Ballistic Coefficient Questions

mmattockx

mmattockx

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I have done some reading and have a grasp of how the BC came to be, as well as what the reference projectiles look like and how they were developed. My question is why do very streamlined target bullets have a BC less than 1, when the G1 reference is simply a semi-streamlined flat base projectile? Is it the sectional density that does it, or is it a scale effect with the Reynolds number being much better for the 1" diameter reference projectile?

Thanks,
Mark
 
For long range projectiles use the G7 reference.
At longer ranges the G1 will not be as accurate at predicting drop charts.
Litz's book - Applied Ballistics for Long-Range Shooting , has a wealth of information on B.C.
 
mjohn7 said:
For long range projectiles use the G7 reference.
At longer ranges the G1 will not be as accurate at predicting drop charts.
Litz's book - Applied Ballistics for Long-Range Shooting , has a wealth of information on B.C.
Click to expand...

I understand that, but the bullet makers use the G1 for all their info. My question is why the smaller projectiles show a BC well below 1.000 when they are a more efficient shape than the reference. You need to get up to a highly streamlined .50BMG match bullet to find a BC over 1.000. To me, that indicates that scale effects or the high SD of the reference are trumping the less aerodynamic shape.

Mark
 
Mark,
You were on to it with the sectional density.
The 'standard projectile' has a mass of 1 pound and a diameter of 1 inch, giving it a sectional density of 1.000, and a BC of 1.000 in reference to itself.

Other more normal sized projectiles have much lower sectional densities, like ~.3. Even if the G1 form factor is .5 (meaning the bullet has 1/2 the drag as the G1 standard), the G1 BC will only be .3/.5 = .600 which is much less than 1.0.

If you had a bullet that was 1 inch in diameter and weighed 1 pound with a 'sleek' profile and a G1 form factor of .5, the G1 BC would be 1.0/.5 = 2.000.

Good question.
-Bryan
 
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