Your actual BC can change with twist rate and velocity. I had a long chat with a Hornaday engineer one time about their 180gr eld. In my rifle with a 9.5 twist at 2850 he estimated my bc would be in the high 6's. That estimate is very close to the actual performance I am getting with 180 vld's.
If I were to re-barrel with a 7.5-8 twist he estimated the high velocity band might get as much as .810 (I don't remember the G7 BC equivalent but I do use it.)
He speculated my 180 vld's would probably have a similar result as the profiles are similar.
This is important as well.B.C.'s are not always as published and are rarely what you'll get after bullet deformation after being pushed through you barrel. Typically they're close enough but to match as shot data, I like to tweak this number first in my calculator rather velocity for this very reason. I am meticulous in getting as much environmental data as possible before I do though. You can be spot on B.C. and velocity but if your calculator is off by 20 degrees the air density will be different and you will end up with a different result than the program kicked out. As stated before I think your target speed and wind values are what's giving you fits in this scenario. Try zeroing those and input your environmental data as close as possible. I bet you'll be surprised by the changes.
I will start of by saying thanks to those willing to help out.
OK, I did find a discrepancy in my data.
sight height was off. 1.5 vs 2.054 actual.
I made changes to the calcs but didn't correct everything.
Below is my chart.
It is still not what my actual data revealed. But better. If someone see's a discrepancy, I'd appreciate it.
I'll double check chrono and zero to see if changes. I haven't checked tracking on this scope. I would hope I can trust a NF.
Regarding hard info.
6.5-06
Berger 140gr VLD hunting. I've used a G1 BC of .6, .612. And G7 of .307
3170 fps, 100yd zero at 320 elevation R26 powder.
Scope- Nightforce NX8 4-32 FFP MOA
You guys will have to check me on this because I only think I understand it
I have had to resort to trial and error and data manipulation in the past because of poor velocity readings or record keeping. Usually this took the form of finding some velocity, or zero, that, when plugged into a calculator made sense of the data.
One problem I had was that TBuckus did not ever say that he verified his zero at 100yds.
1. I started by thinking about the 300 yd correction. His correction called for an up 8.25 MOA correction and it should have been up 6.5., an error of 1.75 inch.
2. I started by looking for a zero with a 6.5 inch drop at 300 yds given the stated velocity. Using a165 yd zero I come up with a 6.55 drop at 300 yds. If the "up" correction errors of 3.75 inches and 6.375 inches are removed, then a 30 inch drop at about 480 yds and a 49 inch drop at about 580 yds are shown on the ballistic chart.
3. it is my opinion that this could be considered a 165 yd zero.
I used the "Ballistic AE" app for my calculations.
I have to say that this was a lot of fun and I will not be wounded in any way if proven wrong.
shoot me down.
How or where do you make corrections to make a drop chart line up to actual data?
3 different drop charts are showing more drop than actual.
Here's my example
300yds
2.75MOA=1.75 high
500yds
6.75MOA= 3.75" high
600yds
9.25MOA= 6.375 high
I switched from G1 to G7 and the numbers change minimally.
That worked for me on several rifles. Once I get a good group at 100 I move out to 7-800 and shoot a rock wall with book BC and a good estimate of the speed. (buddy accidentally center punched the crony) We used a good camcorder and a 7" display from amazon to refine the point of impact. I then use the shooter app and adjust the velocity until my drop matches. Several Rifles later I feel confident the drops match very closely.
At 5000 feet we stoned a cold bore 1515 shot on a 7" target with a 6.5 creedmoor using factory 140 ELD's.
Also shot the mile successfully with the magnum. Drops were right on across many days. Oh the wind, if only for the wind!!
A soultion you may wish to consider. You have 3 unknown variables - Is your chrono accurate? is the published BC accuracte, are your clicks accurate. Trying to figure all that out can be maddening. Consider the the following:
Take your maxium expected shot distance, for example 800 yards
Than actually shoot and record the clicks required to hit at 800, 600, 400, and 200. While the trajectory is of course a curve, click adjustments are reasonably linear over 200 yard increments. In other words, if you know what it actually took in clicks to hit at 800, 600, 400, and 200; you can extrapolate by simple division what it takes for the intermediate ranges. I ran this simulation for you and your worst case was being off by .6 MOA at 500 yards, that is 3 inches.
If you are uncomfortable with factual data points every two hundred yards then use 150 yards and confirm 200, 350, 500, 650, and 800 and extrapolate between those.
B.C.'s are not always as published and are rarely what you'll get after bullet deformation after being pushed through you barrel. Typically they're close enough but to match as shot data, I like to tweak this number first in my calculator rather velocity for this very reason. I am meticulous in getting as much environmental data as possible before I do though. You can be spot on B.C. and velocity but if your calculator is off by 20 degrees the air density will be different and you will end up with a different result than the program kicked out. As stated before I think your target speed and wind values are what's giving you fits in this scenario. Try zeroing those and input your environmental data as close as possible. I bet you'll be surprised by the changes.
