Changing ballistic info to match "real" drop.

After having done some set up with a new load for my 7mmRM and entering all of the relevant information into 2 different ballistic calculators (which both came up with the same trajectory data) I have found a difference in the actual drop (zeroed at 109 yards and tested at 440 yards) and the correction provided by the calculators.
It is shooting 3" high at 440 yards with the recommended dial up of 6.7 MOA.
I have only used one chronograph to check MV and I am relying on Noslers BC data from their web site.
My question is, what is the best way to change the calculation to match the actual drop?
Change the MV? Change the BC? or is there something I am missing?

What ballistic programme are you using mate? Assuming all the atmospherics entered are correct some apps such as Applied Ballistics will allow you to true your data. I.E enter what your actual elevation was at a number of given ranges. The app will then provide a "trued" ballistic table for that environment. My chrony isnt that reliable so use that for ballpark and then fine tune using the above method. Hope that makes sense?

try changing velocity by say 25fps at a time and see what it does.......

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Originally Posted by Link

What ballistic programme are you using mate? Assuming all the atmospherics entered are correct some apps such as Applied Ballistics will allow you to true your data. I.E enter what your actual elevation was at a number of given ranges. The app will then provide a "trued" ballistic table for that environment. My chrony isnt that reliable so use that for ballpark and then fine tune using the above method. Hope that makes sense?

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Thanks for that. I have all the correct atmospheric data for when I sighted it in and current conditions so that shouldn't be an issue.
When you say the app "trues" the data, what does it adjust to true it? This is what I am getting at.

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Originally Posted by Micky Duck

try changing velocity by say 25fps at a time and see what it does.......

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+1 I do the same, just tweak velocity til it matches where the group landed on target

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Originally Posted by Micky Duck

try changing velocity by say 25fps at a time and see what it does.......

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Yeah. Just had a play with that. It's about 100fps difference. Seems a big difference but as I said, only used one chronograph.
The "new" velocity is not unrealistic however and actually more in keeping with what I expected when I first developed the load.

Brian Litz has an excellent chapter on this topic in one of his books - well worth reading.

He basically says change the variable that you have the least faith in.

There is a fair amount of documented research for most of the modern projectiles, and you should be able to get a validated BC (G7) for them.
DO NOT trust what the manufacturer tells you their BC value is, it is marketing. Let me know which projectile you are using and I can look it up in Litz's book.

Chronographs, unless you are using a LabRadar, are not exactly precise instruments, so muzzle velocity would more than likely be the variable I change first.

The other thing to confirm is that your turrets are calibrated correctly. Shoot a "tall target" and get a good 100yd zero, then dial up 10 or 20 minutes, and measure the shot fall.

Cheers @ebf.
Projectile is a .284 150gn Nosler BT. Nosler recon .493

G7 0.229 G1 0.447

Use G7, it should be a better match for that shape projectile

That's from 2nd Edition of Applied Ballistics, someone else may have a later version to confirm recent values.

Cheers. I will use that and see where that puts the velocity.

Unfortunately that just makes it worse. On the bright side my MV is climbing all the time.

Out of interest @ebf, do you have BC info on the .284 160gn Seirra TMK?

Nope sorry, TMK's came out after my book was published.

Hey @Shearer are you using a mill dot scope by any chance? If so have you calibrated it to find out if it's correct.
I.e. The mills are correct quite a few are round about:O_O: someone will be along to say how. Just a thought.

Weird, on JBM Ballistics they have a 155gr TMK and a 168 TMK ?

If you select the (Litz) projectile from their library and look on the result page, it lists the relevant BC

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Originally Posted by ebf

Weird, on JBM Ballistics they have a 155gr TMK and a 168 TMK ?

If you select the (Litz) projectile from their library and look on the result page, it lists the relevant BC

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Thanks again.

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Originally Posted by Sideshow

Hey @Shearer are you using a mill dot scope by any chance? If so have you calibrated it to find out if it's correct.
I.e. The mills are correct quite a few are round about:O_O: someone will be along to say how. Just a thought.

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No. Leupold VX5. 1/4"@ 100yards.

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Originally Posted by Shearer

No. Leupold VX5. 1/4"@ 100yards.

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Ok so not that...like I said just a thought:)

What Ebf said, turret accuracy my first pick, MV accuracy second, then other considerations such as the true BC, how accurate was the zero really, accurate input into the trajectory simulators etc. The accuracy of turrets on scopes cannot be trusted until verified - ideally by having the scope off the rifle and checked against a graduated measuring stick at a known distance.

If you have the space shoot a verification string out to say 600 on a tall target keeping original zero. Measure the drops then repeat a few dialing on the measured drop. Compare with the ballistic calculators results.
Best result is you get to shoot a bit and have faith in the results so ont second guess the results.

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Originally Posted by Puffin

What Ebf said, turret accuracy my first pick, MV accuracy second, then other considerations such as the true BC, how accurate was the zero really, accurate input into the trajectory simulators etc. The accuracy of turrets on scopes cannot be trusted until verified - ideally by having the scope off the rifle and checked against a graduated measuring stick at a known distance.

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Year that's pretty much what I was eluding to in my previous post. I new someone would be along to stop the eluding:thumbsup:
Cheers @Puffin

Most calibrations change the Muzzle velocity (MV) and this is what i would tinker with if I was you.

Below is from the AB manual for the AB MObile App.

"The ballistic calibration feature allows a user to calibrate the ballistic solution based on observed bullet
drop at range. There are two parameters that can be calibrated: muzzle velocity (MV), and drop scale
factor (DSF). The Ballistic Calibration Screen is available from any screen via the Android Action Bar.
Due to uncertainties involved with modern chronographs, velocity measurements are not always as
accurate as we would hope. Therefore the first variable a shooter should attempt to calibrate is muzzle
velocity. The recommended range for muzzle velocity calibration is where the bullet is at Mach 1.2 in its
flight. If you have multiple 'observed' data points, use the farthest high confidence data point available
MV Calibration DSF Calibration DSF Table

for muzzle velocity calibration. The interface will provide recommended ranges range in which to
calibrate muzzle velocity based on the bullets remaining velocity. After you've entered the observed
range/drop pair, the application will automatically calculate the adjusted muzzle velocity and the
application will display the actual MV that results in your observed drop. Click Use MV and the calculated
MV will be applied and the elevation and windage shown at the top of the screen will be automatically
updated.

For long-range shooting, the drop scale factor (DSF) is used to refine the ballistic solution in subsonic flight
using a similar process. By firing rounds at long range, and noting the true drop, a drop scale factor is
computed. Clicking Use DSF populates the DSF value and Mach number into the View DSF Table
Viewing the DSF Table By scrolling down, the full DSF table is presented based upon all DSF values that
have been calculated and saved. The table can be cleared by clicking on the Clear DSF button. "

Shooting a tall target will show you if your scopes mechanical clicks actually are 1/4 moa. If they are not you can input this value into the application for example: 4 clicks might equal .95 MOA.

In short adjust your muzzle velocity until it matches up or shoot at all the ranges you want to shoot game at then measure the actual drop at each distance and write it down to determine a range card.

clear as mud?

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Originally Posted by 300_BLK

Most calibrations change the Muzzle velocity (MV) and this is what i would tinker with if I was you.

Below is from the AB manual for the AB MObile App.

"The ballistic calibration feature allows a user to calibrate the ballistic solution based on observed bullet
drop at range. There are two parameters that can be calibrated: muzzle velocity (MV), and drop scale
factor (DSF). The Ballistic Calibration Screen is available from any screen via the Android Action Bar.
Due to uncertainties involved with modern chronographs, velocity measurements are not always as
accurate as we would hope. Therefore the first variable a shooter should attempt to calibrate is muzzle
velocity. The recommended range for muzzle velocity calibration is where the bullet is at Mach 1.2 in its
flight. If you have multiple 'observed' data points, use the farthest high confidence data point available
MV Calibration DSF Calibration DSF Table

for muzzle velocity calibration. The interface will provide recommended ranges range in which to
calibrate muzzle velocity based on the bullets remaining velocity. After you've entered the observed
range/drop pair, the application will automatically calculate the adjusted muzzle velocity and the
application will display the actual MV that results in your observed drop. Click Use MV and the calculated
MV will be applied and the elevation and windage shown at the top of the screen will be automatically
updated.

For long-range shooting, the drop scale factor (DSF) is used to refine the ballistic solution in subsonic flight
using a similar process. By firing rounds at long range, and noting the true drop, a drop scale factor is
computed. Clicking Use DSF populates the DSF value and Mach number into the View DSF Table
Viewing the DSF Table By scrolling down, the full DSF table is presented based upon all DSF values that
have been calculated and saved. The table can be cleared by clicking on the Clear DSF button. "

Shooting a tall target will show you if your scopes mechanical clicks actually are 1/4 moa. If they are not you can input this value into the application for example: 4 clicks might equal .95 MOA.

In short adjust your muzzle velocity until it matches up or shoot at all the ranges you want to shoot game at then measure the actual drop at each distance and write it down to determine a range card.

clear as mud?

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Cool. cheers for that.
I have adjusted MV to match drop at this stage and when I get a chance will verify again at what distances I can (depends a bit on where stock are).
I have checked the scope adjustments and they do seem to correspond with 1/4" @ 100 yards (0.262 MOA).

The 3" offset would result from only a 20mm error in your zero, not very much, particularly if different shooting positions are used for the two distances. Can I ask from how many shots and what sized groups are the two group centres being arrived at? Moving to using a 200yd zero might provide some insight?

Pretty sure my 109 yard zero was not 3/4" out but I will definitely check again along with shooting more targets at varying distances.
Zero was reached with about 20 shots over 3 sessions. The last session being 3 shots to confirm (two touching and other 1/2" away).
Drop at 440 yards was established in one session with 9 shots (broken into 3,2,2,2). The 'group" for those 9 shots was 3" high x 6" wide. There was a rather variable cross wind and two shots stretched the group horizontally but the others were in a 3" circle. I realise this is a relatively small sample but it was consistent and I was shooting as I would in the field (over my pack). I am primarily a hunter so overall group size is not the most important thing to me. It is where the first shot consistently lands from a cold barrel.