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new twist on b.c.'s

 
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  #1  
Old 09-13-2004, 10:26 PM
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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!
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Old 09-13-2004, 11:02 PM
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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
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Old 09-14-2004, 08:29 PM
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Re: new twist on b.c.\'s

Thanks for the tip Monte. The caps are off.
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Old 09-15-2004, 03:35 AM
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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)??
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Old 09-15-2004, 07:21 AM
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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 ]
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Old 09-15-2004, 07:42 AM
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Re: new twist on b.c.\'s

hey goodgrouper,
if you get the chance to test the 178 Amax from 1-10 twist @ about 2680 fps. Let me know what BC you come up with. [img]images/icons/wink.gif[/img]
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Old 09-15-2004, 08:06 AM
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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.
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