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Up/Downhill corrections

 
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  #8  
Old 06-07-2007, 04:29 AM
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Re: Up/Downhill corrections

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...



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...



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.
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  #9  
Old 06-07-2007, 07:43 AM
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Re: Up/Downhill corrections

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.
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  #10  
Old 06-07-2007, 08:38 AM
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Re: Up/Downhill corrections

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
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  #11  
Old 06-07-2007, 08:42 AM
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Re: Up/Downhill corrections

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..........
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  #12  
Old 06-07-2007, 09:02 AM
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Re: Up/Downhill corrections

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

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.
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  #13  
Old 06-07-2007, 09:33 AM
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Re: Up/Downhill corrections

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.
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  #14  
Old 06-07-2007, 11:13 AM
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Re: Up/Downhill corrections

[ 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.
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