Thread: 7mm 180 ELDMs BC

Seen a few different numbers quoted for the G1 BCs . Hornady website states 0.796 and also 0.777. Very impressive but a tad confusing. Have seen some other numbers quoted for 1:8 vs 1:9 twist.
Any of you chaps got real world data for 1:8 twist ?
Intended use long range hunting out to 1000 yrs.

My Applied Ballistics shows G1 as .703. I am not sure about the bc changing with the twist rate.

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

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 ?

The berger website has some info and calculator on the spin rate vs BC.

If you use a G7 BC it wont be so dependent on velocity. If you want a in more depth read A Better Ballistic Coefficient | By Bryan Litz | Berger Bullets Blog Twist Rate Stability Calculator | Berger Bullets

I was hoping some one might have a simplified explanation

Originally Posted by chainsaw

I was hoping some one might have a simplified explanation

On the shooter app it lets you put multiple different bc figures in at various velocities, often Sierra quote multiple G1 bc figures too. That would be job done

faster spinning projectile should have higher bc due to being more stable.

Any one got real world validation of G1 or G7 bc’s?

Originally Posted by chainsaw

Any one got real world validation of G1 or G7 bc’s?

The simple solution is get the applied ballistics app and then buy the custom drag curve for that bullet that does all that for you.
The best ballistic program by far

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I just use the middle number. Get some crony data. Then get some drop data at intended ranges and true bc to match. It's always been pretty damn close. I don't shoot further than 600 though

The grooves cut by the rifling would differ slightly between 1/8 and 1/9. As would 3, 4 or 5 land rifling patterns. Cant beat the real world results