Question on BC

[shawnba67]

I got to looking at these bullets and formed a question I can't solve.
All are Speer 200 grain Hot Cor bullets
.308 has a bc of .556 sectional density .301
.323 has a bc of .411 SD .274(ok larger frontal area more wind resistance ect)
.338 has a b. of .448 SD .25

how on earth can the 338 have a higher bc ?

 

[sloughfoot]

because.

 

[CMB69]

SD has less to do with BC than form factor and velocity/time of flight. If a high SD projectile is a flat ended cylinder, it is not very aerodynamic and will have a low BC. Create a very aerodynamic bullet with the same SD and it will have less velocity loss/shorter TOF, so will have a better BC. That .338 bullet is more streamlined, has better aerodynamics.

 

[eatsnopaste]

gtfo!

 

[shawnba67]

gtfo!

I have no idea what that is or means.
How can a shorter fatter bullet be more streamlined?

 

[CMB69]

Maybe one of those values is a misprint/error?

If one accepts the values as correct, then the higher BC is [STRIKE]implying[/STRIKE] stating that bullet loses velocity at a lower rate and in fact is more streamlined/aerodynamic.


I know that Sierra, while they use calculated BC when designing a bullet, actually shoot and measure velocity loss and use that info as the final determination of BC. I assume other companies do the same, but maybe not.


Also, BC is not static, and is highly influenced by velocity. An explanation that should be easy to understand would be to take a bullet that does not have a particularly high BC and start it out at 4,000 fps. The rate of velocity loss during the 4,000 - 3,000 fps phase of flight is greater than during the 3,000 - 2,000 fps phase, and so on. Well, the same is true no matter what the stated BC of the bullet is. Sierra provides multiple BC's for each of their bullets depending on the velocity range it will be used at.

 

[Broom_jm]

Shape and location of the ogive on the .338" bullet is different, resulting in a higher BC. Also, which BC are they refering to, and at what velocity?

The numbers aren't that far apart and would present very little difference in ballistic performance.