uphill/down hill shooting

[horse2960]

Just thought I would put this out for those of you that hunt steep country like I do. I know it is comon practise to use horizontal distance or cos of angle gages. from reading Aurther J Pejsa book "modern practical Balistics" I know cos or horizontal distance is wrong ( hear me out). Bullets are not acted on horizontal distance. They are acted on time of flight,only. His chapter on uphill/downhill shooting is an important read for anyone that shoots on steep angles.What I didn't know is how wrong is it. There is a lot of wrong information out there by supposably informed people.
From my research you can use the horizontal distance out to about 400yds with not much more than an inch off. At 500yds and beyoud things get worse. for example I ran a BC of .404 (WSM300 165gr) and at 600yrd @30degress I would be 5.44 inches low, A700yrds @20 degress I would be 5.05" low. this is that it would shoot low if you diled to the rang that the cos of the ange would give you. I think that most long range shooter can see this could result in a miss. I should mention That I am getting this data from Arther Pejsa's balistic softwear.(won't run on Vista could only get is to run on Windows XP). The only one that I tust as I know that he understands that the scope angle has a lot to do with subject.
To sume things up I am confident now to shoot using the cos ange out to 400yrds at 500and 30degress I need to start to ajust. I still have to shoot and verify this data but mathmaticaly I believe it to be right. wouldn't mind if someone else did some testing as well. I hope this is helpful and I am open to coments.
Larry In BC Canada

 

[SBruce]

Hello Larry,

You are correct, in fact we had a quite lengthy discussion of the proper way to account for high angle shots in an older thread.

http://www.longrangehunting.com/forums/f17/long-range-incline-declined-angle-shots-66782/

The old basic "take the cosine of the angle and multiply by the line of sight distance" to get a horizontal distance or "shoot to" distance doesn't work precisely in all situations, and modern ballistic programs don't use this method for calculating angled corrections. As you mentioned, it will be close out to 400 yds, but often not close enough if the angle is pretty steep.

The calculation that some of the ballistics programs appear to use is:

Correction on level ground for said distance minus (Drop from Line of departure at same distance times (1-Cos of shot angle))= New correction for the angle.

If our correction at 500yds on level ground is 22.6", The shot angle is 30 deg, our drop from line of departure is 58.2", then our new correction for a 500 yd shot at a 30 deg angle is only 14.8.

22.6-(58.2(1-Cos30))=14.8

The other method results in a "shoot to" distance of 433yds, and our drop at 433yds on level ground is 11.8" ....................3 inches different in this example.
Certainly close, only a little over 1/2 minute difference, but that 1/2 minute could be a miss on a rockchuck.

 

[Mikecr]

I apply an adjustment to bullet PATH
PATH*(1-COS(AngleDeg*PI()/180))

exterior ballistics