Rifle Scopes Verifying scope to bore centerline

O

on2wheels

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Full Member
Minuteman
Jan 17, 2011
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Phoenix, AZ
#1
I recently was advised to check the centerline of the scope to the bore for a set of rings that I am going to change out due to machining process of the rings. The new rings are Burris ETR and from what I have read on the forum are decent low end rings compared to others. The rifle is a Savage 10fp, 308.
Does anyone have a recommendation on how to verify that the scope, once secured in the rings, is in direct centerline with the bore?
I have an idea as to how one could evaluate this and that is by setting the 12 o'clock datum on the barrel and the
6 o'clock datum on the scope, once the scope is mounted and reticle verified for vertical alignment, or is this over thinking the process?
Any suggestions would be appreciated.
 
#2
Re: Verifying scope to bore centerline

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: on2wheels</div><div class="ubbcode-body">I recently was advised to check the centerline of the scope to the bore for a set of rings that I am going to change out due to machining process of the rings. The new rings are Burris ETR and from what I have read on the forum are decent low end rings compared to others. The rifle is a Savage 10fp, 308.
Does anyone have a recommendation on how to verify that the scope, once secured in the rings, is in direct centerline with the bore?
I have an idea as to how one could evaluate this and that is by setting the 12 o'clock datum on the barrel and the
6 o'clock datum on the scope, once the scope is mounted and reticle verified for vertical alignment, or is this over thinking the process?
Any suggestions would be appreciated. </div></div>

What I read is that you simply go out and shoot it.
Put a large target up and draw a large crosshair on it. Make sure to make them level and exactly perpendicular. You can do that with a long carpentry level or a plumb.

Make sure to line up your crosshairs with the lines on your target.

Take a shot at center (at the intersection)

Dial in some elevation and take a shot.

If your scope is perpendicular to the bore, then you will follow along the vertical lines. If not, then you will move off the centerline at an angle.
 
#3
Re: Verifying scope to bore centerline

Simplicity at it's best.
Prior to the chasing of ghosts my previous scope and ring set-up worked fine so I never gave the centerline issue a thought as the round went where intended. Now that an individual has raised the issue and muddied up the water I thought it best to inquire before I go and run after false alarms.
I think I will run with your suggestion and ask the individual to hold the target for giving me grief over nothing.
On a lighter note... would lapping the rings be advisable or is this more ghostbusting?
 
#5
Re: Verifying scope to bore centerline

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Underwhere</div><div class="ubbcode-body"> If your scope is perpendicular to the bore, then you will follow along the vertical lines. If not, then you will move off the centerline at an angle.
</div></div>

Ummmm, that's to check cant of reticle in scope, not whether the scope's base screws are machined askew in the receiver....

Easy way to tell if you're base (therefore scope)is out of alignment is if you have to use a lot of windage from center to zero....
 
#6
Re: Verifying scope to bore centerline

From what I'm reading the above is not exactly correct. I recently had a similar problem with a set of rings, due to sako's proprietary dovetail. Anyhow, think about what will happen if your scope is out of centerline with the bore.

If you zero your rifle at 100 yards, your zero will be accurate at 100 yards. This is the point (because you zeroed it here) that the scope and bore meet. Another way to see this, although exaggerated, is if you imagine the bore as one leg of a triangle, and the centerline of the scope as another leg of a triangle. The 100 yards "zero" is the point of the triangle, where these two legs meet.

Now what happens if you go beyond 100 yards?

Simply continue drawing these lines formed by the scope and bore. You will see that they diverge, forming a big X.

Long story short, the FARTHER you shoot from your zero, the MORE your PoA and PoI will diverge. An easy way to check this would be to zero at 100 yards, and then shoot at 100 yards increments farther and farther. If you can shoot well, you may notice a difference, but realize it will not be a lot, perhaps not even enough to care about. I imagine you will find the latter.

For instance, if your scope is 2mm out of centerline with your bore, which would be pretty noticeable, your PoA and PoI difference would only be ~12mm at 600 yards, and ~20mm at 1000 yards (by similar triangles).

In short, your centerlines would have to be really far off to make a noticeable difference. Take the Dragunov/SVD. In standard dress, the scope is off center to the bore in order to retain a clear view of the iron sights, and probably more so than any set of rings/base combination would get you. It still shoots (relatively) well in the right hands.

(This example is simplified, assuming a flat trajectory, but should still be applicable.)
 
#7
Re: Verifying scope to bore centerline

So yes, you may be chasing ghosts. That being said, I got an appropriate set of rings and base for my rifle. If nothing else, it will hold up to recoil better.

From what I can see from the rings you described, you should be fine.
 
#8
Re: Verifying scope to bore centerline

Its been my experience that a scope that is properly leveled and does not have a canted reticle when mounted at a minor angle to the bore has tracked just fine once zeroed at 100 out to 700 yards (farthest I shot with the rifle I had that had a misaligned base before fixing it).

Theory-yes it makes a difference. Reality-the error is usually less than the shooter/ammo can hold within medium ranges.

Of more concern is limited elevation travel from the reticle being offset to the side int he erector assembly to compensate for the windage extreme.

The culprit in cases of misaligned scopes is usually the base screws being machined out of line, not rings (there are exceptions such as the dovetail/ring example).
 
#11
Re: Verifying scope to bore centerline

on2wheels, just check the reticle verticality and go zero the rifle, if you do not use a lot of windage adjustment (counting fron the optical axis of the scope) it is OK.

If your action mount holes are offset to the same side but exactly parallel to the bore a small amount, say 0.08" (and this is a very large and noticeable error), when you zero at 100 yds you converge the bore and scope optical axis at this distance and the error is zero. At 200 yds the lines diverge again 0.08", at 300 0.16", at 400 0.24"... and 0.72" at 1000 yds. It would be VERY difficult to notice (this is only 0.07 MOA at this distance). The scope does not lose any windage adjustment in this case.

If your action holes are not parallel but in angle to the bore it is another story, you can lose a lot of windage (and elevation). Say one is 0.02" to the right and the other 0.02" to the left, then for a short action you have about 25 MOA windage error.

Normally you would have a combination of the two above (hopefully small errors), the offset does no mean anything, and the windage error to zero at 100 yds hopefully will not be large enough to worry (less than 4-5 MOA). If you have a lot of windage errors you have to re-drill the holes centered, or use adjustable windage rings, or rings with excentric inserts.
 
#13
Re: Verifying scope to bore centerline

Well I think I have boiled it down to everything that has been said here.
Once the scope is mounted and reticle verified for zero cant I can zero the rifle up to 200 yds at our range after that it's off to the desert as usual for longer distances.
The amount of deflection noted here appears to be minimal and should not be of concern seeing how I am not shooting in match grade competitions. Basicaly shooting for self gratification and defense should life as we know it deals those cards.
I now see why so many speak very favorably about SN Forum vs SS Forum.
Thanks to all that have brought very good information to the topic.
 
#15
Re: Verifying scope to bore centerline

The problem with marking a datum line is unless you do it the entire length then there's a chance the two points converge where you put them but are off in different areas. This is the reason the pointed ring alignment bars from Wheeler are useless. The points can meet but that doesn't mean much. The Kokopelli kit uses flat ends to meet so if there is any gap it's clear there's a problem. That's why you'd need to mark the entire scope at 6 o'clock and the entire area it mounts over at 12 o'clock. Even then, how do you realistically check it?

This is partly why I like the Kokopelli kits so much, they have a tool for doing this...
 
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