new twist on b.c.'s

goodgrouper

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new twist on b.c.\'s

I RECENTLY HAD A CONVERSATION WITH DAVE BROWN AT SIERRA ABOUT BC'S. DAVE IS THE GUY WHO THE OTHER TECHS REFER YOU TO WHEN THEY HAVE BEEN STUMPED BECAUSE HE WORKS FOR NASA MAINLY AND HE KNOWS BC LIKE NOBODY ELSE. HE SAID THAT SIERRA TESTS ALL THEIR BC'S AT ABOUT 400 FEET ABOVE SEA LEVEL AND HE SUSPECTS THAT MOST BULLETS FIRED OUT OF NORMAL BARRELS GAIN SOME SMALL BC WHEN FIRED AT HIGHER ELEVATIONS. NATURALLY HE WAS SURPRISED WHEN I TOLD HIM I TESTED THE BERGER 140/6.5MM AT 500 YARDS AT AN ELEVATION OF 5000 FEET AND FOUND A NEW BC OF .704!! THIS BULLET IS RATED BY BERGER AT .627. I WOULD LIKE TO DO THE SAME TEST AT 10,000 FEET AND SEE WHAT SURPRISES I GET THEN! I WILL LET YOU KNOW!
 

milanuk

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Re: new twist on b.c.\'s

might want to ixnay the capslocks. In general, on the 'Net talking in all caps is considered the equivalent of SHOUTING and generally somewhat rude.

Just a helpful hint.

Interesting info on the B.C.'s. Let us know what you find.

Thanks,

Monte
 

Dave King

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Re: new twist on b.c.\'s

When you are doing the calculations for the BC at these different altitudes are you adjusting for elevation, temp and baro pressure or still using the sea level, 59 degrees, etc. Standard Temperature and Pressure (STP)??
 

goodgrouper

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Re: new twist on b.c.\'s

Dave King
Yes Dave, all I did was chrono the bullets at the muzzle and then at 500 yards and then I called Dave Brown at Sierra to get the formula for calculating bc. He told me that the formula encorporates equational calculus and it was much easier to let the software he uses to calculate everything. So I told him the humidity, temp, elevation, barometric pressure, and wind conditions and he told me the bc over the phone. Another thing he told me which I did not know is that the distance between both chronos has to be measured exactly from center to center not just from the last skyscreen on the first chrono to the first skyscreen on the last chrono!
Just a side note: I actually chronoed several bullets that day out of some of my other wildcats. I did a 75 gr. a-max out of a 22-250 improved, a 55 gr. blitzking out of a different 22-250 improved with a slow twist, a 40 gr. v-max from a .223-4000, and a 180 scirroco from a .300 win mag. The results were .479 for the amax, .299 for the blitzking, .239 for the vmax, and .538 for the scirocco. All bc's were higher than the manufacturer's calculations!

[ 09-15-2004: Message edited by: goodgrouper ]
 

JBM

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Re: new twist on b.c.\'s

I would be careful comparing BCs in this way. I know Sierra doesn't calculate their BCs using velocities 500 yards apart. This will work, but you're calculating an average BC over the 500 yards. If one were to calculate a BC at near the velocity at 500 yards it would probably be lower/higher/different than you're average. Not that it's wrong, but you might be comparing apples and oranges.
 

goodgrouper

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Re: new twist on b.c.\'s

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?
 

goodgrouper

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Re: new twist on b.c.\'s

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!
 

JBM

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Re: new twist on b.c.\'s

<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>

Yes.
 

a.JR

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Re: new twist on b.c.\'s

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
 

goodgrouper

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Re: new twist on b.c.\'s

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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