Range Report I need help with math to calculate scope error.

[MitchInOR]

OK, I used Lindy's method of using a 48" ruler at 100 yards to calculate the reticle and tracking error of my scopes.

Scope number 1: Weaver Tactical Grand Slam 3-10x40. Being a mil-mil based scope, 10 mils on the reticle should equal 36 inches on the ruler and 10 mils of erector travel should equal 36 inches also. This was correct with both measurements so this scope is correct.

Scope number 2: SS 10x42 mil-moa version. 10 mils measured 36 inches with the reticle so this is correct. According to Lindy's article 36 moa should equal 37.75 inches. My scope measured off 38.5, .75 inch off.

Since I am a dumbass at math how can I determine the percentage of error and apply this to various distances?

 

[gstaylorg]

Re: I need help with math to calculate scope error.

If I understand your question correctly, one way would be:
(38.5-37.57)/37.75 x 100 = 1.987 or ~ 2% difference between theoretical and measured values.

This corresponds to the difference between theoretical and measured values in inches (38.5-37.57), divided by the theoretical value in inches (37.75), multiplied by 100 to give percent difference.

As long as the +2% difference is constant throughout the range of the turret adjustment, you could use it as a correction factor if necessary. However, if I understand correctly what you did, for your scope that comes out to an overall difference between theoretical and actual adjustment of less than one click (0.25 MOA) in an overall turret adjustment of 10 MOA (40 clicks). Many factors including wind conditions and your relative shooting skill can affect your POI much more significantly than a +2% difference in tracking. For example, JBM calculates a drop of 11.0 MOA for an SMK 175 @ 2600 fps MV at 500 yd. The +2% difference you measured in the tracking of your scope would put you ~1.2" higher at 500 yd (ie. ~11.2 MOA). Is your shooting that good? My first thought would be to go to the range and shoot a lot of rounds with it at various ranges and determine whether or not you actually need to apply a +2% correction to your turret adjustments. In other words, you may not notice a +2% difference in practice. I'm sure there members here that do shoot well enough to notice the difference. Unfortunately, I'm not one of them (LOL).

 

[roggom]

Re: I need help with math to calculate scope error.

Just for sake of curiosity would like to see data from 50 and 200.

At 100Y there are 3.6 " per mil and 3.438 minutes per mil, therefore there should be 1.047 inches per minute...

36 moa x 1.047 = 37.692

Your scope tracked 38.5 so 38.5/36 = 1.069" per minute

So at 100Y 37.69 / 1.069 = 35.259 moa so for your scope @ 100, reduce elevation by 3 clicks

Get the other data and see if it tracks

 

[gstaylorg]

Re: I need help with math to calculate scope error.

roggom,
I think your math comes out the same as mine (ie. 1.069/1.047 = 1.021, or ~ +2%). The only question I have is simply, for what possible reason (other than to check scope tracking) would you ever dial 36 MOA (or 35.25 for that matter) at 100 yd? A 2% difference is not all that large, possibly even within tolerances for that optic, although I don't know that with any certainty. Using the hypothetical 175 SMK numbers I posted above, a 36 MOA adjustment is going to get you on at very close to 950 yd. Now I'll admit that a +2% tracking error at this range would put you just over 7" high. My only question is whether the OP actually shoots out to that range and/or whether a correction will really be necessary for type of shooting for which the scope will actually be used. If it's going to regularly be 500 yd and beyond, then probably so. If not, it probably won't be nearly as important a consideration as other issues like shooting fundamentals, wind conditions, etc.

 

[MitchInOR]

Re: I need help with math to calculate scope error.

Thanks for the all the help. So my scope has a 2% tracking error? My .308 load is 155 Lapua Scenar at 2900fps. JBM has me at 27.8 moa at 1000yd. A 2% error would be .55 moa off. So this would be 5.8 inches at 1000yds? Not really a significant problem, although farther out the numbers get bigger.

500yds is about the closest I shoot my 308 except when establishing zero. Last year I was shooting out to 1240yds with consistent results. I have since moved my farthest target out to 1275yds or 1325yds (depending on position). I am anxious to see if I can get hits at this range. I am just waiting for my new stock to arrive from McMillan.

The reason for this post is I have discrepancies between my field data and online and portable ballistic calculators. It appears my chronograph data is off and maybe a little can be contributed to my SS scope.

The past month I have been collecting field data out to 750yds with my AR15 and Weaver scope. Once I had come-ups for various distances it was obvious my fps was off. I wanted to check my scopes for error so I could eliminate that as a problem or correct for it.

Thanks again for the help.

 

[gstaylorg]

Re: I need help with math to calculate scope error.

You also might think about contacting SWFA just to see what they can tell you about their tracking tolerances for that scope. If 2% is way out of line, a fix might be in order. Good luck with it!

 

[TiroFijo]

Re: I need help with math to calculate scope error.

I don't think a 2% is out of line, unless you talk about ultra expensive scopes like S&B, etc.

 

[gstaylorg]

Re: I need help with math to calculate scope error.

That's good to know, I wasn't sure about the expected tolerances.