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Old January 16, 2005, 10:30 PM   #1
Kyote
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Balistic coeffecient

What is the formula to determine the B.C. of a bullet? This is one of those things I want to KNOW not guess at. Why? 'Cause!

Thanks guys!
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Old January 16, 2005, 10:35 PM   #2
Cowled_Wolfe
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I don't know the formula... But as I recall, it's not simple...

IIRC, even when they finally had computers strong enough to do the math, aerospace companies still opted to use wind tunnels to find out BCs simply because it was more accurate.

If you had a 100% controlled test area, a few chronos, a buncha free time, and a good mind for math, you could probably determine a BC by experimentation and computation.
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Old January 16, 2005, 10:40 PM   #3
drinks
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Bc

K;
The BC is not even a fixed number, go to Lyman's Cast Bullet Handbook, you will find that the BC depends on initial velocity as well as the shape of the bullet.
I just take a not more than 2 digit number and use it for rough guesses, nothing more.
Don
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Old January 16, 2005, 10:50 PM   #4
Kyote
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I knew that vel. played a part in this and that the B.C. changed with vel. and bullet construction. I was thinking if I could get the known vel. and sectional density the B.C. would just fall out of the sky.

It's late and I don't have anything else to do and I'm dreaming of a prarie dog town!!

Thanks
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Old January 16, 2005, 11:04 PM   #5
Jim Watson
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There are programs in ballistic software and on the net that will figure BC if you plug in the velocity at two different ranges, along with caliber and weight. For best results, you need two chronographs, but I did it once by chronographing ammo out of the same box at the usual 10 feet and again just in front of the target stand at 97 yards.

There is software that will compute BC from bullet dimensions but you have to have very good numbers for the actual shape and contour of the bullet.

BC appears to vary with velocity because the BC tables were figured for military bullets and artillery shells, not sporting rifle bullets. If you paid White Sands to shoot your ammo down the doppler radar range and do the computer time you could have a consistent BC for your brand of bullet.

I got some very peculiar results Friday, either the Sierra BC values are way optimistic or my scope is not giving the claimed adjustment values. Oh, well, I guess I'll just have to do some more shooting. Tough work, but somebody has to do it.
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Old January 16, 2005, 11:04 PM   #6
drinks
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Bc

K;
Lyman gives 3 measured, not calculated , BC's for each bullet, depending on initial velocity.

The BC is just more voodoo stuff for mag writers to use to try to keep from working for a living.
Don
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Old January 17, 2005, 04:44 PM   #7
brickeyee
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It is the ratio of sectional density to coefficient of form. Coefficient of form is a method of expressing how the bullets shape compares to the 'standard shape' used to create a ballistic table. Buy the book by Pejsa if you want a detailed explanation. He has a better model of drag and uses much better equations than the old ingall's tables.
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Old January 17, 2005, 06:04 PM   #8
k in AR
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Kyote,

this isn't the forumla... but if you know the bullet weight, the muzzel velocity, and the 100 yard velocity... "Point Blank" software has a calculator that will figure it. BTW, Point Blank is a free download from WWW.Huntingnut.com that really works and does most all your ballistics calculations. I have been using it for years and it really works.

* Just checked, Huntingnut is moving to a new server and is off line right now. Hopefully he will be back up and running soon.
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Old January 21, 2005, 01:56 AM   #9
Nnobby45
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You'll be ahead of most folks if you understand that sectional density represents a bullet's length in relation to it's diameter, and that when you figure in the shape of the nose (Ogive), you get the coefficent. A spitzer type bullet can't make a long range performer out a bullet with poor SD, and a great Sectional Density can't make a long range performer out of a round nose. Combine good SD with good ogive, and presto---good BC. Wish I could tell what formula would be used to figure in the shape of the bullet's nose. Anybody know Calculus, Trig, or Advanced Mathematics? Did I mention that the BC changes with the bullet's velocity? Better add Rocket Science.
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Old January 21, 2005, 06:33 AM   #10
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the end result is that if you have a bullet that has a B.C. between .350 to .450 it is doubful that you can tell the difference in any hunting situation. in a target situation it doesn't matter as you just add in some more elevation. just look at the bullet company charts and you can see that how much it really takes to see the difference. the numbers that they show are just a guide anyway. if you want to really know what the B.C. is set up a chronograph at different ranges and shoot a lot of ammo. even then it isn't going to prove that much
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Old January 21, 2005, 11:08 AM   #11
Jim Watson
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A 550 grain roundnose .45-100 works pretty well at long range. Takes a little more elevation on the sights than a .30 spitzer, of course.

I shot some at 100 yards the other day and fooled with the scope. Turns out the adustment values are about half what they are marked, and the adjustment range of the scope is not enough to go from 100 to 600 yards at max. A clear but cheap scope. I will have to get something better.
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Old January 21, 2005, 09:24 PM   #12
Robert M Boren Sr
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Check out Sierras relosding manual. Sierra is the only one that I know of that has come close to doing it right. BC changes with velocity and they list different BC for the same bullet at different velocities spreads. They also give the formula if you want it.
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