2003, 05:37 PM
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Join Date: Jun 2001
Location: Palmer, Alaska
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Re: Computerized Ballistic Solutions

Blain,

What is the scope mounting height and altitude you're using, and how have you determined the BC .583 is actually correct out of your gun? According to every other ballistic program out there besides yours, your BC or MV, or both is actually lower than those numbers.

What is the bullet dropping at 400-500-600-700 yards too? Drop at 1000 yards really tells nothing about the curve by itself, and if the scope clicks are not calibrated exactly for MOA the total MOA dialed to zero out there won't be accurate either...

With a 1.5" scope height I have 27.12 MOA with the RSI SL G1 BC using Bergers published .583 BC. Exbal Palm predicts 27.0 MOA, desktop version 27.1 MOA, Atrag1p predicts 27.2 MOA. Oehler BEX predicts 27.1 MOA, LFAD predicts 27.38 MOA...

I wonder why your program predicts such a different number than "all" the others?
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  #12  
Old 10-27-2003, 08:54 PM
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Re: Computerized Ballistic Solutions

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> What is the scope mounting height and altitude you're using <HR></BLOCKQUOTE>
Scope height: 2". Altitude: I ignore the altitude because I have the actual field atmospheric pressure. My program will use altitude only as a means to approximate the actual pressure.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> What is the bullet dropping at 400-500-600-700 yards too <HR></BLOCKQUOTE>



The results from my program from 200 yds to 1000 yds are correct. The first time I used the program's output was in a tactical match which involves shooting from 200, 300, 500, 600, 800, 900 and 1000 yds. At the completion of the match (36 total rounds), the total vertical dispersion of the 36 rounds was 10 inches - and at each station I used exactly what the program output. This is very satisfactory performance as far as I am concerned.


<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> I wonder why your program predicts such a different number than "all" the others? <HR></BLOCKQUOTE>
I wouldn't know, except that I don't believe that any of the programs you mentioned use Arthur Pejsa's work. I can say that generally the use of a drag function similar to Meyevski-Ingalls (essentially the G1 model) will yield more optimistic results (as shown on the above graph) than what a shooter will see on the range. There are a number of ways to deal with this, such as calculating differing BCs for different speed regimes. This is how Sierra deals with the problem. Pejsa has dealt with the problem by using a deceleration constant that can be found for each bullet.
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