Collecting DOPE to True Ballistic Calculators w/ Angled Shots? Will data be flawed?

HumbleEinstein

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I have a long range competition coming up this weekend. My primary goal is to collect dope on my new handloads for the purpose of truing my Kestrel w/ AB and some other iPhone ballistic calculators I use. However, it occurred to me that while there will be quite a few targets at varying ranges, almost all of them will be at a downward angle. Some of the angles are steeper than others. Is there anything I should know before I start collecting dope for truing purposes on angled shots. Will I need to multiply the actual distance to target by the cosine before inputting into the software for truing purposes? Are angled shots inherently flawed for truing? In addition, I will be shooting over a somewhat steep valley that can produce upward drafts very near the firing line. Is that going to screw up my dope for the purpose of truing? Please educate me. I get too few opportunities to shoot, so its important to me I make the most out of it. Thank you.
 

NachtKracht

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I have done the same thing recently, but at a low steepnes of -8.5 degrees max. I just ranged the target and measured the incline with a plrf10c lrf. So the ballistic programm could use all this for the calculations.

For me this worked just as fine as it did at our own flat ranges. For 308 i found that for calibration a range of 850-1000m worked best to get the most accuracte comparison between software predictions and real life target elevations.
I used the bc calibration, but the muzzlevelocity will give just as good results.
After calibration i was almost deadon at all ranges from 540 out to 1606m, for elevation, with an accuracy of .1 mil...windage however......;)

Just put in as much accurate info as possibe when your calibrating; range, heading, inclination, pressure, temp, moist etc.
 

HumbleEinstein

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My concern is that none of my ballistic calculators seem to factor in other data for calibration. The only thing they seem to require is range and elevation. That's why I'm wondering how to use the from the angled shot, if at all.
 

proneshooter

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Use the straight line horizontal distance from you to the target instead of the slant distance to true up your dope. Remember that the straight line distance (slant distance x cosine of slope) is what your bullet sees from an external ballistics perspective (drop and windage).
 

djskit

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Just put one of the angle or cosine indicators on your rifle.

I use an angle indicator because that's what my Iphone ap needs as an input.

My geometry is a little rusty, but the cosine factor is multplied by the your dope. So if your dope is 10.0 mils at a given distance, and your cosine is 0.87, your dope woudl be 8.7. Oversimplified to be sure. Again, I use an angle indicator.
 

CoryT

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>>Use the straight line horizontal distance from you to the target instead of the slant distance to true up your dope. Remember that the straight line distance (slant distance x cosine of slope) is what your bullet sees from an external ballistics perspective

That is not correct. Correcting range by the cosine of the angle is a 'quick fix', and only an approximation. It's referred to as "The Riflemans Rule" The improved version of this rule uses the cosine to correct the elevation setting, which is close, but still not exact.

If the target angle exceeds 5 degrees and the range exceeds 600 yards, you might not want to try and use that data point as a correction factor, but you can use it to check if the current model is correct by seeing if entering the range and angle give the correct setting as actually used.

For further information on this subject, see Inclined Fire on the Sierra Bullets ballistics website.

exterior ballistics
 

HumbleEinstein

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>>Use the straight line horizontal distance from you to the target instead of the slant distance to true up your dope. Remember that the straight line distance (slant distance x cosine of slope) is what your bullet sees from an external ballistics perspective

That is not correct. Correcting range by the cosine of the angle is a 'quick fix', and only an approximation. It's referred to as "The Riflemans Rule" The improved version of this rule uses the cosine to correct the elevation setting, which is close, but still not exact.

If the target angle exceeds 5 degrees and the range exceeds 600 yards, you might not want to try and use that data point as a correction factor, but you can use it to check if the current model is correct by seeing if entering the range and angle give the correct setting as actually used.

For further information on this subject, see Inclined Fire on the Sierra Bullets ballistics website.

exterior ballistics
That was precisely my concern. It seems to me that using angled shot DOPE might be inherently flawed for the purpose of truing common ballistic software. Thank you for the information.
 

Graham

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Collecting DOPE to True Ballistic Calculators w/ Angled Shots? Will data be ...

You likely won't have 'true' any software unless you approach 90% of the bullet's supersonic range.
 

Sevo2300

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The AB Kestrel has a function for entering either cosine or degrees under the target screen. I don't know enough about the algorithm to know if the Kestrel gives the actual true correction or just uses the "riflemans rule". I suspect that the Kestrel may actually solve for the "true" correction because you can enter positive or negative values for slope, and the solution for a given distance changes if you enter the same slope in degrees, but change it from positive to negative. There is supposed to be a slight difference in firing solution depending if you are shooting uphill or downhill, and the Kestrel seems to do this.
 

ptosis

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That was precisely my concern. It seems to me that using angled shot DOPE might be inherently flawed for the purpose of truing common ballistic software. Thank you for the information.
If I understand the problem correctly, you want to true your ballistic curve, but only have an inclined range to do it, right?

Estimating the "flat" dope out of inclined fire results may introduce significant errors; basically, you would be "reversing" one of the known rules of thumb (such as Rifleman's Rule) with all associated inaccuracies.

If you still want to go that way, your best bet is to reverse the "Increasingly Improved Rifleman's Rule" -- mil clicks version or MOA clicks version, depending on your scope. In other words, add [X] clicks per [Y] degrees (see the method description), then divide (not multiply) the result by cosine. Up to 800 m / 40 degrees you should stay within ±1 click from truth (unless using a very exotic calibre).

This being said, the purpose of truing is to calibrate the drag curve of a particular bullet. With common bullets, chances are that someone (with more resources and knowledge) has already done it before (e.g., the AB software has excellent custom curves researched by Bryan Litz). In this case, "truing" is basically knowing your V0 with enough precision, knowing the V0/temperature dependence, and checking that predictions correspond to reality.

I totally second CoryT's opinion re: "use it to check if the current model is correct".
 

ptosis

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The AB Kestrel has a function for entering either cosine or degrees under the target screen. I don't know enough about the algorithm to know if the Kestrel gives the actual true correction or just uses the "riflemans rule". I suspect that the Kestrel may actually solve for the "true" correction because you can enter positive or negative values for slope, and the solution for a given distance changes if you enter the same slope in degrees, but change it from positive to negative. There is supposed to be a slight difference in firing solution depending if you are shooting uphill or downhill, and the Kestrel seems to do this.
AB solver actually does the 3-DOF model calculations of the "true" trajectory, which is very, very close to reality (provided that actual input data, incl. the bullet drag model, is correct). The "Rifleman's Rule" is total crap beyond 400 m.
 

HumbleEinstein

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If I understand the problem correctly, you want to true your ballistic curve, but only have an inclined range to do it, right?

Estimating the "flat" dope out of inclined fire results may introduce significant errors; basically, you would be "reversing" one of the known rules of thumb (such as Rifleman's Rule) with all associated inaccuracies.

If you still want to go that way, your best bet is to reverse the "Increasingly Improved Rifleman's Rule" -- mil clicks version or MOA clicks version, depending on your scope. In other words, add [X] clicks per [Y] degrees (see the method description), then divide (not multiply) the result by cosine. Up to 800 m / 40 degrees you should stay within ±1 click from truth (unless using a very exotic calibre).

This being said, the purpose of truing is to calibrate the drag curve of a particular bullet. With common bullets, chances are that someone (with more resources and knowledge) has already done it before (e.g., the AB software has excellent custom curves researched by Bryan Litz). In this case, "truing" is basically knowing your V0 with enough precision, knowing the V0/temperature dependence, and checking that predictions correspond to reality.

I totally second CoryT's opinion re: "use it to check if the current model is correct".
Correct. I guess I'll use the match to the confirm predictions of the software. I'll have to collect data to true (if necessary) at a future time.
 

Sevo2300

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AB solver actually does the 3-DOF model calculations of the "true" trajectory, which is very, very close to reality (provided that actual input data, incl. the bullet drag model, is correct). The "Rifleman's Rule" is total crap beyond 400 m.
So, is it correct to say that the AB Kestrel can be used to "true" when shooting at an angle since the truing function uses the difference between the Kestrel calculated elevation and the actual elevation.
 

ptosis

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So, is it correct to say that the AB Kestrel can be used to "true" when shooting at an angle since the truing function uses the difference between the Kestrel calculated elevation and the actual elevation.
Yes and no.
You cannot really use it to create a custom drag curve for a bullet, at least not directly.
But you can see if the model that you are using is correct or not.
 

ptosis

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Not true, you can create custom curves.
I'd love to know how to do that -- that is, how to build a custom drag model for a bullet out of measured slope dope with significant angles of inclination.
Any links to documentation, guides, resources, even hard copy books?
Because, short of writing a custom ballistic solver software, I see no easy way -- but maybe I simply fail to see the obvious, my brain is not what it used to be.
(I am not afraid of math, and have some notions of external ballistics, so any level of technical detail is ok.)
 

Sevo2300

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I should be more specific when I say calibration (truing). My AB Kestrel allows me to true both MV and DSF. Since the AB Kestrel trues MV and I think also DSF by comparing it's predicted elevation to the actual bullet impact elevation on the target, it seems like you could true using inclined targets since the AB Kestrel compensates for incline in it's elevation solution and would then allow you to true by using the AB Kestrel incline corrected solution to to the actual target impacts.

On a another note, would I need to calibrate my DSF if I'm already using one of the custom curves?