Range Report Sight height & ballistic calculators

[philodox]

I've been using the center of the bore to the center of the tube on my scope for the 'sight height' number on JBM's ballistic calculator. Been doing it that way for years.

Have I been doing it wrong all this time? Where are you supposed to measure the distance from? Being 1/2 in off on the sight height can make the ballistic calculator way off. Thanks!

 

[ChadTRG42]

Re: Sight height & ballistic calculators

Yes, it can make a difference at longer ranges. Measure from the center of the scope tube to the center of the bore.
Mine is 2.25" from center to center, which is a little high. Most are around 1.75" with the bigger scopes.

 

[Lindy]

Re: Sight height & ballistic calculators

Center of the bore to center of the scope tube is exactly correct.

And, going from 1.5 inches to 1.75 inches makes a 2 inch difference at 1000 yards with my load on a standard day. That's 0.06 mils, less than 0.1 mils, which is as close as I can dial or hold.

So, your measurement is fine. It's highly unlikely that you could be a quarter inch off on that measurement.

Changing parameters in a ballistic program can help you find out what matters and what doesn't.

 

[The Mechanic]

Re: Sight height & ballistic calculators

When doing calculations always keep in mind cumulative error. Or if you are unfortunate enough to have all errors fall in one direction you will be off more than your worst error.

 

[Lindy]

Re: Sight height & ballistic calculators

Yes, of course.

Sources of Ballistic Program Inaccuracies

 

[The Mechanic]

Re: Sight height & ballistic calculators

Didn't mean that for you and Chad, Lindy. You two are way beyond my skill level and I think I am pretty darn good.

 

[philodox]

Re: Sight height & ballistic calculators

Oh I'm not off or anything. I just wanted to make sure I was using the correct measurements

Thanks guys.


And my measurements are 2.125" from center to center using a Premiere Heritage on a FN SPR.

 

[TiroFijo]

Re: Sight height & ballistic calculators

A normal sight height error has a very limited influence in the predicted trajectory donwrange, <span style="font-weight: bold">and the effect is the same regardless of caliber or load.</span>

If you have a 100 yds baseline zero, and a sight error X, the error at 200 yds is X, and then gets larger with distance, but even at 1000 yds is only 9*X.

Since most people can easily measure their sight height within 1/8", this means you have an error in the order of: 1/8" * 9 = 1.125" at 1000 yds, or 0.1 MOA, or 0.03 mils. This is less than a click and completely lost in the noise.

 

[Rob01]

Re: Sight height & ballistic calculators

Here's how I do it and it's always been on for me

 

[skandlus]

Re: Sight height & ballistic calculators

not tryin to be too anal, but if you used a 20MOA base wouldnt there be a difference between the eyepiece to bore and objective to bore... especially if the scope is 14"+???

 

[Lindy]

Re: Sight height & ballistic calculators

The difference between the front and back of a 5" 20 MOA base is 0.03 inches.

I've already noted that a quarter of an inch of that measurement doesn't make a difference that you can hold.

 

[skandlus]

Re: Sight height & ballistic calculators

cool, so I guess it doesnt matter what part of the scope the measurement is taken? at the eyepiece, at the turrets or the objective. am I correct?

 

[Lindy]

Re: Sight height & ballistic calculators

Rob posted a couple of pictures above of the measurement he uses. I do the same thing.

 

[Rob01]

Re: Sight height & ballistic calculators

Measure in the middle. It's basically the average of the .03" difference for you anal people

 

[TiroFijo]

Re: Sight height & ballistic calculators

It doesn't matter the objective height, or its projection at the bore due to a slanted base...

<span style="font-weight: bold">We aim the scope through the internal erector tube NOT the outer case</span>. If the scope is zeroed at 100 yds the erector is at the same height and angle no matter how many MOA you have in the base.

 

[Lindy]

Re: Sight height & ballistic calculators

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If the scope is zeroed at 100 yds the erector is at the same height and angle no matter how many MOA you have in the base.</div></div>

Yep.