Range Report Determining exact sight-height for ballistic calc?

[bp78]

<span style="color: #CC0000">The root question:</span> <span style="font-style: italic"><span style="font-weight: bold">So for true sight over bore height measurement, do you measure at the objective lens or the point on the scope where the reticle is contain</span>ed?</span>

So I just got my new CZ 452 Varmint in .22lr last week. Dressed it up with a DIP 20MOA picatinny base and Mueller 4.5-14x optic, with TPS medium rings. Took it out to the range and got it on paper at 60 yards then immediately went back to about ~210 yards and was able to ring a 3" plate 2/3 shots, and the 8" plate 5 out of 5 shots consistently with just generic Federal Automatch bulk. Very little wind this morning, 60 yard zero put us with a ~5 milrad holdover at 210y and the wind was anywhere from 0.5 to 1.5 milrad right this morning. Great fun with a .22lr.

Now to the question, does anyone have a good simple method of accurately determining the scope height over the bore? It's an issue for the ballistic calculations in the JBM Ballistic software.

Asked on another forum, the easy answer is to measure from top of the scope to bottom of barrel. Subtract 1/2 of scope OD and 1/2 of barrel OD at the point measurement.

The point unanswered over on RFC, <span style="color: #CC0000"><span style="font-weight: bold">is if the front object lens the proper point to take measurements or better to use the point on the scope where the reticle is contained, or midpoint of the sco</span>pe?</span> Might make a nats-hair difference when you have a 20+ MOA base and big long optic mounted.

Thanks.

 

[jasonk]

Re: Determining exact sight-height for ballistic calc?

Have you ever put in one value and then calc'd the dope to 1,000 yards then changed the sight-height by .25" and done it again? Last time I tried it I seem to remember that it made hardly any difference at all even at 1,000 yards. There are 101 other reasons and ways to miss at that range

 

[azimutha]

Re: Determining exact sight-height for ballistic calc?

It doesn't make a lot of difference if you're not off my more than .25". To get close enough, you can just pull the bolt back even with the ocular and measure from the center of the bolt to the center of the ocular. Or, if you want to get really exact about it, use calipers to measure the diameter of the barrel right below the objective and then to measure the objective diameter. Divide those two numbers in half, add them together and then add the distance between them.

 

[bp78]

Re: Determining exact sight-height for ballistic calc?

Ok, fair enough. In my case, .22lr standard velocity at the far end:<div class="ubbcode-block"><div class="ubbcode-header">Code:</div><div class="ubbcode-body ubbcode-pre" ><pre>
Sight Height 1.0"
360yds -211.7 in -16.3 mil

Sight Height 1.5"
360yds -208.6 in -16.1 mil
</pre></div></div>

So 2/10's of a mil difference for a 1/2" in sight height difference at 360 yards.

Still would like to know which is correct in theory though.

 

[gstaylorg]

Re: Determining exact sight-height for ballistic calc?

Regardless of where the reticle is actually physically placed within the tube, if it appears to the eye to be centered within the sight picture/circular field of view, it seems as though the center of the objective lens would be the point at which you'd want to know height above the bore. Let me point out that this is just a guess on my part, but it seems to make sense from thinking about the following example.

Although the reticle crosshair center is typically aligned with the center axis of the tube, which is also centered with repsect to the objective and eyepiece lenses in most rifle scopes, it doesn't have to be. It might not be extremely useful in practice, but you could theoretically design a scope with an offset tube between the eyepiece and objective (like a periscope) using an angled prisms like those found in binoculars. What I'm thinking of would be something like putting the muzzle of a weapon through a really small hole in a concrete wall, then using the periscope-like optic to sight/range the target over the top of the wall, maybe a couple or three feet above. In such an extreme example, the objective center would be markedly farther from the bore than the reticle. The reticle (and the target) would still appear to your eye to centered with regard to the objective. Because critical angular relationship between the light coming from the target (sight picture) would be the distance from the bore to the center of the objective, I'd think that would be the critical measurement in such a case.

I have played around with sight height numbers in JBM and also found it doesn't seem to make all that much difference from a practical viewpoint (as long as your measurement is reasonably close). However, I would also be interested in hearing more about the actual theory involved in the OPs question from someone more knowledgeable about optics than myself.

 

[damoncali]

Re: Determining exact sight-height for ballistic calc?

The correct way is to measure at the muzzle. However, it's rather difficult to do that.

 

[gstaylorg]

Re: Determining exact sight-height for ballistic calc?

I'm assuming you mean distance between centerline of bore at the muzzle to line of sight through optic at same?

 

[damoncali]

Re: Determining exact sight-height for ballistic calc?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gstaylorg</div><div class="ubbcode-body">I'm assuming you mean distance between centerline of bore at the muzzle to line of sight through optic at same? </div></div>

Yes. ballistics calculators start with time = 0 and distance = 0 at the muzzle, the sight height is the vertical distance between the centerline of the bore and the line of sight (center of scope) at that point.

Since the angle between the scope and the bore is so small, it's good enough to just measure at the scope in whatever way it is easiest. And as has been stated, the effect is not very big. If you're talking iron sights, I'd measure at the muzzle because it's actually practical to do so.

 

[Rob01]

Re: Determining exact sight-height for ballistic calc?

Center of scope tube to center of bore which is usually close to stock height. As long as you are within .1-.2" you will be fine. I have done it the way above for years and it has worked from 11 feet to 1650 yards.

 

[jasonk]

Re: Determining exact sight-height for ballistic calc?

But what if you're shooting on a side hill, Rob?

 

[blklabs]

Re: Determining exact sight-height for ballistic calc?

Another way is to measure the bore diameter at the end of the scope and divide by 2. Measure the scope objective and divide by 2. Measure the gap between barrel and the objective. Add up all 3 and you will have your number.

Of course the easier way is to just do like Rob said and you will be fine.

 

[Rob01]

Re: Determining exact sight-height for ballistic calc?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jasonk</div><div class="ubbcode-body">But what if you're shooting on a side hill, Rob?</div></div>

Then I have to get old Cowboy and his magic bubble device to come shoot for me as I don't know how to shoot off angles