AB Kestrel Cosine Correction

[TEDean]

I understand the requirement of using (cosine x dope = corrected dope) to correct for the shooting angle. I use the PLRF 25C to feed data to my Kestrel, thus the angle will also be populated in the Kestrel. So I did some calculations.

If I lock the weather, no wind with 0 angle at 1000 yds it gives me an elevation“x”. If I manually input 45 degrees into the Kestrel, I get a corrected elevation “y”. However if I use the 0 angle elevation times the cosine at 45 degrees (0.707) then I get a “z” mill elevation which is about 0.2 mils difference at 1000 yds for my system.

I certainly don’t expect elevations numbers to be the same, however 0.2 mils at 1000 yds is significant.

Stupid Q?

 

[ballison]

Is your calculated dope less than what the Kestrel is giving you? If so, here is my guess at what's going on. Your method of calculating dope takes into account the adjusted range but also uses the "time of flight" of the adjusted range. The true time of flight should be based on the line-of-sight distance, which the Kestrel takes into account. And with a longer time of flight, the bullet will be decelerating more, requiring more elevation (.2 mils?). Hope this makes sense.

 

[Deleted member 113831]

When you apply the cosine to your dope, it is closer than applying it to the distance. However, it will not be exact. The Kestrel is giving a solution using a much more complicated ballistic calculation that accounts for the ballistic trajectory, time of flight, velocity decay etc...

You are never going to be exact when using trigonometry to answer a calculus problem.

 

[TEDean]

Is your calculated dope less than what the Kestrel is giving you? If so, here is my guess at what's going on. Your method of calculating dope takes into account the adjusted range but also uses the "time of flight" of the adjusted range. The true time of flight should be based on the line-of-sight distance, which the Kestrel takes into account. And with a longer time of flight, the bullet will be decelerating more, requiring more elevation (.2 mils?). Hope this makes sense.

1000yds, DOF 0,
+45 degrees 4.69U
-45 degreees 4.61U
0 degrees 7.04U
7.04 * 0.707 = 4.98U

....You are never going to be exact when trying to apply trigonometry answer to a calculus problem.

: ) Well said on Calculus vs geometry.

45 degrees is an extreme, probably not realistic. I was just thinking it would be a bit closer.

I was shooting up in WY last year at 30 degrees using the dope x cosine, hitting 14” rounds 500yds. I was just curious.

Thanks for the replays!

 

[Deleted member 113831]

1000yds, DOF 0,
+45 degrees 4.69U
-45 degreees 4.61U
0 degrees 7.04U
7.04 * 0.707 = 4.98U




: ) Well said on Calculus vs geometry.

45 degrees is an extreme, probably not realistic. I was just thinking it would be a bit closer.

I was shooting up in WY last year at 30 degrees using the dope x cosine, hitting 14” rounds 500yds. I was just curious.

Thanks for the replays!

The dope x cosine method does work unbelievably well for angles 30 degrees and under out to 800+ yards. The difference isn't likely to cause a miss. I still use it for hunting and field shooting when I want to practice NOT using by ballistic app. As you have already shown, even at 45 degrees at 1000 yards, you are only 1-1.25 moa ( .3- .4 mils) off.

 

[DocUSMCRetired]

The reason for the difference is because we are not simply using the advanced rifleman's rule and calling it good. The AB Engine accounts for differences in altitude and air pressure as the bullet travels at an inclination. Our engine also accounts for the different effects gravity will have based on the angled shot.