A minute high at 610 and 1050?

[DIY Mtn Hunter]

I tried some BC confirmation testing today and ran into some puzzling conditions. First, I zeroed my 338 edge at 100 yards while gathering roughly a dozen velocity readings. I then backed out to 610 yards and 1053 yards, using my Kestrel with Horus, I plugged in the G1 of .818 for the OTMs. I also set up all of the parameters I could find, condtions, etc. The results I had were the center of my 5 shot group being 6.5" high at 610 yards and ~12" high at 1053 yards.

First, this sounded like a bad zero issue. I went straight back to 100 and it was still on. Next, I ran the solution on JBM and got the same solution (13.2 MOA & 30 MOA). What could I be missing here? Surely Berger didn't underpublish this BC by this great of an amount? From what I've heard, their estimates are usually accurate. I'm also running a relatively slow load of 2650 fps, so I wouldn't expect that to be the case..

Please help.

 

[nso123]

Could it be that your chronograph is not reading correctly? I am new to ELR, but it seems that the velocity being off by a substantial margin could cause this.

 

[DIY Mtn Hunter]

Thanks nso, I am seeing rather significant velocity swings in various calibers (old beta chrony), but I typically just exclude the major outliers from my spread and chalk it up due to reading errors (see attached image). On my 6.5, I am seeing exact matches at both distances with the actual velocity readings I'm getting, so I think its right most of the time and just throws out occasional erroneous readings. It does tend to do this more early morning or late evening, so I try to shoot with good, but not too bright lighting when possible.

 

[Lowlight]

There is nothing wrong with being 1 MOA off, these devices are just a starting point until trued to your system.

There are many places where inaccuracies can be found, from the shooter to the bullet. The computer cannot see every variable and is only giving you a general average that works somewhere in the middle. From there you have to true and adjust. This process is far from perfect and your kestrel is not calculating anything it just spitting out an average based on some basic principles.

Consider yourself in good shape, adjust you MV as necessary to match up your results and drive on.

These are starting points for the process and not cast in stone end results.

 

[MitchK]

The bullet doesn't lie. Make your adjustments off of the impacts.

 

[Dylanss180]

Ill throw something else in to consider. You are using G1 bc for your calculations on a g7 bullet shape. That is why if you look at Sierra's bullet b.c chart they will usually have 3 values for different speed ranges. They give G1 instead of G7 because it is a larger number and as we all Americans are we like big things. If you read Bryan Litz book you can see the drag profile difference between a G1 and a G7 and they are nowhere near the same. All being said I found using a G7 for your calculations especially when you are getting closer to the transonic range is where you will really see the difference.

When I am going to extreme distances 2000 yards plus I actually weigh each bullet and sort them. Then measure the ogive length of each bullet and sort them again. Then I will coat the riding diameter of the bullet with a dry lube and seat the bullets in the case using a micrometer seating die. I know that is super overdone but It does make a small difference at those distances. Some food for thought.

 

[DIY Mtn Hunter]

Thanks to all who have replied, I will make a few tweaks to the velocity initially to make the profile fit at 600, then see if I can't figure out how to adjust the G1 on my Kestrel Horus to reach a slightly lower BC once it falls to the lower velocities.

 

[zenbiker]

Ballistic Coefficients given to us by manufacturers may be mathemateical calculations or actual testing of velocity at diferent distances. Various internal and external ballistic factors determint the B.C. which can vary slightly weapon to weapon. You can calculate the B.C. from your gun and plug that into your ballistics program for more accurate results. Just rest assured what you are experiencing is not unusual, and personally I'd be thankful for getting a free MOA of elevation...


Sent from my iPad using Tapatalk HD

 

[varmint slayer]

A minute high at 610 and 1050?

That's not terrible like Lowlight said. Could be bad BC,shooter errors, or scope adjustment (mechanical) error among other things.

 

[Canuck257]

That's not terrible like Lowlight said. Could be bad BC,shooter errors, or scope adjustment (mechanical) error among other things.

This prompts me to ask, have you calibrated your scope using the tall target test and applied that correction factor to you inputs?

 

[DIY Mtn Hunter]

I have not. I guess I assumed a brand new March would be on.

 

[Canuck257]

I have not. I guess I assumed a brand new March would be on.

Scopes are mechanical devices and therefore have construction tolerances. It may not be huge but it becomes significant over distance. Both my Sightrons move 0.975 for each 1 moa on the knob. My S$B moves 0.987 per moa input.

Ivan