# Applied Ballistics correction value math help!!

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by bigngreen, May 30, 2012.

1. ### bigngreenWell-Known Member

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OK, I thought I had this all straight but then I started reading and figuring and one thing lead to another and now I don't know what I know.

I tested an optic by turning it up 120 clicks which puts me at 30 whole numbers of some value I'm trying to determine, the optic moved 31.5 inches so I took and divided actual movement of 31.5 inches by dialed movement of 30 and it came out at 1.05 which I'm thinking is what my actual turret movement is in since it was done at 300ft and MOA should be 1.047 at that distance. So I'm thinking that's .003 over correcting at 300ft for MOA and so the correction factor would be 1 minus .003 which is .997, but then I read the directions and they seem to have you dividing dialed value by actual value and that is your correction factor which comes out as .952 so I think that's it but it does not look right!

JacknSD put it like this in another thread trying to help but my brain ain't gettn it so I though I best move to another thread!! Maybe I need the why you did it the way you did written out? Thanks for helping!!

"If you meant 30 MOA moved it 31.5 inches at 100 yards then it would be: .997

30*1.0471996=31.415988

31.415988/31.5= .997

So almost no correction seems to be needed. "

2. ### KRPWell-Known Member

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What is the claimed adjustment value of the scope? The dialed adjustment and actual adjustment numbers in your equation need to be in the same unit of measure, MOA or inches...just both the same. If the scope is labeled in MOA don't worry about it, unless the rifle can shoot and you can accurately measure groups that are in the thousandths.

3. ### EagletWell-Known Member

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Friend, you either still know what you know or it makes two of us that don't know
what we know!

Here is part of the instructions: For example, if you've come to realize your .25MOA per-click scope is actually .23MOA per-click then you'd put a correction factor of 1.08695652 (.25/.23) because you actually need to adjust more

Needles to say, (The application will always multiply by the correction factor), so if we fall short the correction factor needs to be higher than 1.0 and if it falls on the high end it needs a number smaller than one. (This is more for me to understand what I'm saying. )

See instructions portion and it makes sense.

In your case, you dialed 30 MOA and obtained 31.5/1.047198) = 30.080MOA

Then our correction factor has to be less than "one" 1.0

30/30.080 = 0.997 Now, we both know what we know!!!
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Another way to look at it:

you said:

I tested an optic by turning it up 120 clicks which puts me at 30 whole numbers of some value I'm trying to determine, the optic moved 31.5 inches

31.5"/1.047 = 30.080 MOA

30.080 MOA / 120 CLICKS = 0.2506667 MOA per click... we need a correction factor
smaller than 1.0 ---->(0.25 MOA/ 0.2506667 MOA) = 0.997. And again
we both know what we know.
***********************************************************

You also said: "So I'm thinking that's .003 over correcting at 300ft for MOA"

That's correct; it's over correcting 0.003" per MOA.

3 Thousands of an inch??????? At 100 yd????? NO CORRECTION NEED.

As a matter of a fact, you're good at doing that, you should come over and check
my scopes.

JacknSD is right, NO CORRECTION NEEDED!

BignGreen, what optics are those? Is it a NightForce?
***********************************************
It could also be that the three of us don't know garbage! Don't know what we
think we know!

4. ### BryanLitz<b>Official LRH Sponsor</b>

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I agree with the answers already given; your turrets are marked in MOA, and you need no correction factor being that close. It would amount to about 1/10th of one click over 30 MOA.

But just for the sake of clarity, you did the math properly, and the correction factor should be slightly less than 1.0.

Maybe I'll take some time and elaborate in the instructions with an example of this calculation.

Take care,
-Bryan

5. ### EagletWell-Known Member

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bsl135,

I'm starting to like you!

6. ### Bart BWell-Known Member

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Here's what I've done to check how much a scope's click moves point of impact.

Clamp your scope solidly with it at maximum power aimed and focused at a ruler exactly 50 yards away from the front of the scope and at exact right angles to the line of sight. Make sure the ruler's aligned straight with a reticule wire. Align the reticule on an even inch line. Count the clicks as the reticule moves to another inch line 2 inches away.

If it takes 17 clicks to move the reticule exactly 2 inches at 50 yards, each click's worth .2353 MOA. Check both elevation and windage to verify they're both the same. If 17 clicks don't put the reticule exactly on the ruler 2 inches away, keep moving the adjustment in the same direction until it aligns exactly on an inch line; counting clicks all the way. Then divide the inches moved by the clicks to get there. 1 MOA in the shooting sports is exactly 1/2 inch at 50 yards. But some say it's worth .5236 inch. Whatever.

The main thing is to find out how much the scope moves impact for each click. Then you can convert that to whatever angular unit of measurement you want.

Note that all scopes of the same make and model will have a small tolerance in their adjustments. That's 'cause their lens elememt's focal lengths vary a small percentage and that effects the target image size in the reticule plane; longer focal length of the objective lens system makes larger images, shorter ones make smaller images. While the same make and model all move the reticule tube the exact same amount per click, If the target image on the reticule is 5% bigger than spec, your click value will be 5% less than spec. And sometimes, the mechanics of a scope are not exact and that causes more errors.

7. ### LouBoydWell-Known Member

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To add to the confusion the angular image scale of a riflescope is not constant with the focus adjustment. The optical equation for the focal spacing of a simple lens (like the objective of a telescope) is 1/d1 +1/d2 = 1/f and the image scale is at the target compared to the scale on the reticle is d1/d2. Related to a riflescope f is the focal length of the objective lens, d1 is the effective spacing of the objective lens to the target, and d2 is the effective spacing of the objective lens to the reticle. If d2 was constant, then the image scale would be exactly proportional to d1, But d2 changes as you focus a scope to new parallax settings as the target distance is changed. So the distance you choose to set up your scope will affect the observed image scale, d1/d2. The error is not linear with distance d1 and becomes greater at shorter distances as d2 becomes longer. The difference between d2 and f is too small to notice at reasonable shooting distances. I think that's what causing bigngreen's confusion. Where the riflescope manufacturer decides to make the image scale "correct" is not determined by the rules of optics. That's just the manufacturers choice limited by tolerances of the optics that are used.

There's also a question of where you're measuring the target to scope distance. To the objective lens or to the reticle, or somewhere else. .003 error at 100 yards is equivalent to a 10.6" spacing measurement error. I'm not saying that's the source of the .003 error, only that the calculations aren't quite as simple as are being assumed in this thread.

I personally think it's nit picking. Other sources of error like bullet velocity uncertainty, difference between actual drag curves and the G() functions, and uncorrected wind deflection residuals are generally greater error sources.

Last edited: May 30, 2012
8. ### Bart BWell-Known Member

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I've never had a rifle scope that focuses to any parallax settings. Nor do I think any scope does that. Do parallax settings have any unit of measurement? Does parallax stay the same once set? I don't think so.

Parallax is apparent movement of the reticule on the target image caused by the shooter's aiming eye being off the scope's optical axis behind the eyepiece and the scope's not focused on the target. When the aiming eye's on the scope's optical axis, there will never be any parallax between target image and reticule regardless of what range the scope's focused at. And parallax varies with how far the aiming eye's off the optical axis as well as the difference between target range and the range the scope's focused at. The only thing that adjusts where the eye's at is the shooter moving his eye. And his eye ain't connected to the scope's focusing adjustment.

9. ### bigngreenWell-Known Member

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OK, I think I got it I agree it's a small value but I can measure it and verify it so why not, it was not long ago when Coriolis was the big non issue and guys shouldn't worry about it but when you look at it is a factor that one can dial so it needs to factored in when ELR shooting. If I'm shooting far enough to see a difference in the correction the program gives me so I want to nail it down.
I tested optics that were way more of, this Sightron Slll is the tightest I've tested so far.
My test rig allows me to set up the optic in the rings it's going to be mounted one the rifle with so I can test the value of the turrets, set it to dial vertical and lock it in the rings then set the scope level then transfer it to a rifle ready to rock.
The rig has allowed me to dial up and down 80+MOA and check repeatability multiple times with accuracy at 300ft from the target face to the center of the turret, I repeated the test at different power levels and it does not change the value.
I tested an optic that was way of but I've worn 4 barrels out shooting under it and I know what it is, in a half hour with no shots fired I tested it and it came out to the same value I've been using from trial and error. So if I test every optic I instal it will just make life go easier and barrel life can be used on important things!!
I know if your value is of you'll fight velocity and BC trying to get the trajectory right but it will screw with you till you find the value!!

10. ### MikecrWell-Known Member

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I think it's important to ballistically account for scope adjustments in the field(for single shot kills).
So I test adjustments & set level like bigngreen.
So far I've found (3)Mk4s to be MOA(to the best of my measure), and (2)NF NXS to be ~1.09 IPHY.

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