Rifle Scopes Scope slope angle corection

[wadcutter]

If my scope is at an incline of say 20 degrees fow does this effect the turret setting. ie a 25MOA dial up would only give me 23.5 MOA.............. Whats up?
Does Exbal etc take this into account?

 

[J.Boney]

Re: Scope slope angle corection

Your impact, whether incline or decline, will always be high.

Click on the link below and save it. Also, Lindy has an article link in the reply to the thread. Great Article...

http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=922474&page=2

 

[wadcutter]

Re: Scope slope angle corection

Ok,I had a look at the link but it realy didn't address my thoughts. I was thinking more along the line of the scope adjustments rather than just correcting the slope distance back to the horizontal.

i.e.
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.

Should shoot low I think?

 

[Lindy]

Re: Scope slope angle corection

You're confusing yourself.

The scope base is irrelevant, but, so you'll know, the base is angled in MOA, the same as the scope. But the scope base doesn't matter, if you have achieved a short-range zero like 100 yards.

So, forget the slope base.

For shooting on an angle, just use the scope setting calculated by Exbal. It's more accurate than just multiplying the actual range to the target by the cosine of the angle.

If you don't want to or cannot use Exbal, multiply your dope for the range to the target as if the shot were flat by the cosine of the angle.

In your example, if the shot were 800 yards at a 20 degree angle, and the dope for 800 yards was 26.5 MOA:

Cosine of 20 degrees is 0.94.

So, take 26.5 * 0.94 = 24.9

That's only 0.15 MOA off what Exbal calculates.

I expect the 23.5 MOA you got from Exbal resulted from your forgetting to take the 20 degree angle out when you put in 752 yards. Try that again.

 

[wadcutter]

Re: Scope slope angle corection

I think I am confusing myself?

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?

 

[Lindy]

Re: Scope slope angle corection

You're not barking up the wrong tree, but you're just slightly misinterpreting things.

See this reference:

Angled Fire Explained

 

[CoryT]

Re: Scope slope angle corection

The issue is not how much up you get in the horizontal plane, but how much up you get from the zero point, which for practical purposes is the 'horizontal' plane of the bore. Yes, the bore has a small (very) angle related to the scope, but it's too small to matter.

 

[Lindy]

Re: Scope slope angle corection

There are two angular components to a short-range - say, 100 yard - zero.

One is compensation for the height of the scope over the bore. That angle may be computed as:

angle = arctan (height above bore/distance to target)

In the case of my AI, for example, with a height above bore of 1.75 inches, at 100 yards (3600 inches) it's:

angle = arctan(1.75/3600) = .03 degrees or 1.67 MOA

The other component is the drop from the muzzle to the zero range. With my load at 100 yards, that's about 2.7 inches, or, using the same formula above, about 2.58 MOA.

Add those two up, and it's about 4.25 MOA or about 1.25 mil.

Which is probably more than anyone really wanted to know. But that's the amount of elevation which is consumed by a short range zero, which the interested readers, if there are any, can compute for their own setup.