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# Slope angle shooting at Long Range.

#1
10-07-2009, 10:37 PM
 Platinum Member Join Date: Nov 2008 Posts: 1,087
Slope angle shooting at Long Range.

Ok,

There is something that I have been wondering about when taking long shots where some slope angle is involved. It involves the turret adjustment in MOA to compensate for the slope angle.
If I tilt my scope at a 20 degree angle, and adjust my scope 25 MOA. Then I am getting 25 MOA of adjustment perpendicular to the slope plane, but I am only actually getting 23.5 MOA of adjustment in the vertical plane.

Now my Exbal program will give me a corrected turret setting for the slope distance. But is this corrected setting only taking into account the slope distance or does it also take into account the fact that the scope is also now at a slope angle?
With the scope being at an angle to the horizontal, the true vertical turret movement should always be different than if the scope was horizontal.
Is this confusing you? because it is confusing to me.

Eg. If I was shooting at say 800 yards, on flat country then Exbal might give me say 26.5 MOA of adjustment.
If the 800 yard distance was down a slope of 20 degrees, then Exbal would give me a slope corrected setting of 24.75 MOA (i.e. for a slope corrected distance of 752 yards).
Now if I punch in the same 752 yards as if it was a flat distance, then Exbal only gives me a setting of 23.5 MOA.
So the question is, where did the extra 1.25 MOA of adjustment come from? Is it a correction for the scope slope?

If this is the case then using an angle cosine indicator and simply reducing the slope distance to the horizontal in the field is sure to give you incorrect turret settings for most long shots at any sort of steep angle.
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#2
10-08-2009, 12:45 PM
 Silver Member Join Date: Jan 2005 Posts: 338
Re: Slope angle shooting at Long Range.

Topshot, even though the 800 yd 20* sloped shot is corrected to 752 yds because of gravity effect, the bullet is still traveling 800 yds with the effects of air drag and time of flight.

The flat 752 yd shot has less time of flight and less time for the air drag to effect it's drop.

I may not have the terminology correct but I think it is the basis for what you've noticed in the Exbal programing. ---- RHB
#3
10-08-2009, 01:25 PM
 Platinum Member Join Date: Jul 2007 Location: Arkansas (Home of Record) Posts: 1,929
Re: Slope angle shooting at Long Range.

+1, The bullet will still have the time of flight as with a 800yard shot and so drag and wind will affect it accordingly.
#4
10-08-2009, 04:46 PM
 Gold Member Join Date: Sep 2009 Location: Washington Posts: 782
Re: Slope angle shooting at Long Range.

I was thinking of a problem similiar to this. Lets take the 800 yard shot. it is corrected to 752 yards as stated. How about windage. Do you figure your windage off the corrected 752 or the 800. By using the logic above I assume since it still travels the 800 yards you would use that distance to determine the windage and not the corrected 752, right.

Last edited by pyroducksx3; 10-08-2009 at 04:47 PM. Reason: misspelling
#5
10-08-2009, 06:45 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,830
Re: Slope angle shooting at Long Range.

Quote:
 Originally Posted by Topshot;319506[FONT=Arial Now if I punch in the same 752 yards as if it was a flat distance, then Exbal only gives me a setting of 23.5 MOA.[/FONT] So the question is, where did the extra 1.25 MOA of adjustment come from? Is it a correction for the scope slope?
Lets not forget that the MOA value/inch relationship is different from 752 yards versus 800 yards.

EXAMPLE 1 MOA at 752 yards = 7.87"

1 MOA at 800 yards = 8.38"

Say a bullet trajectory is 124" at 800 yards at 0 degrees. If you add a 20 degree slope and it has a 12" cut it falls 112" @ 800 yards. The bullet still travels 800 yards even though it acts like it travels less. For your scope to work properly you still compensate for 112" at 800 yards. If you use the yardage it acts like on level ground youre screwed. This is where the extra 1.25 MOA is coming from.

Clear as mud??
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
#6
10-08-2009, 10:18 PM
 Platinum Member Join Date: Nov 2008 Posts: 1,087
Re: Slope angle shooting at Long Range.

So if I take my scope off my rifle and put it on the kitchen table. Tilt the scope 20 degrees and dial up say 25 MOA. What do I get?

I get 25 MOA of adjustment in the same plane as the scope but only 23.5 MOA in the Vertical plane.

So the way I look at it. A field tactical shooter who is using a simple angle cosine indicator and reducing the slope distance back to a horizontal distance. Then treating the shot as a simple flat 752 yard shot should miss the target low due to the error in not taking the slope of the scope etc into account.

Am I barking up the wrong tree on this?
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#7
10-09-2009, 11:03 AM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,830
Re: Slope angle shooting at Long Range.

Quote:
 Originally Posted by Topshot Am I barking up the wrong tree on this?
Yes.

You cant treat it as a 752 yard shot.

Lets say a given bullet drops 152" at 800 yards and 124 @ 750 yards.

Then add the 30 degree slope. 800 becomes 124" of drop. Yes the drop in inches is the same as 750 yards on a level plane. However if you treat it as a 750 yard shot using MOA youre going to be off. 124" at 750 yards = 15.75 MOA. 124" at 800 yards = 14.75 MOA regardless of slope. The inches of drop is the same between the two yes. The MOA used to compensate is NOT the same. The bullet's path changes due to the slope as you know. Those changes are not compatible with any distance other than the line of sight to target. You still have to figure your MOA from the inches of drop at the range fired. The MOA to inches of drop relationship is different for every range even if the inches of drop was the same from range to range.

I understand what youre saying about the scope being less than 25 MOA on the vertical plane when adjusted to 25 MOA on a slope not 0 degrees. This is not the issue. Regardless of slope, when youre looking at a target at 800 yards line of sight your scope when moved 25 MOA will move the reticle 25 MOA at 800 yards in relation to the target regardless of where the bullet impacts. When the bullet impacts at less drop than normal due to this slope you compensate accordingly. When the bullet falls 152" at 800 yards the appropriate MOA is 18.25. When it falls 124" the appropriate MOA becomes 14.75. The slope doesnt matter at this point. The slope dictates a bullet trajectory. Even though the slope will cause a bullet to drop less than a shot at 0 degrees the line of sight distance is NOT null and void for calculations. Line of sight is line of sight, inches are inches and the MOA at range is MOA at range.

I hope that helps.
__________________
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

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