Thread: Changing ballistic info to match "real" drop.

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

Thanks again.

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 someone will be along to say how. Just a thought.

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

Originally Posted by Shearer

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

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.

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.

Year that's pretty much what I was eluding to in my previous post. I new someone would be along to stop the eluding
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?

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?

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.