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# G1, G5, G7 curves. BC's, Drop, curves and putting it all together

#8
02-24-2004, 10:55 AM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Please explain how you got your corrected BC from G1 to G5 (EX..495bc @ G1 and .320bc @ G5)
Is there a formula?
<HR></BLOCKQUOTE>

Yes there is. I'm going from memory here, so take it with a grain of salt. You have to G5/BC5 = G1/BC1 so that BC5 = BC1*G5/G1 where G5 and G1 are the drag models calculated at the same mach number. This gives you a G5 BC at the mach number.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>
Aslo MV? &gt;meaning Mach Velocity is that velocity divided by 1126? (Vel./1126)
Thanks for the help.
<HR></BLOCKQUOTE>

I thought that MV was muzzle velocity. Brent?
__________________
JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm
#9
02-24-2004, 01:13 PM
 Platinum Member Join Date: Jun 2001 Location: Palmer, Alaska Posts: 2,544
Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

JBM,
Could you run an example with that formula, I can't make heads or tails of it for some reason.

Convert a .680 G1 BC to a G7 BC if you will.

308win,
I've got two programs that will convert the BC's, RSI Shooting Lab is the better one http://www.shootingsoftware.com/ Oehler Ballistic Explorer is another.

MV is muzzle velocity the way I've know it. Max, where the hell are you? Come answer this one! [img]images/icons/smile.gif[/img] Mach number abrev... is ???

Mike,
Blaine Fields has a program called "Precision Shooter's Workbench" that uses a "deceleration constant", or DK. It allows you to use the published BC and your MV, you just change the DK number to modify the trajectory curve independantly until it matches your drops. It works very well. http://www.precisionworkbench.com/2.htm

The Exbal Palm program has a sight in condition page, and a field condition page to deal with altitude, density altitude, temp or MV changes. There's another way to skin this cat though. Compare the bore line drop at your conditions before you leave (on a ballistic program), verses the altitude at your hunting location to see how far your 100 yard zero will shift.
__________________
Brent
#10
02-24-2004, 04:54 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,840
Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

Yeah I think the constant they are talking about equates to Pejsa's retard coefficient rate. Will mess with it.
#11
02-24-2004, 05:39 PM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

To convert a G1 BC of .680 to a G7 BC, find the values of the G functions. G1 is 0.359 and G7 is 0.169. You didn't specify a mach number, so I used 3.0. Then BC7 = 0.680*0.169/0.359 = 0.320 (at mach 3.0).

Note that I'm using G function values calculated from CD vs. Mach number tables provided by Bob McCoy at ARL. They don't agree completely with the values published in Hatcher's notebook, etc. I'm also using an ICAO atmospheric density instead of the older Army Standard Metro value.

The idea here is that the CD is the same no matter what drag function you use (it's the same bullet). That's all the equation G1/BC1 = G7/BC7 is -- a statement that the CDs are the same (for the same mach number and density). Then you just solve for G7.

[ 02-24-2004: Message edited by: JBM ]
__________________
JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm

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