Thread: Truing a Ballistic Calculator

Originally Posted by JT_NZ

I am not sure if some of you are trolling here.........

When it comes to truing I would refer to the "Grandfather of Truing" (Todd Hodnett) in the first instance:

https://www.youtube.com/watch?v=KaOEyGeWDdk

Secondly, what is wrong with this article?

https://www.snipershide.com/precisio...aponized-math/

1. Ignores the effect of density altitude. Due to the design of the protracted methodology, there is every likelihood that the density altitude will change over the duration of the truing exercise. During the recent Alpine Long Range Shooting Comp (ALRSC) there was a sharp weather change over 15min. For me, this resulted in a 0.3 mil change in elevation at 800m shooting the 260 REM when I updated the environment in my Kestrel 5700. By ignoring DA and conducting a protracted truing process, you run the risk of corrupting your data.

2. Ignores the effect of MV Migration - very important when it comes to magnums. A rifle's MV will not remain constant over the life of the barrel, even if all other variables remain constant. For example, over the first 150-200 rounds of a 338LM's life you will observe approximately 50fps in MV migration (eg. 2700 fps to 2750 fps). Then pending cleaning regime, you may observe drastic MV migration over the first 50 rounds following a robust clean prior to settling down. Once again, prior to the ALRSC I trued the 260 (CAL MV) and expected my average MV to change over the course of the match because it was a new barrel - which it did! When engaging the target at 1000m I noticed that I was approx 0.2mils high and I updated the Kestrel accordingly. But by conducting a protracted truing process as described in the article, you are once again exposing yourself to the risk of creating corrupted data as you MV may not be consistent between all of the recommended 100m/yd increments.

3. Adjusting both MV and BC pre trans is asking for trouble. I don't think I need to go too deep on this, especially if you take the above into consideration and do the math on the compounding effect. Furthermore, if you start playing with both MV and BC in your Kestrel for what would be a adhoc CAL MV process, it will most likely throw out your post trans true - CAL DSF. Listen to Grandad (Todd) and just adjust your MV at approx Mach 1.2.

4. Their math is poor. I quote:

"A 308 going about 2550fps is about 1 mil every 100 yards from 400 to 700 yards, give or take a tenth or two"

So plugging that into the Kestrel with the following parameters; 175gr SMK, 2550fps, DA = 716ft, G7 = 0.243 we get the following elevation holds:

100yd = 0.00mils
200yd = 0.61mils
300yd = 1.49mils
400yd = 2.51mils (delta = 1.02mils)
500yd = 3.67mils (delta = 1.16mils)
600yd = 4.98mils (delta = 1.31mils)
700yd = 6.47mils (delta = 1.49mils)
800yd = 8.16mils

When truing, you want to be as accurate as possible. There are times for gross generalisations in LR/ELR shooting. Truing is not one of them.

5. Aerodynamic Jump (AJ) matters. I quote:

"An impact using 4.2 Mils doesn’t need to include Spindrift, Coriolis, or Aerodynamic Jump, because 4.2 is the final answer; it’s not 4.2 plus, it’s just 4.2; that is what we true too."

The author is not really clear in what they are saying here so I will emphasise - account for AJ! Truing is all about the vertical component of the trajectory and we need to account for the variables that affect this to a quantifiable degree that matters. For example, using the gun profile provided above (175gr SMK, 2550fps, DA = 716ft, G7 = 0.243), this setup will approx 0.1 mils of AJ per 6mph of wind. Therefore, if you are truing in 12mph wind, you will have 0.2mils deviation high or low pending which way the wind is coming from. This delta needs be removed from the calibration input.

In summary, keep your truing methodology as simple and accurate as possible. And like the Grandfather of Truing says - "know what matters".

JT

That's what I meant!

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