Thread: 7mm 180 ELDMs BC

Originally Posted by GWH

Dont hornady quote 3 different bc's at different mach ratings.

Yep and hence my question/confusion. Hornady website updated info says
Expanded BC Values for ELD Match Bullets
Mach 2.25 (2512 fps* and above) - Velocities above 2,500 fps and distances out to 300 yards.
Mach 2.0 (2232 fps*) - Mid-range shooting where the bullet spends time in flight after slowing down.
Mach 1.75 (1953 fps*) - Long-range shooting where much of the bullet flight occurs after velocity loss.

7mm 180 gr. ELD Match 1 in 8.75" Twist

> 2500 fps 0.777 G1 0.391 G7
2230 - 2500 fps 0.748 G1 0.376 G7
1950 - 2230 fps 0.731 G1 0.368 G7

7mm 180 gr. ELD Match 1 in 7.5" Twist
> 2500 fps 0.816 G1 0.411 G7
2230 - 2500 fps 0.812 G1 0.408 G7
1950 - 2230 fps 0.782 G1 0.394 G7

So if i have a 1:8 or 1:8.5 twist bbl and want to use a simple ballistic calculator with one BC (G1) input how do you derive that with some degree of confidence ?
It seems like you need to determine an intended max range, then work backwards to find a weighted average BC ??? or does that just introduce more error ?

On another level/topic - why does BC drop off with speed ? and twist ?
I'm guessing faster twist = higher spin = more stable (less wobble or yaw). (up to some limit based on projectile design/construction)
But how does slower speed impact BC. Does spin rate also drop along with speed ? = more wobble/yaw ?