Range Report Calculating zero shift in different conditions?

targetterror

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I am familiar with how to use the ballistic calculators, and have used JBM's online calculator extensively to good effect. One question I have is how to calculate the zero shift for a given load when the atmospheric conditions change. Ie, say I zero my rifle at 200 yards in 95 degree heat, with a given humidity and pressure (so at density altitude X, if I understand things correctly). If I change nothing on my rifle and shoot the same ammo at 200 yards when it is 30 degrees out (or otherwise a different density altitude Y), the POI is likely to shift since the air mass the bullet travels through is different.

Ballistic calculators seem to be limited strictly to calculating trajectory when a bullet is <span style="font-style: italic">zeroed for any given density altitude</span>. Is there anyway to calculate the zero shift for shooting the same zero in different density altitudes?
 

The Mechanic

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Re: Calculating zero shift in different conditions?

Since you did not specify your powder you are dealing with interior ballistics otherwise you should already have your changed info with your exterior ballistic calculator.
 

targetterror

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Re: Calculating zero shift in different conditions?

I realize that powders may be temperature sensitive, which affects velocity. So, for my question, assume you have a stable powder, or the pressure or humidity is changed for a given temperature, such that the velocity is constant, but the atmosphere changes.
 

Graham

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Re: Calculating zero shift in different conditions?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TargetTerror</div><div class="ubbcode-body">the POI is likely to shift since the air mass the bullet travels through is different...</div></div>At what range?<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TargetTerror</div><div class="ubbcode-body">Is there anyway to calculate the zero shift for shooting the same zero in different density altitudes?</div></div>A short range zero is not a function of density altitude.<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TargetTerror</div><div class="ubbcode-body">Ballistic calculators seem to be limited strictly to calculating trajectory when a bullet is <span style="font-style: italic">zeroed for any given density altitude</span>.</div></div>Then change the density altitude: Run the numbers on your ballistics program. Given an initial 100 yard zero, calculate your 200 yard DOPE for 5 degrees F. Compare that with your 200 yard DOPE at 125 degrees F.
 

targetterror

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Re: Calculating zero shift in different conditions?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">Then change the density altitude: Run the numbers on your ballistics program. Given an initial 100 yard zero, calculate your 200 yard DOPE for 5 degrees F. Compare that with your 200 yard DOPE at 125 degrees F. </div></div>

If I understand how the programs work correctly, you would actually have two different zeros to run that comparison. You would have a 100 yard zero at 5 degrees, and a 100 yard zero at 125 degrees. The program assumes that you are in fact zeroed for the new weather conditions and then gives you dope based upon that.

My question is slightly different. I would assume that if I zero my gun at 100 yards at 5 degrees, I will be zeroed for that weather condition at that range. If I then come back when it is 125 degrees, without changing anything at all on my rifle or scope (and assuming the internal ballistics stay constant), I would again assume that there should be a POI shift because the atmosphere the bullet travels through has changed. So, is there a way to calculate this <span style="font-style: italic">change in zero</span>, NOT simply the new dope once this zero has been set?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">
A short range zero is not a function of density altitude.</div></div>
Why not? Isn't any zero inherently related to the density altitude at which it is set?
 

Graham

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Re: Calculating zero shift in different conditions?

That the zero of the rifle changes with the temperature of the air, like is written in Mike Lau's book (circa 1997), is a theory left over from the days when powders were very temperature sensitive.

The difference in a one hundred yard zero due solely to a change in density altitude is well-within the diameter of a bullet. Therefore you can neither adjust for it not hold for it.

The difference between POA vs POI on a zero'd rifle at five degrees vs one hundred degrees will be a function of ammunition temperature, and therefore of internal ballistics. That's why it isn't in the external data portion of the ballistics program.

If you want to account for this change, and you have a program that calculates FPS deviation with temperature, record your muzzle velocity at different ammo temps and enter the claculated FPS per degree deviation in the program. If the program does not have this feature, record the muzzle velocity at different tempa and use those results.
 

jbmillard

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Re: Calculating zero shift in different conditions?

I agree, it is a very small effect. I don't think I'm a good enough shot to see it at my typical zero range. It obviously does affect your shooting for longer ranges.

As for ballistics calculators, all the range cards on my website account for this.

Here is a short article on how to do it with my other calculators:

Elevation

The bottom line is that the elevation angle (angle between line of sight and bore) is set when you zero. This angle is constant so you use that value when shooting at other conditions.

Brad
 

Graham

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Re: Calculating zero shift in different conditions?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JBM</div><div class="ubbcode-body">I don't think I'm a good enough shot to see it at my typical zero range...</div></div>Let's think about this for a minute: What is the minimum adjustment on your scope?
 

bohem

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Re: Calculating zero shift in different conditions?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">The difference in a one hundred yard zero due solely to a change in density altitude is well-within the diameter of a bullet. Therefore you can neither adjust for it not hold for it.</div></div>

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JBM</div><div class="ubbcode-body">...it is a very small effect. I don't think I'm a good enough shot to see it at my typical zero range. It obviously does affect your shooting for longer ranges.</div></div>


These two guys nailed it (and JBM wrote the gold-standard software that all the other ballistic apps have been lately compared to). The ToF involved for a modern, supersonic rifle round at 100, 200, even 300yd is small enough that a change in DA from -2000 to +12,000 feet Density Altitude creates a DOPE change (again, ignoring rifle effects and shooter effects) that are inside the correction capability of a scope with 1/4 MOA turrets and in many cases 1/8th MOA turrets.

Now, functionally if you can correct for it at 400yd and you're shooting a 1MOA target or animal vital zone and the DOPE shift is off by 0.25MOA from what you SHOULD be using, it only correlates to a 1.047" impact point difference from the atmospherics alone. It's a small thing to consider up close.

At 600yd that effect is on the magnitude of 6-10" most times and at 1000yd you can easily get 40+" of change. So out long it quickly matters more and more.


 

Eaglet

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Re: Calculating zero shift in different conditions?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TargetTerror</div><div class="ubbcode-body">I am familiar with how to use the ballistic calculators, and have used JBM's online calculator extensively to good effect. One question I have is how to calculate the zero shift for a given load when the atmospheric conditions change. Ie, say I zero my rifle at 200 yards in 95 degree heat, with a given humidity and pressure (so at density altitude X, if I understand things correctly). If I change nothing on my rifle and shoot the same ammo at 200 yards when it is 30 degrees out (or otherwise a different density altitude Y), the POI is likely to shift since the air mass the bullet travels through is different.

Ballistic calculators seem to be limited strictly to calculating trajectory when a bullet is <span style="font-style: italic">zeroed for any given density altitude</span>. Is there anyway to calculate the zero shift for shooting the same zero in different density altitudes? </div></div>

Most applications that have the ability to enter "zero conditions" and "Field Conditions" are suppose to calculate the difference properly. Understanding that as the temperature changes more likely BP and humidity will also change, but for the purpose of this example I'll only change the temperature leaving everything else the same.
I'm a ColdBore 1.0 user and in this program I enter all the conditions for my zero and as I travel and the weather changes I enter my new conditions in the "Environment Conditions - Field" and the application tells me what adjustments I need taking in account the Zero conditions and the new Conditions.

Let's start by entering all data and conditions for Zero.

<span style="font-weight: bold">Note: Field conditions are off</span>
f1xhm0.jpg


<span style="font-weight: bold">Field conditions are off</span>
235ezn.jpg


For comparison purposes I entered the zero conditions only and saved this data to "Track 1"; then with the original data on "Track 1", I recalled it on "Track 2", turned ON the Field Conditions and entered the new info.

<span style="font-weight: bold">Note: Field Conditions are on</span>
2ij476f.jpg


Now I run the two Tracks side by side so that the results can be compared up to 1500 yards.

<span style="font-weight: bold">Note: Track 1 and Track 2 are first compared in inches and then in MOA.</span>

2nw1ts4.jpg
fvbj9f.jpg


As others have said, the difference is too small to be noticed at 200 yards, only 0.1", but as we shoot farther away we need to care more and more.

At 900 yd, the difference will cause a complete miss on a mule deer.