Explain On-the-Fly Ballistic Coefficient Changes

S

Swifty Morgan

New member
The other day, I was shooting with a pro doing my spotting. The ballistic coefficient on my ammo box was 0.462. After a few shots, he told me to change it to 0.420 in my ballistic calculator.

How did he know to do that?
 
There are 2 ways to do your truing.
Velocity, or BC.
Either can only be done while shooting longer distances.

If i'm correct in my thinking, you were hitting low before he told you to change the BC in your calculations, and redial.

Essentially telling the calculator the bullet doesn't fly as well as the advertized BC would have you think.
 
The muzzle velocity was determined using an accurate chronograph, and the spread was very low at 8.1 fps. Something else had to be wrong, so an error in the published BC would make sense.

I don't understand how he came up with 0.420, though. Maybe he was familiar with this ammo? It's very popular.
 
Here's a hint your kindergarten teacher never told you:

*Nothing* in ballistics is linear. Air flow is both laminar and turbulent and the ratio and values change with speed, attitude and more than a dozen other variables.

In general, BC drops as speed drops.

As such, a single number for a BC is likely a 'best case': Hornady says the G1 on their 750 gr AMax is 1.05 (G1), but Brian Litz measured it at .991

In general, and that's more like 'much of the time' a BC is going to be higher at much higher speeds. Until you start chasing X-15 rocket planes, then the tertiary and quaternary effects take over and ballistics math as we commonly use it doesn't work any more.

But kudo's to your instructor, you found an excellent one.
 
Last edited:
He told us a lot of stuff which goes against "expert" advice. He told us to ignore at least one thing they teach military snipers. He said the military has to get people trained fast, so corners get cut.
 
That's a very popular bullet and my guess is he has experience with it and what he found to be more accurate.

He also could have been throwing out an educated guess to help you get your drop table trued up with actual drop. It doesn't take that much time playing with variables on a ballistics calculator to get a feel for how a given cartridge and bullet will be affected by making changes.
 
Interesting--did you take shots at a different distance with the new BC entered to confirm if the projected trajectory was more accurate?
 
Hey Swifty, I see that in your location it states Florida. So I am thinking that since your so close to sea level that he was thinking that the air is denser due to higher barometric pressure and that it is why he lowered the BC on the app so that compensation added the extra clicks on the scope for elevation.
 
Last edited:
ms6852 said:
Hey Swifty, I see that in your location it states Florida. So I am thinking that since your so close to sea level that he was thinking that the air is denser due to higher barometric pressure and that it is why he lowered the BC on the app so that compensation added the extra clicks on the scope for elevation.
Click to expand...
Most good apps compensate for barometric pressure.
 
Here is your answer straight from Sierra:
"The gun can affect the measured BC value in two important ways: spin stabilization and tipoff moments. A bullet is gyroscopically stabilized by its spin, which is imparted by the rifling in the barrel. If a bullet is perfectly stabilized by its spin, then its longitudinal axis (which is also its spin axis) is almost perfectly aligned with its velocity vector. If a bullet is not perfectly stabilized (which is usually the case), the bullet may not be tumbling, but its point undergoes a precessional rotation as it flies. In previous editions of Sierra’s Reloading Manuals we have described this precessional rotation and have called it “coning” motion to aid in mental visualization of the motion. As the bullet flies, the point rotates in a circular arc around the direction of the velocity vector. Coning motion results in increased drag on the bullet, and any firing test method then yields an effective BC value for the bullet that is lower than the theoretical value. The rifling twist rate in the gun barrel and the muzzle velocity together control the spin rate of the bullet, and therefore control its degree of stability."
 
1. Advertising.
2. The bullet is not an exact match for the form factor used to figure BC.
3. When in doubt, print the higher number, see #1.
 
Right, see #2 above, why Sierra shows velocity brackets. But strangely, G7 is not as much of an improvement as I would have expected... and it is a lower number for the same bullet, therefore not good under #1.

It is a lot like Quickload, you have to tweak the "fixed" values to get it to "predict" what you are actually getting.
 
Interesting--did you take shots at a different distance with the new BC entered to confirm if the projected trajectory was more accurate?
Click to expand...

I'm trying to remember exactly what happened. I believe we made two adjustments. Anyway, I ended up shooting accurately.
 
You must log in or register to reply here.
Back
Top