# Ballistic Coefficient Calculation Question

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Lightvarmint, Apr 9, 2008.

1. ### LightvarmintGuest

During the last week, I have been testing some prototype .338 265gr hunting bullets. I am down here at virtual sea level (92' elevation) and our calculated BC using velocity over our Oehler 35P at 600 feet and wound up at a very solid .770......... Similar weighted and shaped bullets indicated .776 with the Oehler at 1204 feet downrange.

My question is what would the BC be at 2500, 5000, 7500 and 10000 feet using the same initial velocity and bullet? Or, a better way to ask the question is:

What would be the calcualted BC at the above altitudes. Thanks in advance.

James

Last edited by a moderator: Apr 9, 2008
2. ### P KUNDAWell-Known Member

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Calculated BC and a real BC are two totally different things

Every bullet has different BC at different velocity. (look at Sierra website) I would even go as far as saying, that any bullet fired in a different barrel/twist has its own BC. that will differ from a published data and will be changing differently with downrange velocity.

Peter

3. ### goodgrouperWell-Known Member

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Every individual bullet has a different bc and can only really be accurately calculated for the particular rifle they are to be shot in at the actual elevation and distance that day but ballparking the number is possible. I can tell you that the 300 grain SMK (.338) has a sea level rating of .768 and when I tested it at 11,000 feet from a fairly worn barrel and a crosswind that it raised to .810 at just over 2000 yards.

4. ### LightvarmintGuest

Thanks for the answer and I have spent many hours on the Sierra website over the years, but this is beyond what they offer especially since it is not one of their bullets.

You are correct about all of the factors (temp, altitude, speed, twist rate, bullet engraving depth etc) affecting the BC . But maybe you misunderstood the question. I was just wondering what the ballpark average BC would be (from the muzzle to say 1000 yards) with a 3100 fps initial velocity in the gun I tested it in when I got the .770 assuming I changed altitudes from 92 feet to the ones I had listed above and basically kept the same initial velocity + or - a few fps.

I thought you all may have a function that did it in one of those pocket brains that have the different ballistic programs loaded.

James

5. ### edgeWell-Known Member

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On a ROUGH basis:

Assuming 265 grains @ 3100 fps at Sea Level and a BC of 0.770

2500 = BC = 0.829
5000 = BC = 0.8938
7500 = BC = 0.9632
10000 = BC = 1.038

This assumes that the pressure, temperature and everything else is the same! This is HIGHLY unlikely!

That is the reason the you need to shoot for yourself under known conditions.

edge.

6. ### LightvarmintGuest

Edge,

Thanks for the calculation...... Now, if I can get the bullet manufacturer to bump the BC up a little from .770 at sea level......

Again, thanks.

James

7. ### P KUNDAWell-Known Member

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James I misunderstud the question completely