How do I calculate ballistic coefficient?

[bobinpa]

If a bullet maker advertises a BC of .xxx at 2850 FPS, how do I determine or at least be very close to the BC at 2450 FPS?

I just want a starting point.
Thanks in advance.

 

[fx77]

http://gundata.org/ballistic-coefficient-calculator/

https://www.sellier-bellot.cz/en/products/ballistic-coefficient-calculation/

 

[El Matador]

I'd probably just take off 2-3% and call it good. BC will also vary based on your altitude and twist rate, so verifying your drops at long range is a fairly standard practice. Scope adjustments can also be off by a few percent. Unless you're shooting at insane distances you can just make small changes to your velocity, BC, and/or "elevation correction factor" (available on some calculators to account for possible error within the scope). Fiddle with it until the drop chart matches your actual shooting results.

 

[bobinpa]

I'd probably just take off 2-3% and call it good. BC will also vary based on your altitude and twist rate, so verifying your drops at long range is a fairly standard practice. Scope adjustments can also be off by a few percent. Unless you're shooting at insane distances you can just make small changes to your velocity, BC, and/or "elevation correction factor" (available on some calculators to account for possible error within the scope). Fiddle with it until the drop chart matches your actual shooting results.

Thanks. That's basically what I planned on doing, I was just hoping to get a pretty close chart to start with.

 

[El Matador]

I believe Hornady and Sierra both give a few different BCs as velocity changes. Looking at those numbers should give you a good approximation of how much to change the BC for your application. According to Berger Bullets you'll get far less variation in your BC if you use the drag model that most closely matches your particular bullet, which would be the G7 for most boat tail, spire point, or tipped rifle bullets.

 

[338 dude]

If a bullet maker advertises a BC of .xxx at 2850 FPS, how do I determine or at least be very close to the BC at 2450 FPS?

I just want a starting point.
Thanks in advance.