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# BC calc.'s

#1
03-25-2004, 12:58 AM
 Platinum Member Join Date: Feb 2003 Location: Pueblo, CO Posts: 1,244
BC calc.\'s

Are there any ballistic programs that can calculate BC from downrange drop figures (maybe comparing actual trajectory to theoretical), and/or is this even possible??
#2
03-25-2004, 01:50 AM
 Platinum Member Join Date: Jun 2001 Location: Palmer, Alaska Posts: 2,539
Re: BC calc.\'s

I do have one that will let you enter the drop, but you also enter the BC and it gives you the MV...

Just enter your known MV and BC into a program, and look at the drop in inches to compare to your actual drop. If its predicting flatter trajectory than you're seeing on the target, lower the BC number incrementally until they match. Go the other way if yours shoots flatter than the program predicts. That's all I know that can be done. I have some programs that will give you the BC from velocities at two ranges, and this works the best.
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Brent Moffitt
#3
03-25-2004, 06:28 AM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: BC calc.\'s

I get asked this all the time so I put my opinion in my FAQ:

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>(Q) Why can't I calculate BC from bullet drop at different ranges?

(A) You can, but not very accurately. If you look at the bullet drop for two bullets, one with a BC of 0.5 and the other with a BC of 0.55, you'll find that the bullet drops at 300 yards with a muzzle velocity of 3000 f/s and a 100 yard zero, are -10.9" and -10.7" respectively. So if you can shoot groups less than 0.2" at 300 yards, have at it. Of course you'll still only know the BC within about 10% (the BC calculation CGI can be as accurate as 2%!).
<HR></BLOCKQUOTE>
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JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm
#4
03-25-2004, 07:19 AM
 PUBLISHER Join Date: May 2001 Posts: 6,518
Re: BC calc.\'s

Good topic.

I'd welcome a discussion of how accurate do we really need our BC calcs to be. And how does one get really accurate velocity measurement at 2 ranges. How much does this velocity accuracy limitation affect the BC calc compared to a BC calc done with drop method inherent inaccuracies.
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#5
03-25-2004, 03:03 PM
 Platinum Member Join Date: Jun 2001 Location: Palmer, Alaska Posts: 2,539
Re: BC calc.\'s

Len,
I haven't looked at the accuracy issue for a two range velocity derived BC. It is pretty easy to get if you have a rangefinder and a chrono though, two chronos makes it even easier, and a tad more accurate if they're calibrated together. I think it's more accurate than the Oehler 43's acoustic target because the distance is less critical, much less.

The most important thing for me is simply to use the one that matches my fired drops, I know what the MV, temp, alt and BP is, so BC is the only other variable that will modify the trajectory.

Once I establish this baseline as my sight in perameters, the program is used to modify them as the other variables change from day to day, air density being the MAJOR influence if you have a stable MV with temp insensitive powders.

After 800 yards you really notice the subtle MV or BC changes.
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Brent Moffitt
#6
03-25-2004, 04:28 PM
 Platinum Member Join Date: Feb 2003 Location: Pueblo, CO Posts: 1,244
Re: BC calc.\'s

You know guys-- i'm starting to get into the technical aspects of this more and more-- where can i go to read up on internal/external/terminal ballistics that's not overtechnical?
#7
03-26-2004, 10:27 AM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: BC calc.\'s

There is some error analysis applied to the Siacci methods in "Exterior Ballistics" by McShane, Kelly & Reno (see my bibliography page). It shows equations that give changes in position, time and velocity for small changes in BC, density, mach number, etc.

If anyone is interested, AbeBooks lists five copies of "Exterior Ballistics" for sale. It's definately an older book, but there is still a lot of good information in it.

[ 03-26-2004: Message edited by: JBM ]
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JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm

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