It’s generally accepted that it is desirable to have your rifle scope’s internal adjustments something like centered when the rifle is sighted in. Or in other words, you DON’T want either the elevation or windage adjustments to be near the limits of their travel, because that can do nasty things to both the optics and the adjustments.

But what really constitutes ‘centred’?

The instructions that came with some scopes I have owned said to count the number of turns/division/ clicks to take each knob from wound fully in to wound fully out, and then go back half of that number, and that is ‘centred’.

The instructions with at least one other brand of scope recommended setting up the scope in some sort of improvised (but stable!) V blocks, where some at least moderately distant object could be sighted through the scope. Then rotate the whole scope, quarter-turn at a time, and check if the centre of the reticle stays on that same object at each position. If it does not, turn the adjustment knobs to the positions where the point of aim DOES remains the same at each position, and the scope is then ‘centred’.

A variation on this method, but the same in principle, is to mount the scope on a rifle with the rings halves loose enough to allow the scope to be rotated, put an optical collimator in the muzzle, and check if the reticle centre stays in the same position on the collimator grid. This can work quite well for some set-ups, but low rings and/or tall turrets can prevent the scope from being fully rotated.

A while back I read on some internet forum – it MAY have been this one - that the correct method is to stand the scope objective lens cone down on a mirror, with good all-round lighting. When you look through the scope, you see the primary image of the reticle and also a reflected image. If the two images do not coincide, turn the adjustment knobs until they do, and the scope is then ‘centred’.

All three methods sound quite plausible, at least to an ‘optics layman’ such as myself. But which do you give the most credence to if you try all three methods with a particular scope, and get three noticeably different results?

This is what happened to me recently, and the scope in question is a Leupold Vari-X III, 6.5-20X, with target turrets, not some ‘el cheapo’ job from an unidentified part of Asia. I bought it second-hand, but it appears to be in good condition – a few small scratches and scuff marks on the body, but no signs of any real knocks.

The readings were:
‘Mean travel’: Elevation – 49’ Windage – 50’.
‘V’ blocks: Elevation – 38’ Windage – 38’
‘Mirror’: Elevation – 42’ Windage – 44’

I was able to do a bit of tweaking of the mounts, got the rifle zeroed with the scope close to its ‘mirror centre’, and it seems to work perfectly well.

With another Leupold scope I have, a Vari-X II, 4-12X, all three methods agree within a couple of minutes, in both windage and elevation. With an older Tasco 6-24X (a ‘target’ model) the windage difference between the mean travel centre and the V block centre was over 20 minutes! I was able to get the rifle zeroed at something like the mean of the two ‘centres’. Optically, that scope is not as good as a Leupold, no matter what, but it isn’t bad either – it’s been on my Sako 222 for decades, it has never malfunctioned, and that rifle has taken so many rabbits that the barrel is showing significant wear.

So what is the true ‘centre’ of a scope? How critical is it, really?

And what does the ‘mirror centre’ really tell you? If you tip the scope even a tiny fraction on the mirror, the two images of the reticle move a LONG way apart. I tried putting a shim 0.005†thick under the front of the objective cone of the 6.5-20X, and it moved the ‘centre’ over 10 minutes. It seems to me that if the front of the objective cone of a scope was slightly off-square to the axis, due either to faulty manufacture or to taking a bit of a knock, then using the ‘mirror method’, you could perhaps run out of travel on the adjustment knobs trying to get the two images of the reticle to coincide. That would be plain silly!

Comments, anyone?

(I also have posted this on the Optics forum at Rimfire Central, because I cannot remember if it was here or there that I first read about the mirror method)

I can't answer you question with any certainty, but I would think you correct with the mirror. If the end wasn't square to what's inside it would seem pointless.
In a way, just guessing here, that being close to center is the main point in it all as a starting place for adjustments only. That being "perfect" center probably doesn't matter all that much? There would seem to be so many variables in tolerance from the rifle drill points, base/rings, scope, etc. anyway, that getting a perfect center is an illusion when all complete...then you start turning knobs and take it all out. You have me wondering now though, ha ha. Sorry for the babble and no help.

I have centered many of my ~50 beater scopes.

I used to need daylight.

Now I am using this tool.
It is a basic an optical collimator with a magnet instead of spud set and an LED illuminator. It comes with a holster so one can see in the field if he has knocked his sights off.
http://www.swfa.com/c-538-leupold-boresighters.aspx




Some old Weaver K-4s will not adjust to be co linear with the tube.
Leupolds not only will center, but if they have 2.5 turns of elevation and windange, the center will be at 1.25 turns.

Okay, Redrover, you write the English language far too well to be posting on this site, and you obviously have too much time on your hands if you are obsessing over "centering" your scopes. You're my kind of guy.

Now let's talk real theory. The ostensible purpose for mounting the scope so that its opto-mechanical (just made that term up) center coincides with the center line of the bore is to allow the maximum adjustment of windage and elevation.

But wait! The scope should never need adjustment so that it is pointed above the bore center, as the bullet always drops, never rises, when it leaves the muzzle (this assumes that you never turn your gun upside down to fire it, which, considering you're from New Zealand, may not be a safe assumption). So, it would make more sense to mount the scope such that it was "centered" for purposes of windage, but for purposes of elevation, it points slightly below the bore's centerline, thus allowing you to use most of the vertical adjustment to elevate the bullet, there being little need to point the bore below the line of sight. The advantage here is that you would be less likely run out of elevation adjustment if zeroing for extra long distances.

However, this is largely simply theoretical, anyway. My experience is that with modern, high-quality scopes there is no degradation of the quality or shape of the sight picture when the scope is adjusted to the extreme. Years ago this was not true with all scopes and it was possible to get a "lopsided" sight picture (that is, an out-of-round sight picture with a black area taking up one or another side of the field) if you adjusted the reticle too far to one or another extreme. To my knowledge, there is no degradation of performance with a Leupold when it is used with its adjusment mechanism at or near the maximum limit.

By the way, we drive on the right-hand side of the road on this forum, so it's "centering", not "centreing". Besides, the "ei" looks funny when juxtaposed and I'm not sure it doesn't violate some cardinal rule of spelling.

What does kiwi taste like, anyway?

Thank you for your VERY interesting response, Stonecreek. I hope the formatting of this reply comes out as I intend it to.

(I wish there was a 'Preview' function available - are you reading this, Don G?)

>> Okay, Redrover, you write the English language far too well to be posting on this site, and you obviously have too much time on your hands if you are obsessing over "centering" your scopes.

It’s getting into winter down here. The deer finished roaring ages ago, the evenings are getting colder, the rabbits are getting hard to find, and to top it all off, my club’s rifle range is closed for repairs to the bullet stop area, so I can’t even go and shoot there! Total disaster!!

But no, despite having some time on my hands, I’m not really ‘obsessing’ over the set-up of my scopes. If the makers of some of them, and optics gurus like Wally Siebert and the late Dick Thomas (of Premier Reticles fame) say that it is advantageous to have them at least approximately ‘centred’ when the rifles are zeroed, I’ll take their words for it. I’m sure they know infinitely more about scopes than I ever will. I’d rather put time and effort into setting up my scopes as they suggest than put even more time and effort into trying to prove them wrong.

>> Now let's talk real theory. The ostensible purpose for mounting the scope so that its opto-mechanical (just made that term up) center coincides with the center line of the bore is to allow the maximum adjustment of windage and elevation.

Agreed. Also, according to at least some of the gurus, it prevents degradation of the optics. I wouldn’t know from personal experience whether this claim is true or false, as I have never tried to check it out.

>> But wait! The scope should never need adjustment so that it is pointed above the bore center, as the bullet always drops, never rises, when it leaves the muzzle (this assumes that you never turn your gun upside down to fire it, which, considering you're from New Zealand, may not be a safe assumption).

Yes, well – the soles of my feet are facing somewhere in Spain, not the USA, but what’s a few thousand kilometres matter? Presumably it is in some way due to this effect that my old faithful, push-feed, Sako L61 rifle will cycle cartridges perfectly even when I work it while hanging by my heels from the branches of a tanekaha tree. (I understand that if I tried to do this while hunting in the USA, it would jam immediately and I would get eaten by a grizzly bear)

>> So, it would make more sense to mount the scope such that it was "centered" for purposes of windage, but for purposes of elevation, it points slightly below the bore's centerline, thus allowing you to use most of the vertical adjustment to elevate the bullet, there being little need to point the bore below the line of sight. The advantage here is that you would be less likely run out of elevation adjustment if zeroing for extra long distances.

Dead right. I have gone to a lot of effort to set up the scopes this way on a couple of 308 target rifles that get used all the way from 100 metres to 1000 yards. The scopes are about ‘centred’ when the rifles are zeroed for 600 yards.

>> To my knowledge, there is no degradation of performance with a Leupold when it is used with its adjustment mechanism at or near the maximum limit.

I’ll take your word for it. The Leupold 6.5-20 I mentioned has gone on to a Sako AI, and the integral tapered dovetails on the action provide the means for a little bit of ‘windage adjustment’ of the rings. It has finished up with the rifle zeroed with the scope about half way between the ‘mean travel’ centre and the ‘mechanical’ centre, and the picture looks pretty good. Of course with the range closed, I don’t know EXACTLY how targets will appear through it, but that shapely young lady who lives down the hill from my place looks …

OK, just kidding, I wouldn’t really point a rifle at her – if she even existed …

>> By the way, we drive on the right-hand side of the road on this forum

I thought this was a United Arab Emirates forum. Do they drive on the wrong side of the road there, too?

Ooops, I’d better not say that – the Emirates airline gave a big pile of dosh to our yachting team that is currently thrashing the Italian Luna Rossa boat in the races for the Louis Vuitton Cup and the right to challenge the Swiss for the Americas Cup.

>> it's "centering", not "centreing".

Hmmm. We speak and write English down here, not American. I can’t find the word ‘center’ or drivatives of it anywhere in my copy of the Oxford Dictionary. Though I can’t find ‘centreing’ in it, either … It does look a bit funny.

>> Besides, the "ei" looks funny when juxtaposed and I'm not sure it doesn't violate some cardinal rule of spelling.

You are reminding me of spelling lessons at school, many decades ago, and hearing endless repetitions of the rule: ‘i’ before ‘e’ except after ‘c’ (EXCEPT – yadda-yadda-yadda-yadda) I can’t remember the rules for what all the other exceptions are.

However, you know what I mean. Shades of that classic line by Professor Higgins: ‘The French don’t care what they DO, actually, as long as they pronounce it right’. (that doesn’t sound like good grammar, either – maybe the last word was ‘correctly’?)

>> What does kiwi taste like, anyway?

I’ll take this in the spirit I’m sure it was written in.

Anyone who was caught deliberately hurting a kiwi would probably get treated more harshly by both the law courts and the general populace than they would if they did the same thing to a fellow human.

quote:

my old faithful, push-feed, Sako L61 rifle will cycle cartridges perfectly even when I work it while hanging by my heels from the branches of a tanekaha tree.

There is more than a single grain of truth in your post, Redrover, but the quote above stands out as the most sage of observations. Talk about obsessing, those "controlled feed" guys have to be absolutely the worst of all. I am loathe to say publicly the number of Sakos I own, but if there is such thing as a feeding failure inherent in their design, I am yet to finesse one into exhibiting it over 40-odd years of shooting them.

As to which side of the road they drive on in the UAE, you'll have to check with Saaed. But my impression is that everyone there attempts to take his half out of the middle. If you happen to bang up your Bentley or Rolls, you just go down to the store and get a new one.

Speaking of which side of the road . . . I'm off to Namibia in ten days for my first-ever African trip. In researching the country, I find that while many countries have switched (modernized, corrected, gotten right with God) from driving on the left to driving on the right, Namibia is the ONLY country in the world ever to switch from right to left. That may explain why, as the second least densly populated country in the world (following Mongolia), Namibia has among the highest traffic injury rates.

I wish you continued good luck with your scope centering (centreing? centring? centre-ing?). Since I shoot virtually nothing but Sakos with their dead-square, straight, plumb, and even dovetail bases, I have had little experience with reticle centering problems. Such problems are usually endemic with those who shoot rifles on which scope mounting is an afterthought, like Mausers and M70s (hmmmm.... controlled feed models).

Well, I must be off to fix my wife's breakfast. I think she wants scrambled kiwi eggs.

Quote (Stonecreek)

>> Speaking of which side of the road . . . I'm off to Namibia in ten days for my first-ever African trip. In researching the country, I find that while many countries have switched (modernized, corrected, gotten right with God) from driving on the left to driving on the right, Namibia is the ONLY country in the world ever to switch from right to left. That may explain why, as the second least densly populated country in the world (following Mongolia), Namibia has among the highest traffic injury rates.

An Irish friend of mine tells me that the Irish government are intending to experiment with getting cars to change over to driving on the right side of the road instead of the left, as they do at present. If it works out OK, they will then get trucks and buses to change sides as well ….

>> I wish you continued good luck with your scope centering (centreing? centring? centre-ing?). Since I shoot virtually nothing but Sakos with their dead-square, straight, plumb, and even dovetail bases, I have had little experience with reticle centering problems.

I haven’t had any serious problems with my various Sako rifles, either, using Sako, Burris, Buehler (sp?) and Leupold mounts/rings. But I’ll freely admit to having spent a bit of time tweaking them to get the scopes as closely CENTRED as I could. I bought those rifles and scopes with every intention of keeping them – and using them – for the rest of my life, so as far as I was concerned, it was time well spent.

I certainly HAVE encountered problems with some other rifle, mount and scope combinations. In a few of the most extreme situations, I initially ‘ran out of clicks’ before getting on target, and had to employ drastic remedies. Nothing new or hitherto unheard of here – if you hang around internet forums for a while, you regularly see people reporting this problem, and asking what to do about it. Sometimes it’s due to a definite defect in one of the components, other times it’s just all the manufacturing tolerances stacking up on the same side.

I was hopeful that somebody here with more knowledge of the inner workings of scopes than I possess would explain what the ‘mirror’ test really does – or does not – prove, but it looks like that isn’t going to happen. Not to worry, though – I have got all my own scopes lined up to my satisfaction.

>> Well, I must be off to fix my wife's breakfast. I think she wants scrambled kiwi eggs.

That (to put it in good NZ vernacular) would give her crook guts – if only due to sheer size. For those who don’t know, kiwi eggs are HUGE in relation to the size of the bird. It’s a bit like average-sized women giving birth to a 50 pound babies - OOOUUW!

Have fun in Africa. Maybe your good lady can try scrambled ostrich eggs over there …

Centreing is a good idea to start with, but the sky won't fall in if you have to wind in a bit of correction. I've just shimmed my Ruger mounts a little because I ran out of windage adjustment with our howling easterlys, but the scope performed well otherwise.

Another example with my 24x target scope, it was wound from 100yards to 900yards regularly, where it was at full travel. Lets face it, that's what the adjustments are for.