# How do I calculate the ballistic coefficient?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Troutslayer, May 13, 2005.

1. ### TroutslayerWell-Known Member

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So I checked out some of the programs that can calculate bullet performance, and they all require the BC be entered. Is this a number that I can reach through math using the energy in foot pounds, possibly divided by some magic number?? I'm trying to figure this out for Winchester ammo and they don't publish that information on their web site or in the catalog. Is it there and I don't know what I'm looking for?

2. ### sniper2Well-Known Member

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What is the cartridge and what grain bullet?

3. ### sniper2Well-Known Member

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go to beartooth bullets web site they have a BC calculator

4. ### TroutslayerWell-Known Member

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The cartridge I'm trying to figure out now is Winchester .300 mag. 180 Grain accubond. I would really like to know how to get the number, not just have one of you tell me the answer.
Thanks

5. ### BuffalobobWriters Guild

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The Accubond is a Nosler bullet.

http://www.nosler.com/chartaccubond.html

For factory ammo you usually have to decode it down to the real manufacturer of the bullet to get the BC

6. ### BrentWell-Known Member

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[ QUOTE ]
Is this a number that I can reach through math using the energy in foot pounds, possibly divided by some magic number??

[/ QUOTE ]

No.

[ QUOTE ]
I'm trying to figure this out for Winchester ammo and they don't publish that information on their web site or in the catalog. Is it there and I don't know what I'm looking for?

[/ QUOTE ]

It's a Nosler bullet and they publish the BC.
The published BC for the 200gr Accubond is .589 and it was close to that when I tested it with 2 chrono's at .585.

I've not tested the 180 AB but I'd guess that the published BC for it is also pretty close too.

If you want to figure true BC's for yourself you'll need an accurate chronograph, period. Trajectory is determined by MV and BC.

You will need to record MV and then downrange velocity then match these up in a ballistic program output by modifying the BC until velocities match... this is your true BC. You must also factor in the atmospheric pressure at the time of testing for it to be accurate though.

RSI Shooting Lab is the only program that will calculate a theoretical BC from a bullet's form (dimensions). It works very well for a starting point and is only sveral points off on any bullets that I've checked.

7. ### HoytemanPAWell-Known Member

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Kind of the same as Brent said and I agree that you need very accurate equipment to come up with "true" BC but....

For practical purposes even if you do not have a really accurate chronograph you can get very repeatable results by shooting over whatever you have and back calculating the results based upon actual bullet drop. I use Exbal for this purpose. I get a hundred yard zero and then shoot it out at 500 yards and adjust the scope until centered. Then I adjust the BC figure until the program calculates the scope correction to equal the actual correction needed using the average velocity the cheapo Chrony displays. Then I test at 3,5,8 and 1000 and I have very repeatable drops based upon these values in three rifles. If I change a bullet, I have to do it again. Likewise if I were to change the Chrony, I believe I would have to readjust.

In my case I use .550 for the Nosler 200gr Accubond and I have found a value of .600 to work well for a 220 SMK.

This probably means that the chrony is giving readings that are higher than "true" or that the Bullet BC changes based upon velocity in flight. I do not know, I just know that it works. So to me the only reason to need a very accurate chronograph to calculate BC would be by a bullet manufacture so the published data was as correct as it could be for consumer comparison. But for the average Joe, poking holes in paper while shooting over a \$60 Chrony works quite well provided you shoot enough at the extended ranges to be confident in "your system".

PS: Don't use the BC values that I have listed as I feel the adjusted BC is proportional to the accuracy of speed of the chronograph that you are using. That said, I use the manufactures listed BC as a starting point and go from there.