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Up/Downhill corrections

 
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  #1  
Old 06-05-2007, 08:16 PM
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Up/Downhill corrections

It would seem that I have made some errors in the way I have instructed up/downhill shooting correction. I would like to post this to get everyone headed down the right track. I have forever instructed that to correct for up/downhill shooting with a cosine indicator you take the lazered yardage and multiple by the cosine. This number would be the distance that the bullet is affected by the force of gravity. After determining this distance you simply look on your chart at the corrected distance and dial the dope for the corrected distance. This is the correct way to “triangle” the shot if you were shooting a lazer. For the rifleman this method does not take into account that the bullet, while only being affected by gravity over the corrected distance still has a longer time of flight to be affected by. IF YOU USE THE ABOVE METHOD YOU WILL BE LOW EVERYTIME, if you are setup correctly (more on that later).
The second method I have tried is to lazer a yardage find your drop and multiply the drop by the cosine. While this method will work better that applying it to yardage it is still off as much as 4 or 5 moa at ranges inside of 1000 yards. The odd thing about this method is that most of the time it to will make you shoot low but not as much as applying to the yardage. However with some calibers and some angle ranges it will make you shoot high.
Now then how all this came before me, I was talking to Jeff Huber (of Nightforce fame) about their new angle indicator and mount. The NF mount reads in degrees, when I questioned why you would want it setup like that the debate was on. On my side I had the fact that I had been using my method of correction for along time and had taken game at some extended distances using it. On the other side is Jeff & Perry (Perry is the developer of the Exbal program) with a pile of computer and formula knowledge. First some facts about how I roll with respect to LRH, I use the pocket PC when ever possible but like to know and teach how to manually figure drops etc for times when a PC is not working or available. As with most issues concerning LRH the more distant, the steeper, the slower the bullet the more compounded the problem. I based most of my knowledge and experience from shots with a 260 Remington and a 338 Edge at slight angles of around .90 or less. Most of these shots were under 1300 yards. With those kind of ballistics and slight angles even given the long distances, applying the cosine either way or using the PC all got results good enough to hit moa sized targets. The core of the problem is that there is no way to apply the cosine in the field and factor in the additional time of flight. The only way to get correct dope is to enter everything into the PC or have a chart with drop values for different angles (the Exbal program has this function under the Excell button) The problem with another chart is it probably won’t match the conditions your shooting in at that moment.
I ran numerous charts in various calibers and angles to check my theory and then ran some field testing to confirm my findings. I shot the 260, a 308 Winchester 20” barrel, and a 338 Allen Mag. I shot distances from 300-1300 yards and angles from 0-45 degrees downhill. I shot every shot with cosine corrected yardage first, then cosine corrected dope next, and lastly pocket PC dope. Here is what I found in the field:
• The cosine corrected yardage method impacted low on every shot over 400 yards, however the cosine corrected dope worked well .90 and above at all distances, not perfect but well.
• The 308 was helpless the only way to get good hits past 400 with any real amount of angle was to use the ACI and the pocket PC.
• The Allen as you would expect was the most forgiving but still needed PC input to make solid hits past 800 yards if the angle was more than .90. If the angle was less than .90 even out to 1300 yards I could still make good hits.

In the end the solution is fairly simple always use the PC at angles over .90 (cos), distances over 600, have the printed angle sheet for your best guess on hunting conditions when the PC doesn't work and shoot something with as much ballistic advantage as you can. As always do everything you need to make the shot the best you can or pass on it.
Also I will post a picture of the new NF angle indicator and mount that prompted all this very soon, the mount is the nicest base mount I have seen.
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  #2  
Old 06-06-2007, 12:38 PM
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Re: Up/Downhill corrections

Shawn,

I look forward to the pictures of the new Nightforce AI and mount.

When I'm shooting at angles, I've always used a Mil-Dot Master. You can use it to find your angle and then after finding the angle it will tell you the compensated distance for that angle, via it's analog calculator.

Devin
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Old 06-06-2007, 03:11 PM
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Re: Up/Downhill corrections

Using the corrected distance method no mater how you get to corrected yardage will still make you shoot at least little low. If the angle is slight and or shooting a very flat rifle you can still get hits based on this method out to a cretain yardage, but if the angles get steep or long you will shoot low.
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Old 06-06-2007, 04:40 PM
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Re: Up/Downhill corrections

Shawn,

Good tenacity! That's what separates the wheat the misses from the hits.

Hey, bb, recall those extra clicks you cranked in. Good ju ju!
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Old 06-06-2007, 07:52 PM
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Re: Up/Downhill corrections

Roy,

It was somewhat embarassing to find that I had been teaching was not technically correct (or correct in certain applications). The method I learned and subsequently instructed worked within the limited ranges I had used it so I was none the wiser. I am glad to have gotton it squared away and able to pass it on to everyone that is interested.
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Old 06-06-2007, 09:41 PM
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Re: Up/Downhill corrections

Shawn, it takes a big man to admit mistakes. I for one thank you for it. Looks like it's time to buy the pocket pc... I look forward to seeing this new angle indicator from NF. Are they currently for sale? Thanks. -- Don Ferguson
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Old 06-06-2007, 09:53 PM
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Re: Up/Downhill corrections

I guess I'm not following your train of thought here Shawn. Are you saying that multiplying the distance by the cosine and looking at a chart is not as accurate as doing the same thing only punching it into the pocket PC? What if your chart is printed from the same Exbal program that is on your pocket PC? While shooting many targets at angles dialed in off of the pc's suggestion, I have never hit low as a cause of this. I would suspect that the Exbal program would equate the TOF factor and recalibrate the triangular model. If it does not, the difference must be so minuscule that you would not be able to distinguish that factor from all the others we have to take into consideration.

For all intense purposes, I think telling your students what you have been telling them is correct enough that they will never miss an angled shot on big game unless they are shooting over a mile supposing that everything else is in harmony.
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