How much does the angle of the target and/or angle of the scope matter on a tall target test? (Do the scope and target need to be perfectly parallel?)

[richthe1]

I went to my scope manufacturer when I found my scope to have 7% error after conducting a tall target test. I set the test up similar to how Lowlight’s demonstration: https://www.snipershide.com/snipers-hide-scope-calibration/. However, the scope manufacturer said tall target tests in the field can’t really be done accurately, they have to be done in a laboratory environment because you have to get the target and the scope perfectly parallel to each other. I don’t remember the exact percentage he said, but it was something like every 3% angle difference between the scope and the target will cause 1.8% error at 100yds (Honestly, I can’t remember the numbers, but I remember being surprised about how an minimal angle different would account for a significant error). I honestly don’t know how to do the math behind this to check it.

Any feedback would be much appreciated! I’d love to continue tall target testing my scopes, but if this is true how will I ever know if the error is in my scope or was it the way I set the test up?

 

[SLG]

I don't know for sure, but that amount of error has been seen by at least a few people in a few places. I believe that Ronnie Barrett has seen that at his facility for a long time now. NF has also seen this issue, and one of them told me about it, as I was having a similar issue at 100 and he thought it might account for it.

 

[_Windrider_]

It needs to be level and plumb.

 

[Gunfighter14e2]

Different colors will effect people eyes differently in bright light as well. Always use flat dull colors

 

[gunsnjeeps]

Since angle up or down requires aiming below the target doing this on a target that approximates range would add that angle into you estimates. Changing that angle and multiplying the distance would multiply the error.

 

[TripleBull]

I don’t remember the exact percentage he said, but it was something like every 3% angle difference between the scope and the target will cause 1.8% error at 100yds (Honestly, I can’t remember the numbers, but I remember being surprised about how an minimal angle different would account for a significant error). I honestly don’t know how to do the math behind this to check it.

Maybe the first thing to do is dig back in and find those numbers. Are the units degrees? A 3 degree error is pretty large. If they are degrees, then you should be able to work at better tolerances than that. Use good tools and be systematic in setting up your target plumb line, and likewise for your scope level.

If the units are percent, then you need to find out which percent standard is in use. You can find scientific references where 360 degrees = 1%, so 3% would be about 0.8 degrees, With careful setup and a digital level, you can work at tolerances of less than a half degree. You can find people that claim they work at tolerances of 0.1 degree, but I'm not convinced.

 

[richthe1]

Maybe the first thing to do is dig back in and find those numbers. Are the units degrees? A 3 degree error is pretty large. If they are degrees, then you should be able to work at better tolerances than that. Use good tools and be systematic in setting up your target plumb line, and likewise for your scope level.

If the units are percent, then you need to find out which percent standard is in use. You can find scientific references where 360 degrees = 1%, so 3% would be about 0.8 degrees, With careful setup and a digital level, you can work at tolerances of less than a half degree. You can find people that claim they work at tolerances of 0.1 degree, but I'm not convinced.

I’ll see if I can email the company to see the numbers they stated

My target was plumb (I just used a 4ft level for the target) and my scope was level.

Sorry if I didn’t explain that well. Does this drawing help illustrate the point?

At the top where both are the scope and the target are 90 degrees, and assuming that they are directly parallel to each other if the scope dialed from 0MOA 30MOA, his scope should have dialed 31.41” (30MOA*1.047=31.41”) down on the target.

However, in the second case, the target is angled slightly back at a 3 degree angle. Using the numbers I gave (I will email to see if I can get the scope manufacturer’s exact numbers) will appear to cause a 1.8% error in scope tracking, but it’s really just because of the angle of the target. Instead of dialing 0 to 30MOA and coming 31.41” down, it will instead appear to have 1.8% error. 1.8% of 31.41” is 0.57”, so you would get 31.41”+error (0.57”)=31.98”. (You might subtract error in this case instead of add, but hopefully this helps explain my point).