Up/Downhill corrections

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Shawn Carlock, Jun 5, 2007.

  1. Shawn Carlock

    Shawn Carlock Sponsor

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    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.
     
  2. steelhead

    steelhead Well-Known Member

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    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
     

  3. Shawn Carlock

    Shawn Carlock Sponsor

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    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.
     
  4. royinidaho

    royinidaho Writers Guild

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    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!
     
  5. Shawn Carlock

    Shawn Carlock Sponsor

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    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.
     
  6. Just Learnin

    Just Learnin Well-Known Member

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    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
     
  7. goodgrouper

    goodgrouper Well-Known Member

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    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.
     
  8. Eaglet

    Eaglet Well-Known Member

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    Goodgrouper,

    This is what Shawn is saying if I may:

    Let's say your shooting at a target at 800 yards and your ACI says the cosine of the angle is right at 0.90 . You have a ballistic chart that you printed at home for level shooting, for horizontal shooting. Now because you're shooting on an angle, for one of the methods we do this:
    800 X 0.90 = 720 yards. Now in the chart we look for 720 yards on the horizontal and tells us this...

    [​IMG]

    The chart says that for the corrected horizontal distance of 720 yards you need to go up 15.9 MOAs.

    Now using an angle of 26 degrees for which the cosine is 0.90 and letting the program calculate the right ajustment for those same conditions we find out that we need...

    [​IMG]

    16.4 MOA's which makes for a difference of 0.5 MOA at 800 yards... boils down to your bullet impact will be 4.2 inches low; which is what Shawn was saying.

    This error is just for 26 degrees, the steeper the angle and or longer the distance the error will multiply big time.

    Yes, if you imput in Exbal all the info including the 0.90 for the cosine of the angle, you'll get the right info.

    Now on the original Horizontal ballistic chart that you printed at home, if you check at 800 yards it calls for a correction of 18.6 MOA; we know it would be too much because we're shooting at an angle, so the other method we use would be you take the 18.6 X 0.90 = 16.7 MOAs which is much closer to what it should be; it will impact 2.5" high in this particular example. Shawn reffered to this method saying it was more accurate, and he's right! We need to remember that all this is at 26 degrees, not 40 or 60 which would make it easy to miss. Again, the most accurate method is the one in which you imput all the info into Exbal.

    Hope this helps.
     
  9. Shawn Carlock

    Shawn Carlock Sponsor

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    Here is a sample I shot to confirm all of this: 1276 yards, downhill .90, 3630 ft., 62 degrees, 55% etc., I have shot with my 338 Edge (of course). If you look at standard flatline drop for 1275 yards you need 33.50 moa. If you correct the yardage with the cosine to get 1148 yards and get dope for that distance you need 28.50 moa. If you apply the .90 cosine to the dope for 1275 yards you get 30.00 moa. The exbal program gives you 32.00 moa even. When I shot this I needed 31.75 moa. The applyed cosine methods will result in a low miss of 26" and 47" at this distance. So as you can see any method other than using the PC will make you shoot low at this angle and distance with this caliber. The printed Exbal angle chart like the one above is what I now use for a backup to the PC. I still need the ACI but apply it to the PC. Applying the cosine to anything else is not as accurate and on some distances & angles it can be quite a difference.

    Let generate one more example take the 308 Win 175 gr SMK match round. 800 yards, 45 degrees (.71 cos), 0', 78%, 29.53 bar, 59 deg temp.

    Flatline dope for 800 yds 26.50 moa
    800 x .71 = 568 yds dope for 568 15.75 moa
    Dope for 800, 26.50 x .71 = 18.75 moa
    Exbal generation 17.50 moa

    This is more of the original post the cosine applied to the distance always generates low dope, cosine applied to dope for the yardage can give both high and low dope depending on distance angle and the round being used. Like I said above out to a certain distance depending on caliber any of the methods will work for hunting / field shooting, but after a certain point you have to use the PC or Exbal generated sheet to be accurate enough to make good solid first round hits. The simple fact is that while applying cosine to drop or distance will get hits out to a distance it is not the most accurate way to do it and I just wanted everyone to be aware of the margin of error in the other methods. If you don't shoot past 600 yards this info may mean nothing to your hunting performance.
     
  10. steelhead

    steelhead Well-Known Member

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    So are you guys using the Nightforce program or the Exbal? And are you using it on a Laptop or a Pocket PC? Out in the field it would be easier to use the Pocket PC, but I've heard you get a lot more features with the PC version of the program.

    Thanks,

    Devin
     
  11. royinidaho

    royinidaho Writers Guild

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    My mind is mush! I comprehend it all but in an actual shooting situation there is way too much to think about. It looks like there is no free lunch here.

    Exbal and Pocket PC/Palm is the way only way to go! At least for this old knoggin!

    In fact it appears futile to even practice on LR rocks with out it. I'd just be establishing bad habits and planning for a poor first shot followed by an even more poor second shot..........
     
  12. edge

    edge Well-Known Member

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    I don't have quite enough data from your post, but if I make some guesses /ubbthreads/images/graemlins/smile.gif

    I don't have Exbal but I am using SMI Shooting lab.

    2850 fps
    BC 0.768
    Sight Height 2.0"
    100 yard Zero

    1275 = -33.68 MOA ( this seems close enough)

    I don't think that for angles that that is the number you need!
    IMO the number is from the drop column.

    DROP = 501.14 inches
    501.14 x .9 = 451.01 inches

    451 inches of drop equates to 1220 yards
    1220 yards requires 31.42 MOA

    Since this is the only example I ran I don't know if this is right or not.

    edge.
     
  13. Supermag

    Supermag Well-Known Member

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    I think the variation that you guys are seeing from using a printed chart vs. a computer program stems from a couple of minor details (we all know that the little things get you at extended ranges).

    The first is rounding errors, having no experience with the NF cosine indicator, I would guess that even a reading of .9 is not as exact as it could be (how many numbers can you fit on one of these things?). For example, I noticed that someone used 26 degrees for an angle. The cosine of 26 degrees is actually .898794046.

    Second, if you use radians instead of degrees (how cosine was designed to be used) you get a more accurate cosine measurement. For example, converting 26 degrees to radians you get 0.453785606 radians. The using the cosine function to this gives you a measurement of cos=1.099787373. This would actually indicate a shallower angle and a flatter shot than using a cosine indicator, therefore raising your point of impact.

    Wow, now I'm starting to confuse myself but will someone with exbal put in these numbers to check the calculations to the actual outcome.
     
  14. Shawn Carlock

    Shawn Carlock Sponsor

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    [ QUOTE ]
    The using the cosine function to this gives you a measurement of cos=1.099787373. This would actually indicate a shallower angle and a flatter shot than using a cosine indicator, therefore raising your point of impact.


    [/ QUOTE ]

    How do you get a cosine function of over .9999 to work? If you apply a cosine of over 1 it will certainly make you shoot higher, as in way over the target. Your corrected dope like this would be more than the lazered distance to the target by .09978.... How in the world would tht work? To safe guard against this the Exbal program won't take a whole number. I am sure this is not a rounding errors issue the rounding errors won't even account for .25 moa and we have errors of several MOA. The core of the issue is that cosine applied methods do not account for additional time of flight on corrected yardage. There are other ballistic issues not accounted for with cosine applied correction as well but I won't get into them. My key issue to the post is that cosine applied correction is not as accurate as the Exbal program no matter how you apply it, but out to a certain distance it will work well enough for certain shots beyond that you better have a PC or a generated angle drop sheet to work from.