JBM,
I'm afraid I don't quite understand. Can you elaborate? I don't know if it is apples and oranges because the Oehler model 43 acoustic chronograph comes with software to calculate bc's clear out to 1000 yards. It seems bc's can be calculated at any range even though Sierra does their's at 300 meters and JBM's website calculates at 200 yards. I remember reading an article in a Precision Shooter magazine some years ago about a group of guys who tested many of the popular vld's at 1000 yards with the help of 2 Oehler model 43's. (wouldn't that be nice!). It showed that the majority of the tested bullets actually IMPROVED the farther downrange they went. It intrigued Dr. Oehler so much that he even flew out to the range to get in on the experiment himself firsthand! So that led me to conclude that it might help to chrono at 500 yards instead of 200 so I would get a little more accurate read. I would have tried it at 1000 if I had long enough cables! Was this thought wrong in your book? Also, are you "THE" JBM of JBM Ballistics?

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308WIN,
I would love to accomodate you on your request but unless I have your gun with your lot of bullets and your geographical conditions, my own info wouldn't do you any more good than that of the manufacturer's published bc's. One really has to calculte his own bc to get his own bc. And even at that, bc will change from day to day even if all the other things remain constant. If the wind blows from the rear one day, and then from the quartering left the next, the bc's will be slightly different! Kind of sucks, but that's the way it is. I could tell you though that when it is all said and done, the more I learn about this stuff, the less I think I know!

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<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I'm afraid I don't quite understand. Can you elaborate? I don't know if it is apples and oranges because the Oehler model 43 acoustic chronograph comes with software to calculate bc's clear out to 1000 yards.
<HR></BLOCKQUOTE>

I'm sure it can. I was just commenting that if you calculate a BC with velocities measured at the muzzle and 500 yards, you get an average (weighted) BC for that velocity range. This may not be the velocity range that the manufacturers use so the BC may be different. That's not to say that either are incorrect, just different. -- it's hard to know what the manufacturer did.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>It seems bc's can be calculated at any range even though Sierra does their's at 300 meters and JBM's website calculates at 200 yards. <HR></BLOCKQUOTE>

That's up to 200 yards. If I were doing it (and I will be when I build my chronograph for measuring BCs) I would get multiple BC measurements at different velocities instead of getting an average over long velocities. Actually, I want to measure CD anyway... I don't really care about BCs.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I remember reading an article in a Precision Shooter magazine some years ago about a group of guys who tested many of the popular vld's at 1000 yards with the help of 2 Oehler model 43's. (wouldn't that be nice!). It showed that the majority of the tested bullets actually IMPROVED the farther downrange they went.<HR></BLOCKQUOTE>

That wouldn't surprise me. The pitching and yawing should dampen as the bullets go down range leading to a lower effective CD (and higher BC). Typically the CD is CD0 + d^2*CDd2 where d is sin^2(a) and a is the angle of attack. So as a lessens, CD should go down and BC up. Did they say how much it changed?

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> It intrigued Dr. Oehler so much that he even flew out to the range to get in on the experiment himself firsthand! So that led me to conclude that it might help to chrono at 500 yards instead of 200 so I would get a little more accurate read.<HR></BLOCKQUOTE>

That's fine, but if you only measure one velocity at 500 yards, and one at the muzzle, your measure BC is an average of everything that happend from the muzzle to 500 yards. I'd be more interested in the numbers if you measured 2 velocities at 500 yards and calculated the BC there.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Also, are you "THE" JBM of JBM Ballistics?<HR></BLOCKQUOTE>

JBM& Grouper,I think the tests your both refering to is the one conducted by Larry Bartholome with Dr Ohler and others helping out ..About 2 years? ago Larry sent me a copy of those tests that varied from the PS mag reports but very close..Reading these it is not a given that the BC reported will increase as the distance expanded, they checked the various rifles first at muzzle to 120 yds(because thats where the screen had to be placed) and then muzzle to 1000yds..Not definitive but interesting!Still have that list on email,could send if your interested..JR..Jeff Rogers

JBM,
Thanks for your post. It was very informative. One thing I didn't catch though was the "CD" you mentioned. What is that, and how do you calculate it? Also, since you obviously know a lot about bc and so forth, I was wondering if you could maybe tell me the formula for figuring bc. I called Art Pejsa and asked him for the formula, but he was such an arrogant bugger I couldn't stand being on the phone long enough with him to get a straight answer. Thanks again.

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