Rifle Scopes Tall Target Test. Check my math.

[powereng]

From Bryan Litz's video, I wanted to verify the click values of my scope and performed a tall target test. I started by shooting a 5 shot group at the bottom of a large vertical target. I then adjusted my scope for 10mils of elevation. I then shot another 5 round group, again aiming at the original aiming point of the first group. From there I measured the distance between the two groups on the target.

Formula:
Range x Adjustment x Correction Factor = Adjusted POI
1mil = 3.6” @ 100yds
Correction Factor = 0.036

Perfect World Scenario:
100yds x 10mils x 0.036 = 36”

Since I was not shooting at exactly 100yds, but rather 323' or 107.6 yards, my formula would be:
107.6yds x 10mils x 0.036 = 38.736"

Unfortunately when I ran my test my adjusted POI came out to 41.625”. Using the following formula:

Actual POI - Expected POI x 100 = Percent Error
Expected POI

Giving me a result of:

41.625” - 38.736” x 100 = 7.458% error.
38.736”

I ran the test with 5mils of elevation as well which in theory should give me:
107.6 x 5mils x 0.036 = 19.368” of spread.

However my actual spread was 21” or:

21” - 19.368” x 100 = 8.426% error.
19.368”

For those that have ran the same test, what did you come up with for percent error in your clicks. I'm trying to determine if I have an unusual amount of error in my clicks.

I have already contacted the manufacturer of the scope and they have been terrific! I have no doubt that they will take care of me. I just want a second opinion to see if my math and logic is sound.

Thanks, Powereng

 

[BryanLitz]

You're calculating the expected POI shift correctly (that's the hard part). But you're doing the actual correction factor wrong. The correction factor is Expected/actual

So in your 10 MIL example, the expected POI shift is 38.73", and actual is 41.625". The correction factor in that case is 0.93

In your 5 MIL example, the expected POI shift is 19.37" and the actual was 21". The correction factor in that case is 0.92, which is the same as 0.93 for practical purposes.

You know the CF should be less than 1.0 because you're scope is moving *too much*, and you need to cut it back.

Applying it back to your example, if a ballistics program called for 10 MILS, you would apply your CF of 0.93 and only dial 9.3 MILS to *actually* get 10 MILS because your scope moves too much.

Here's a worksheet that steps thru it: http://appliedballisticsllc.com/m-ftr/ABDOC123_TallTarget.pdf

Take care,
-Bryan

 

[sib1948]

Thanks Bryan. Thatmakes it much clearer!

 

[_Windrider_]

Bryan,

The fact that you give direct feedback like this is why I buy your products.

 

[jbell]

Bryan,

The fact that you give direct feedback like this is why I buy your products.

Same here...

Not to hijack the thread, but I would like to say thank you to Brian for all his info! I have read your books, precision shooting articles, and have your video. Although some of it was hard for me to digest and took a few rounds to get in threw my thick skull it opened my eyes to what is important in LR shooting. I really learned a lot from your data provided with hit percentages comparisons. That is a great way to represent the more important factors in shooting. Thanks - Jesse

 

[powereng]

You're calculating the expected POI shift correctly (that's the hard part). But you're doing the actual correction factor wrong. The correction factor is Expected/actual

So in your 10 MIL example, the expected POI shift is 38.73", and actual is 41.625". The correction factor in that case is 0.93

In your 5 MIL example, the expected POI shift is 19.37" and the actual was 21". The correction factor in that case is 0.92, which is the same as 0.93 for practical purposes.

You know the CF should be less than 1.0 because you're scope is moving *too much*, and you need to cut it back.

Applying it back to your example, if a ballistics program called for 10 MILS, you would apply your CF of 0.93 and only dial 9.3 MILS to *actually* get 10 MILS because your scope moves too much.

Here's a worksheet that steps thru it: http://appliedballisticsllc.com/m-ftr/ABDOC123_TallTarget.pdf

Take care,
-Bryan

Thanks a bunch Bryan! Your books and videos are awesome!

Since I don't have a lot of other scopes in this price point to compare to, do you think this is an acceptable amount of error? I ask only because I rather not send my scope off to be "looked at" only to have it come back the same because the error is engineered into the scopes mechanical erector. To put it plainly, is this type/amount of error inherent in all scope and I should just learn to live with it, or can it be mechanically corrected? Does that make sense? Being in the industry, I understand if you would rather not answer.

Thanks again.

 

[BryanLitz]

Thanks for the comments guys.

powereng, If I were you, I wouldn't send the scope back. I would keep it and remember to apply the CF to all predicted drop numbers. In fact I have a number of scopes that need CF's applied. It's not such a bad thing as long as you manage it. The only thing I might do is confirm the CF by re-measuring the target range (maybe with another instrument) and re-shooting the tall target test just to be sure of your CF.

Both Shooter and the AB Mobile apps have a place in the firearm profile to input a CF (it's called SSF in the apps: Sight Scale Factor) and it's automatically applied to all calculations so you don't even have to worry about it once it's set up.

-Bryan

 

[powereng]

Terrific! I run Shooter on my phone and saw the CF inputs so I should be all set. I plan on running a wide target test as well to see if I need to apply a CF to my windage.

Thanks again.

 

[nfoley]

I have done this with many of my scopes and also find most have a very small variance when dialed to 10 mil.
Nice job explaining the math in a simple manner.
Just curious, what scope were you testing?

 

[mdesign]

Thanks for the comments guys.

powereng, If I were you, I wouldn't send the scope back. I would keep it and remember to apply the CF to all predicted drop numbers. In fact I have a number of scopes that need CF's applied. It's not such a bad thing as long as you manage it. The only thing I might do is confirm the CF by re-measuring the target range (maybe with another instrument) and re-shooting the tall target test just to be sure of your CF.

Both Shooter and the AB Mobile apps have a place in the firearm profile to input a CF (it's called SSF in the apps: Sight Scale Factor) and it's automatically applied to all calculations so you don't even have to worry about it once it's set up.

-Bryan

Bryan - I have my CF but cannot find a place to enter the value in my Kestrel w/AB. I do not see SSF in either the unit or the AB Profile Loader on my PC. Am I looking in the wrong place?

 

[Chanonry]

Ok, similar theme. Is this logic correct?

I need to confirm this with a tall target test next week, but as a sighter i put up a vertical steel rule, checked with a level and ranged it with a laser at 51 yards (numerous times). Heavily sandbagged rifle. The reticle is NF MOAR.

I used the reticule to measure 20 MOA = 11.125" I also dialled 20 moa and got 11.125". I repeated this several times.

So at 100 yards I would have seen a measurement of 21.813" while a true 20 moa at 100 yards is 20.96. So both the reticule and the turrets are 'out' by a factor of 1.04069 of one click equals 0.2601??

Is that likely? It is at short range so there may be meaningful errors. On the other hand the distance I 'should' have measure was 10.679", an error of .446", and while the experiment may not have been perfect it was not that inaccurate.

I was just a bit surprised that the reticule and turrets were both suggesting the same error, which made me question the validity

Thoughts??

 

[powereng]

Chanonry

Just for clarification the tall target test is meant to test the accuracy of your scope clicks, not your reticle.

I'm going to assume for now that each click on your scope represents 1/4moa of adjustment at 100yards. 1moa = 1.047" at 100yards therefore 1 click equals a quarter that, so 1 click = 0.26175" at 100yards. So in your 51 yard example your formula would be:

51yards x 20moa x .01047 correction factor = 14.994" of elevation change

At 100 yards your formula would be:

100yards x 20moa x .01047 correction factor = 20.94" of elevation change

You can change the yardage and moa values based on your actual range and test clicks, you just need to make sure the correction factor is based on the actual click values of you scope (moa vs mils).

Just remember you need to aim at a point on the very bottom of your target and shoot a group. Then make your scope moa adjustment and shoot a second group, again using the same aiming point of the first group. You then measure the distance between the two groups on your target. So in your 100 yard, 20moa example your going to need a target taller than 20" to be able to see you second group impacts. If you need less spread at a given range, use less moa adjustment for your test.

Hope this helps,

 

[Chanonry]

I appreciate the response.

20 moa at 100 yards gives 50"?? With a .25" click 20 moa gives 20" at 100 yards, No?

 

[powereng]

Sorry Chanonry, I had to correct my math. I'm use to working in mils. Re-read my post now. Sorry for the confusion.

 

[Chanonry]

Sorry Chanonry, I had to correct my math. I'm use to working in mils. Re-read my post now. Sorry for the confusion.

Thanks, I had a head melt there for a minute...

You have identified an error in my sums. On the basis of my crude test, one click equals 0.278 not the 0.260 I had thought (Forgot that one click is 1/4 moa not 0.25')

It will be interesting to see if this stacks up on the tall target test

 

[woodland-ghost]

Thanks for the comments guys.

powereng, If I were you, I wouldn't send the scope back. I would keep it and remember to apply the CF to all predicted drop numbers. In fact I have a number of scopes that need CF's applied. It's not such a bad thing as long as you manage it. The only thing I might do is confirm the CF by re-measuring the target range (maybe with another instrument) and re-shooting the tall target test just to be sure of your CF.

Both Shooter and the AB Mobile apps have a place in the firearm profile to input a CF (it's called SSF in the apps: Sight Scale Factor) and it's automatically applied to all calculations so you don't even have to worry about it once it's set up.

-Bryan

I have the shooter app and in mine it has elev, and wind correction factor...I'm assuming that's it and it just got updated on the new iOS and doesn't say SFF anymore?? Just making sure in not missing it somewhere else. Going to finally run this test on my 3 main guns this weekend and finally fix this the "easy way" haha.

 

[gigamortis]

Thanks for the heads up on the elevation and windage correction factors in the Shooer app. I never knew what those inputs did until reading this post.

By the way, I'm running Shooter on IOS and its not labelled as SSF. Its labelled as elevation and windage correction factor.

 

[woodland-ghost]

Just to be anal about this test...do you measure distance from end of barrel, shooter position, or center of scope?

 

[powereng]

I measured from a fixed reference point on the concrete slab at my local range to my target which is also set up at a fixed reference point. This allows me to repeat the test at the exact same distance every time without have to re-measure. But to answer your question it was from muzzle to target.

 

[powereng]

I got my scope back from the manufactured and wanted to give an update to any that were following this thread. Plus I want to used this thread to document my results. Setting everything up as before I performed the same tall target test.

I ran the test again with 10mils of elevation which in theory should give me:
107.6 x 10mils x 0.036 = 38.736” of spread.

My actual spread was 38.625"

This time using the correct formula per Bryan Litz

Expected POI = Correction Factor
Actual POI

Giving me a result of:

38.736” = 1.002 CF
38.625”

I also ran the test again with 5mils of elevation which in theory should give me:
107.6 x 5mils x 0.036 = 19.368” of spread.

My actual spread was 19.375"

19.368” = .999 CF
19.375”

I think it's safe to say my scope is now tracking properly.