Where to Measure Scope Over Bore

[Ledzep]

I've been asked this a few times over the years and a few months ago I realized that I have the perfect rifle to demonstrate this for the most accurate measurement possible. Last weekend I finally got it out and took some pictures and a video to prove the point.

So where does one properly measure the scope over the barrel to get the most accurate ballistic calculations? What if I have a 30 MOA base? Over the turret because that's where the adjustments are? Over the Objective lens? Over the ocular?

Answer: At the Objective lens.

Why? Because that is where the image you see is formed. You can move the erector around on that image all you want with the turrets, you can slant the scope up or down with angled bases, but where the image you see is formed is always from the objective lens.

This is a Swiss K31/42 sniper rifle with a Kern 1.8x scope dovetailed onto the receiver. What is special about these scopes is that the objective lens assembly is a rotating periscope. Rotate down for protection during movement/storage, rotate up for use. Behind the rotating objective assembly is functionally very similar or the same as a typical 2nd focal plane scope. The only curve ball is how the elevation is adjusted, but for this demonstration you could do the exact same thing by placing and rotating a periscope in front of a traditional optic and see the same thing for yourself.

Here is the rifle with a business card with a sharpie dot on it (just for a reference point) stuffed onto the muzzle.

A shot of the optic for reference

Here is the optic in the "up" position with the view through the optic overlayed

Here is the same shot with the optic in the "down" position (slightly obscured by the sling on the right)

And here's a video of me flipping it up (live action!)


This shows a direct relationship between where the objective lens is and the image generated through the optic that your eye sees. Again, you can move an erector around on that image and change the angle you look at the image, but you will not change the height that the image is generated from. That height is the center of the objective lens.

The best way I've found to measure that with most rifles is to remove the barreled action from the stock (remove handguards for AR's), measure the widest part between the objective bell and the O.D. of the barrel with calipers, then subtract half of the barrel diameter (at the point you measured if it's tapered) and half of the objective bell diameter.

The errors caused by goofing this measurement are usually pretty small, but they are still errors and you can eliminate them. Happy Shooting!

 

[Baron23]

I've been asked this a few times over the years and a few months ago I realized that I have the perfect rifle to demonstrate this for the most accurate measurement possible. Last weekend I finally got it out and took some pictures and a video to prove the point.

So where does one properly measure the scope over the barrel to get the most accurate ballistic calculations? What if I have a 30 MOA base? Over the turret because that's where the adjustments are? Over the Objective lens? Over the ocular?

Answer: At the Objective lens.

Why? Because that is where the image you see is formed. You can move the erector around on that image all you want with the turrets, you can slant the scope up or down with angled bases, but where the image you see is formed is always from the objective lens.

This is a Swiss K31/42 sniper rifle with a Kern 1.8x scope dovetailed onto the receiver. What is special about these scopes is that the objective lens assembly is a rotating periscope. Rotate down for protection during movement/storage, rotate up for use. Behind the rotating objective assembly is functionally very similar or the same as a typical 2nd focal plane scope. The only curve ball is how the elevation is adjusted, but for this demonstration you could do the exact same thing by placing and rotating a periscope in front of a traditional optic and see the same thing for yourself.

Here is the rifle with a business card with a sharpie dot on it (just for a reference point) stuffed onto the muzzle.
View attachment 8777745

A shot of the optic for reference
View attachment 8777749

Here is the optic in the "up" position with the view through the optic overlayed
View attachment 8777750

Here is the same shot with the optic in the "down" position (slightly obscured by the sling on the right)
View attachment 8777751


And here's a video of me flipping it up (live action!)


This shows a direct relationship between where the objective lens is and the image generated through the optic that your eye sees. Again, you can move an erector around on that image and change the angle you look at the image, but you will not change the height that the image is generated from. That height is the center of the objective lens.

The best way I've found to measure that with most rifles is to remove the barreled action from the stock (remove handguards for AR's), measure the widest part between the objective bell and the O.D. of the barrel with calipers, then subtract half of the barrel diameter (at the point you measured if it's tapered) and half of the objective bell diameter.

The errors caused by goofing this measurement are usually pretty small, but they are still errors and you can eliminate them. Happy Shooting!

Ah yeah, small bus to school and being pedantic is my super power.

Just so understand, you measure from top dead center of the end of objective bell to the bottom dead center of the barrel at that point then do the subtraction….correct?

 

[Ledzep]

Yep

 

[_Windrider_]

I disagree on a technicality. The solutions are based on the point of adjustment and the heavy over bore should be measured from the center of the turret. Now theoretically the center of the objective should be the same as the center of the turret but as long as we are being overly granular in this I believe the turret is the proper place to measure as is the same for measuring distance in theory. All ballistics are calculated at the point of adjustment.

Again though this is being ridiculously granular. Reality would tell you it doesn’t matter as long as you are accurate in the measurement

 

[JBoomhauer]

With a 30moa rail what would the difference in data be at 300, 600, and 1000 yards when measuring height from the Objective, the ocular, and at the turret? All zero at 100 in the app.

 

[lash]

Now theoretically the center of the objective should be the same as the center of the turret

Not at all if you have a tapered rail, which in long range shooting is much more common than not. That’s actually his point if you want it simplified.

 

[davsco]

I thought we measured at the chamber/turret, center of chamber to center of scope.

Under 'your' measurement, you get different results with a longer scope vs a shorter scope if your base is canted. It is more accurate though as you're not estimating the centers, especially of the chamber.

 

[_Windrider_]

Not at all if you have a tapered rail, which in long range shooting is much more common than not. That’s actually his point if you want it simplified.

That’s true. But again we are talking about differences that do not translate to actual measurable changes in the solution.

 

[lash]

That’s true. But again we are talking about differences that do not translate to actual measurable changes in the solution.

Is that true though? Especially for those shooting .22 long range or those shooting ELR? I’m just curious if you have data to back that up or are just guessing.

 

[_Windrider_]

Is that true though? Especially for those shooting .22 long range or those shooting ELR? I’m just curious if you have data to back that up or are just guessing.

I’m pontificating. I mean I suppose we could easily do some tests and see if it matters. I just don’t believe any human could make that micro difference the deciding factor in a hit or miss. I mean the turrets and holds in mils are generally 1/10. That difference has got to be smaller than that.

 

[lash]

I’m pontificating. I mean I suppose we could easily do some tests and see if it matters. I just don’t believe any human could make that micro difference the deciding factor in a hit or miss. I mean the turrets and holds in mils are generally 1/10. That difference has got to be smaller than that.

Okay, I get that. I do know that when shooting .22LR past 100 yards, like say 300+/-, scope height over bore errors can cause issues quickly with ballistic miscalculations. I’ll have to do some measurements on mine and see what differences manifest next time I’m out.

 

[davsco]

I’ll have to do some measurements on mine and see what differences manifest next time I’m out.

I just ran my ballistics with 1.25" over bore and 1.50", so a quarter inch difference, which is probably way more than it would be at the various measuring points (someone can do the math). At 1000 yards (6.5cm), it said a drop of 323" vs 321".

I guess you do need to shoot it to see which measuring point should be used. But again with tenth of a mil or quarter moa adjustments, probably gets lost in the rounding. Plus shooter error.

 

[Ledzep]

I disagree on a technicality. The solutions are based on the point of adjustment and the heavy over bore should be measured from the center of the turret. Now theoretically the center of the objective should be the same as the center of the turret but as long as we are being overly granular in this I believe the turret is the proper place to measure as is the same for measuring distance in theory. All ballistics are calculated at the point of adjustment.

Again though this is being ridiculously granular. Reality would tell you it doesn’t matter as long as you are accurate in the measurement

The adjustments you make on the turret move the erector tube inside the scope. The erector tube is "looking at" the image that is collected by the objective lens. Your dope corrections are angular adjustments that pivot off of the pivot point of the erector tube (approximately, there is some small parallax up close-- that effect really quickly diminishes with realistic range to negligible). The sight over bore is NOT the same subject as the adjustments you make for dope, though. It is a linear space reference point. That is why it's most important to grab it from where the image is generated. Again you can test this for yourself by putting a 4" periscope in front of your objective lens. Do you think your scope over bore will be more correct with the same measurement you have (~2-2.5"), or with that measurement + 4 inches?

Hint: There is effectively zero difference between a 4" tall periscope and 4" taller rings.

With a 30moa rail what would the difference in data be at 300, 600, and 1000 yards when measuring height from the Objective, the ocular, and at the turret? All zero at 100 in the app.

The slanted base changes the angle you are looking at, but the image is still collected from the objective lens. The objective lens is still the point to measure from.

I thought we measured at the chamber/turret, center of chamber to center of scope.

Under 'your' measurement, you get different results with a longer scope vs a shorter scope if your base is canted. It is more accurate though as you're not estimating the centers, especially of the chamber.

Consider this as a thought experiment. If you "fix" the objective lens center location at a nominal 2" over the bore, but you are able to pivot the scope off of that point (the center of the front of the objective lens). If I pitch it 30 MOA left, I can adjust the turrets 30 MOA right and see the same image. If I pitch it 30 MOA down, I can adjust 30 MOA up and see the exact same image. If I pivot it 30 MOA left I can adjust 30 MOA right and see THE EXACT SAME IMAGE.

Separate in your thought angular and positional offsets. The MOA on the base and the adjustments in the turrets are angular shifts. The physical height over bore ONLY matters relative to the objective lens because the image you see inside the scope, and the image that the erector tube "looks around at" is all collected from the objective lens.

 

[Ledzep]

I just ran my ballistics with 1.25" over bore and 1.50", so a quarter inch difference, which is probably way more than it would be at the various measuring points (someone can do the math). At 1000 yards (6.5cm), it said a drop of 323" vs 321".

I guess you do need to shoot it to see which measuring point should be used. But again with tenth of a mil or quarter moa adjustments, probably gets lost in the rounding. Plus shooter error.

The errors caused by goofing this measurement are usually pretty small, but they are still errors and you can eliminate them. Happy Shooting!

ETA: All small errors add up to bigger errors. Sometimes they cancel, some times they add. If you do your due diligence in mitigating/eliminating them you start seeing less and less random misses.

 

[Ledzep]

Another visual to think about. If you put a few prisms in the scope to offset a height, which one changes the effective height over bore? The objective or the turrets?

It doesn't matter where the turrets are. You can artificially exaggerate the situation with higher turret, but you can have nearly the exact same situation, only less severe with a 20 or 30 MOA base. Where the turrets are physically does not matter. Where the objective lens is, does.

The video and pictures in my first post directly show that the top situation (periscoped objective lens) moves the height over bore point of reference directly.