by Tom Gaylord, a.k.a. B.B. Pelletier
This series was started for blog reader David Enoch; but after reading the comments many of you have written, I have to think it’s for most of you. Today, we’ll look at optically centering a scope — what’s involved and why you’d want to do it.
What is optical centering?
Optical centering means adjusting the scope until the center of the crosshairs is actually in the center of the field of view. This is difficult to understand; because when you look though a scope, the crosshairs always look like they’re centered. That’s because they’re permanently fixed in the center of a tube called the erector tube. It’s this tube that gets moved when the scope knobs are adjusted.
Not all scopes work this way, I’m aware, but the majority of modern scopes do; so let’s not get into discussions of German scopes and Russian scopes whose reticles actually do move. They’re sufficiently uncommon that there’s no need to confuse the average shooter with their differences.
An optically centered scope is one whose crosshairs remain fixed on a target as the scope is rotated 360 degrees on its axis. I’ve never seen a scope that was perfectly centered, and I doubt one exists. The closest I’ve seen was a scope whose reticle moved about one-eighth inch when rotated 360 degrees while focused on a target 20 yards away. Most scopes can get only to within three-eighths of an inch under those circumstances.
When the scope tube is rotated, the intersection of the crosshairs moves against a distant target. The object of optical centering is to get the movement as small as possible.
Why optically center your scope?
This practice started and died with field target. Shooters discovered that if their scopes were not parallel with the axis of the rifle’s bore, not only would the pellet impact rise and fall as the elevation knob was adjusted for different targets — it would also move from one side to the other — typically from right to left, though not always. That’s because the scope was right on at the sight-in distance, but off to one side when the scope was adjusted closer and to the other side when it was adjusted farther.
This drawing of a top-down view of a scoped air rifle is greatly exaggerated, but it shows how a scope may not be aligned with the axis of the bore.
When the scope isn’t aligned with the bore, this is how the rifle can shoot. You can adjust the vertical reticle for elevation to get all the groups level with the target, but they’ll still land to either side if the scope isn’t aligned.
I was writing The Airgun Letter (1994-2002) when I competed in field target. Although I started out using springers and the holdover method of sighting, I switched to PCPs, which gave me a better chance to compete. I also started adjusting the scope’s elevation for every change in distance. That was when I discovered optical centering.
The way to optically center a scope in those days was to put it on a solid rest that did not move but allowed the scope to rotate around its axis (in this case that means the scope tube) 360 degrees. Then sight at a target at some distance and watch the center of the reticle move against the target.
I started with actual machined Vee blocks, until I realized that precision isn’t required to optically center a scope. A cardboard box with 2 Vee notches works just as well.
As you rotated the scope tube, you adjusted both the vertical and horizontal reticles until the center of the crosshairs appeared to move as little as possible against the target. I used graph paper with a quarter-inch grid and a tiny black dot aim point that was about half the size of a pencil eraser.
Adjusting the reticle was not straightforward. If often took longer than an hour to get the reticle moving as little as possible against the aim point. And you never got it perfect. There was always some perceptible movement as the scope tube rotated.
Avoid this trap!
Some people would read about optical centering, then go to the range with thoughts of performing it at twice the distance to get even greater precision. It never worked because at 40 yards you can’t see the movement of the crosshairs shifting by one-sixteenth of an inch against a target.
Others were simply never satisfied with the results they got from optical centering. They knew their scopes were not perfect, and they couldn’t live with that. So, they kept swapping scopes and returning to the range again and again, searching in vain for the scope whose crosshairs could be adjusted to remain centered when the tube was rotated.
In the end, those who’d been proponents of optical centering realized they were chasing their tails. Perfection was impossible and there were other easier things that could be done that would deliver the same results. Mounting the scope in line with the bore is just as successful as optically centering it.
Why did optical centering die?
Many shooters are still not aware that they don’t need to optically center their scopes, so it hasn’t really died…but most field target competitors — at least the ones that win — don’t do it anymore. Instead, they take great pains to align the scope with the axis of the bore so centering becomes a non-issue.
If your scope is not optically centered but the scope is aligned, you can correct any misalignment of the reticle during the sight-in. You aren’t fighting the angles of the line of sight and axis of the bore. So, extra time spent mounting the scope pays off in not needing to go through this cumbersome procedure. The results are the same either way. As you adjust the vertical reticle, the shot group remains centered at all practical distances.
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I would think keeping the recticle level would be just as important.
I have messed around and tilted the gun to the left or right and made shots at the target. I actualy have got pretty good at making shots like that now.
I believe the more you try to make shots different ways the better you are when you hold the gun level and correct.
So optical centering is not needed. Barrel never ever perfectly straight. Scope rail almost always not aligned with barrel. Scope rail rarely aligned with action either.
Maybe I need to sell all my airgun and back shooting slingshot. Oh wait, rocks doesn’t come sorted in caliber and grain.
Now my head hurts and I blame you for it. I recalled the old days, before reading your blog, the
joy of just hitting beer can at 10 m.
Seriously BB, is there any way to adjust scope for shooting at different range without screwing up too much ?
Lee,
I don’t understand your question.
B.,B.
Sorry for babbling. You said that by aligning scope to bore, optical centering is not necessary. But in your old blog entry you stated that no barrel perfectly straight, action and scope rail almost always off axis with barrel. I find it hard to believe until I see it myself that on one of my rifle, the scope rail definitely ran off few degree to the left.
So my question is this:
how to align scope parallel with bore if my rail (and my action) is off axis with the bore.
In my case, the scope rail defect is visible to naked eye. I don’t know how to measure if the action also not centered though.
Lee,
Now I have it!
Okay, this will be covered in more depth in the report I write about how to align a scope, but the short answer is that not all airguns are suited to this alignment. I will have a lot more to say about it in the report.
B.B.
Thanks BB,
Will wait for it.
Maybe you’d like my ancient Marlin Glenfield 60c — in which the /barrel/ is not aligned with the receiver.
As a result, I had to shim the rings (on the /sides/) so that the scope forms the hypotenuse of a triangle that consists of a shot side of the receiver length, long side the barrel to target…
B.B.
Bit confused here. Do all scopes have this problem? I’m thinking of the best names in the business like Weaver, Bushnell etc. Not all will be able to get it right as it obviously needs a lot of patience. I thought that with todays high precision systems of manufacture such issues could be overcome.
Errol
Errol,
Optics are far more precision than we allow for. Remember, the main mirror on the Hubble Space Telescope was ground a few microns off and it ruined the image.
No, modern manufacturing cannot solve this, but it isn’t a problem. It’s simply a fact, the same as any fact we shooters have to deal with. Like all rimfire ammunition is subject to variations that occasionally throw a bad shot — even the $50 a box stuff.
B.B.
B.B.
Got it now. Thank you.
Errol
It would seem to me that you would at least want the crosshairs/erector tube centered in the adjustment range when you start. That would hopefully have you close to optical center.
Of course you left us hanging on how to properly align the scope. Just don’t forget to get back to this blurb. I have a sproinger that I will be mounting another scope on soon.
RR,
Oh, I had hoped to avoid this! No, centering the adjustments WILL NOT optically center the scope! It only centers two screws. The optics have to be centered by some other means.
Think of it like this. Centering the optics is like filling a glass of water. In relation to that, centering the adjustment screws is more on the scale of opening one of the locks of the Panama Canal.
B.B.
So, without optically almost centering the scope, how do you know the bore and the scope is aligned?
You align the scope so that it looks in the same direction as the barrel. It can be just as difficult as centering the optics, or it can be easy. It all depends on the accuracy of the scope mounts and of the accuracy of the scope base on the rifle.
B.B.
I’m sorry. I didn’t answer your question. To know the scope is aligned with the bore you shoot it at different ranges and watch for groups that move left or right. When the groups stay centered, the scope is aligned with the bore.
Of course when you do this you must ensure every shot is shot with the scope leveled, so a scope level is an important item.
B.B.
B.B.
I have a scope on one of my R9s that does something squirrely…
Vertical and horizontal adjustments interact with each other. Both horizontal and vertical adjustments have to be made to move straight up/down/left/right. It’s a bit strange at first, but once figured out it’s not too hard to deal with. Once it’s set, then forget about it….and leave it there.
twotalon
TT,
That is a classic symptom of a misaligned scope.
B.B.
B.B.
The scope would have to be off quite a few degrees from square to show this much interaction, and it is not. I don’t know how close the scope is to optical center . It does work (not particularly convenient ) so it is not a big deal . If I clicked for range differences, then it would be another story.
twotalon
Could it be that the erector tube is so far to one side that the supports are riding up the arc of the tube?
That’s badly phrased.
You’ve got (in one dimension) a circle sitting on some flat pad, with a spring pushing it against said pad. Within a small range of movement, the circle can move side to side without a shift in the perpendicular direction…
But what happens if the side-to-side movement has gone so far that the contact point is falling off the edge of the pad (for elevation, spring is on bottom; slipping sideways far enough could result in the spring pushing the tube upwards as it moves off the pads).
BB,
I remember years ago when this first came up that a second method of optically centering a scope was available. That involved putting the scope on a mirror and adjusting the reticle until one image was obtained. Apparently a double image was observed when viewing the scope when it was placed on a mirror. I believe this was shown on the Leupold site but is no longer there (just checked). However, there is a Youtube video on doing this which i’m reluctant to view just now since I’m at work.
I believe kevin L. uses this technique.
Fred DPRoNJ
Fred,
Yes, I was aware of this other technique. However, as I said — people don’t do this that much anymore. so I declined to mention it in my report.
B.B.
I looked up this method on you tube. Then I tried it on a scope that has been a problem child on an older chinese gun that I was having a problem getting to hit a target. It turns out this scope, also hinese made wasn’t even close to centered. Thanks to this mirror technique I know at least the scope is centered and should at least get the gun on target. I never did figure out why an airgun maker would put a cheap crossbow scope on an airgun. But since the gun isn’t even made anymore and the parts are harder to find than an Antartic army medal I figure I’ll keep the gun as original as I can.
B.B. –
I’m glad to see that you are writing about this topic. It’s obvious that you’ve been around this particular block more than a few times. Several of my scopes exhibit these symptoms, and I fight it all the time. Your post today outlines in sufficient detail how to get a scope optically centered, at least enough for practical use. Then it goes on to explain that optical centering isn’t necessary, as long as the scope is aligned with the axis of the bore. So… how do you do that? Are we talking about adjustable mounts here? Shimming? Ring reversal? Are you planning another installment on this topic to explain the mounting process, now that we understand the problem?
After reading this post, I get the feeling that a possible solution is at hand for an aggravating issue that I’ve dealt with for years, but I don’t yet know the full story. Can you shed some light on proper scope mounting?
Thanks,
– Jim in KS
Jim,
I was hoping to get a comment like yours today. In fact, I started to answer it for the anonymous poster, above.
The short answer is you have to find the right mounts that fit your rifle in such a way that they align perfectly. As I said before, that can be easy or hard.
The longer answer is probably another blog. It’s a worthy topic, so let’s do it.
B.B.
Jim…
I will leave it up to B.B. as to the “how” of the situation.
The question that this alignment (or misalignment) problem poses is this ….
Is it really necessary to worry about it or is it so bad that a gun is just about unusable without doing something about it ?
I doubt if any of my rifles have the scopes perfectly straight over the bore. If the scope is offset by 1/16-1/8″, and I zero for 25-35 yds, then there is no problem for me (hunting and plinking) . For a field target shooter, then there IS a problem.
The worst rifle I have is an 853. The barrel is not installed into the receiver straight…it points to the right. At least the peep sights adjust far enough to use it (but not by much). Second worst is a RS2 that has pretzel barrels …..crooked as a politician. The opens adjust just far enough…..
twotalon
TT,
That is the position of most airgunners. Those last few fractions of an inch are just not that important to them. But to the field target competitor who has to go through the small hole without touching the sides, they are. That’s why all of this started with them.
B.B.
Wouldn’t a horiziontally and vertically adjustable scope mount, with precise adjustment capabilities, do away with all of these burdens?
Mel,
Yes, that would work. But such a mount has never been built, to my knowledge. I’m not talking about adjustable mounts, but mounts with precision adjustments that will hold zero once adjusted.
B.B.
Not seen anymore, but old target scopes had no adjustments — the rear ring had the adjustment turrets.
Obviously not something useful if one expects to remove the scope/rings between uses.
http://www.ebay.co.uk/itm/LYMAN-SUPER-TARGET-SPOT-20x-UNERTL-REDFIELD-TARGET-SCOPE-/190699517223
I could swear the rings were still shown in things like Shooters Bible in the 70s
B.B.
I have never exactly understood what defines high quality scope mounts. After this post it seems that having the ability to allow you to align the scope with the barrel is certainly one of the characteristics.
What else defines a quality scope mount and how do you know one when you see it?
G&G
G&G,
High quality scope mount are bought by reputation. For example, there was a time when B-Square mounts were all high-quality and made in America. Then the company changed hands and they moved their mount production to China. Immediately the quality fell off and B-Square mounts were no longer high quality.
UTG mounts are all made in China or Taiwan, yet they are high quality. Leapers maintains rigid quality controls over the products they don’t make in this country. So the country of origin is no guarantee of quality.
I don’t want to get into writing the blog about this yet, but there is more to say about mounts than just what I have said.
B.B.
B.B.
I don’t know why my reply with the question about what a quality scope mount is referred to me as Anonymous. Also, spell check is missing from the reply section.
I found that scopes don’t always center like it says here. But it isn’t much of a problem for me even with my condor. I simply learned the scope and figured out how I needed to compensate my sight picture on different distances. So at 20 yards I line up the target at one point in my crosshairs. At 50 yards I set the bullseye at a different point in the crosshairs, and at 75 or 100 I line things up at other points in the crosshairs. Keep in mind that there isn’t a bullet or pellet made that flies in a laser straight line. they all arc and curve in flight. Also wind pushes the pellet or bullet around in flight. You have to read the range and figure out how that bullet is likely to fly to the target and then set up the shot in the scope for all of that. So lining up the bullseye won’t always guarantee the shot will hit the bullseye. When a shot is fired there’s around a thousand things happening all at once between your shoulder and the target. All of those things affect how that shot lands. So you just have to accept that if you are landing the shots inside a dime you are doing well.
Most rimfire mounts I’ve used suck mightily and cant/tilt one way or the other, not as big a problem with weaver/picatinny but rarely do I find a set that centers over the action. The BKL’s are the one exception, as long as they’re mounted carefully.
And yes, I remember when B-squares were the beez kneez : )
Mechanically aligning the scope tube with the bore still doesn’t mean the erector tube is centered…I’ll be interested in the follow up report.
G&G,
Thanks for alerting us. Spell check is back!
Edith
Edith,
You are very welcome.
G&G
BB
I made a tool that mounts in 1″ scope rings that help to give a idea where things are. I figured I should wait to say for your next part of the blog.
And is there really a way to know if the erector tube is centered?
GF1,
The only way I know is to shoot the gun and see what happens.
B.B.
I’ll come clean and admit that even after all the years, that I don’t get this business about optical centering of scopes. It could be that I have a lot of things on my mind right now and can’t give this the necessary attention, but still… Anyway, start slow. I’m confused by this rotation of the scope that is referenced in the first diagram. What is the axis for this rotation? The second diagram can help me out here. Is the axis vertical–perpendicular through the scope body and down through the top of it? Is the axis horizontal and perpendicular through the scope body from the side? Is the rotation around the axis of the scope? I would tend to discount the latter. I visualize the scope tube rotating around the vertical or horizontal axes at right angles to the scope body to get elevation and windage, respectively. But then how can you have a scope rotating through 90 degrees as it says since that is physically impossible? I imagine the scope rotating through fractions of a degree for adjustment. As Huck Finn would say: “I was up a stump and up it good.”
Well, I was all set to ask about adjusting my Mosin scope since I am gearing up for the inaugural range trip. Okay, I’m warned off, but how about a simple procedural question: How do you adjust the darn thing when there are no clicks or any way to measure how much you’re moving? After a certain amount of thought, I have come up with a sort of reverse procedure. Basically, if the reticle moves instead of the scope in response to adjustment, it follows–through a series of logical leaps–that the adjustment procedure should be inverted. To wit, I will put the reticle on the bullet hole and while holding as steady as possible, adjust the reticle to where I want it to go. FTW?! Is this right? 🙂
On the subject of scopes, I was talking to a fellow while standing in line to register my guns in Honolulu. He was quite an enthusiast about his equipment. But it seemed that he didn’t know the name or make of his scope or the magnification or the click adjustment ration. With what he described as a three inch deviation left at 100 yards, he described spinning the dials of his scope through several revolutions to correct. That would make for a long day at the range.
BG_Farmer and Victor thanks for your thoughts on follow-through. I don’t think you quite appreciate the task you’re taking on with my Dad but I am armed and ready as the saying goes. BG_Farmer, I would think that blackpowder shooting what with the unimaginably long lock time and the explosion in the periphery would make you a real master of follow through. Modern guns must seem like nothing by comparison. To pretend as if you are aiming is a good mental image. Once you get down to details, it is just about impossible to spell out exactly what to do and probably not possible to execute precisely even if you did. An alternative is to think in terms of mental images which can lead to a discourse that sounds touchy-feely but is actually a very effective means to deal with the problem at hand.
Victor, natural point of aim is intellectually satisfying but, in the case of pistols, something that I continue to struggle with myself. In contrast to a slinged-up prone with the rifle, a pistol, even with my Isosceles stance, still feels like it’s swaying in the breeze and the proper position is to be worked toward rather than relaxed into. And I can see from my Dad’s shooting that if I asked him where his pistol was returning after the shot, the answer would be: all over the place. But I think that is a very important and promising concrete point about concentrating on the sights instead of the bull. I am almost certain that he is looking at the target because of his obsession with getting a bullseye.
You’re both right to wonder about the shooter psychology at work here and I’m sure that it is playing a role. As one of the unhandiest people that was ever born on this earth, my Dad is not a problem-solver of any kind. What he is instead is someone who had reasonable athletic skills and who is a hopeless romantic. He has a deep faith that by just trying hard enough, he can pull out some exceptional performance. This is buttressed by an experience in the army where some recruit who didn’t know anything about M1 Garands almost broke the range record. More evidence of the power of ability and concentration over mere rules! And probably the worst thing for his shooting career was shooting a bullseye with the 1911 at 25 yards with one of his first shots. That may have ruined him permanently by imprinting the idea that by concentrating hard enough, he can reproduce that feat… I’ll let you know how it goes.
Thanks for the info about automobiles. Is the Nitrous Oxide set-up illegal? Once you throw the switch, I don’t see how you would remain inside of any speed limit. Interesting about drift. The prominence of the Mustang and the Camaro raises another question. I read that dedicated sports cars like the Corvette have carefully engineered suspensions and other things that allow them to take turns at high speed. Which means that your cheaper muscle cars do not and really can only go fast in a straight line. That was kind of disillusioning to learn. So, does drifting count as turning? Maybe not.
One other thing about the movie is that when the Japanese Yakuza villain pulls a handgun, everyone freezes like he’s revealed a nuclear bomb. In America, the criminal equivalents of these characters would probably laugh and pull out machine guns…
Also more about the Navy Seals from the Chris Kyle book. He said their snipers use Black Hills match ammo which they consider the best in the world. So, that’s whose been gobbling up all the Black Hills ammo! It’s those darn Seals. I’ll resign myself to not getting more anytime soon. Another weird moment is that it is possible for a Navy Seal to just cut and run. You would think that they would have selected or drilled this behavior out of these guys but no. Kyle talks about one guy who keeps running away and telling everyone that they’re about to be overrun and killed. It seems to be a case of a guy who just cracked at some point–sort of like the guy in the Top Gun movie who says, “I’ve lost the edge”….
Matt61
Matt,
When you rotate the scope you hold it in your hands like a clarinet and rotate the body so all the keys go around once. The clarinet keys are analogs for the scope’s adjustment knobs that get turned around on the long axis of the scope. Look at the box. See the clearance provided for the knobs to rotate? That’s how it works.
RE: Russian sniper scopes. If you have a real Russian sniper scope the reticle probably moves with the adjustment. So you can see it move when you look through the scope. If so, end of problem, except to note that you want the reticle lines to go opposite the direction of the intended bullet movement. I.E., if the gun shoots to the left and you want to move it to the right, move the reticle to the left.
B.B.
Matt61,
The whole point of learning and training is to remove, or at least reduce, the element of luck. Beginners luck is one of the worse things that can happen to an individual. I knew guys who started shooting at the same time as me, but quit. During the first several months to a year they could on occasion beat me, which wasn’t saying much because I stunk to high Heaven, like most beginners. But then they quit. However, their selective memories of when they did beat me established in their minds that they were, and always will be, better than me, even years later. Others who did not quit so soon will go on for the rest of their lives saying, “I could have beaten you if I practiced as much as you.” (I wasn’t “gifted” so I practiced a lot.) For some, the thought, or selective memory, is adopted and relished as if it IS reality.
In any case, the development of skill requires both a good teacher and a good student. If the student does not have a real desire (felt need) to improve, then they will likely not be a very good student. Sometimes the student surpasses the teacher, and sometimes the student isn’t ready to learn some lessons. As I’m sure everyone who went to college has experienced, some professors refuse to come down below a certain level to reach their students. Instead, they demand that the student work harder to reach up to their level (however nebulous that determination is). That’s acceptable, but only to a point. Your better professors are creative and insightful enough to try to see what’s going on in their students minds, to a point.
As I’m sure you know, a solution from a professor always makes A LOT more sense when the student tried really hard to get the answer themselves. When a student didn’t try, the answer makes little or no sense, and so everyone’s time is wasted. (For the most part, there are no shortcuts to success.)
The thing about shooting is that it is not entirely a one size fits all kind of activity. What works for me, may not work for you. But that is actually only true beyond a certain point, when you start to get into “advanced” material. The progression starts at the fundamentals, which are common to EVERYONE, and moves through phases of self-discovery and problem solving which only the shooter can determine, or solve, for themselves.
That’s why it’s hard to teach or coach at a distance, as I have tried. The issue isn’t so much distance as it is visual inspection, observation, and timing. It’s just a lot easier to work with someone in real-time, and where you actually see what they are doing. Otherwise, things seem to be much more of an abstraction.
I highly recommend that pistol shooters work with a buddy, or coach. While dry-fire may be the most important thing that a shooter can practice, it’s also very useful to work with a partner who has exclusive access to a spotting scope, and where the shooter is asked to “call” his shots. Just as a shooter gets better with a lot of practice, they also get better at calling shots as they master the fundamentals, like the Cardinal Rule that I keep mentioning. In general, either you can call a shot, or you can’t. If you can’t, then you are probably flinching because you are not following through.
Again, the short road to success is to know what you are trying to achieve (i.e., the Cardinal Rule). The long, and possibly never ending, road to success is to try to figure it all out by yourself. That’s probably what most end up doing. Best case, you get lucky and arrive at the Cardinal Rule. Worse case, you learn the wrong lessons and try to develop your skills based on incorrect principles (something that a surprising number of shooters do).
This is why being a competitive marksman is a tough road that isn’t for everyone (it can’t be). Even if you think you know everything, you still have to overcome yourself both physically and mentally. That’s why all the discussion about shooting through (ignoring) your wobble area, while focusing exclusively on sight alignment. That one thing alone is tough for probably everyone.
Victor
Regardless of the mechanics, if you have a steady enough rest: Shoot a group while centering the rested gun on the bull…
Then, again center the rested gun on the bull and, WITHOUT MOVING THE GUN, adjust the scope to put the cross hairs in the center of the group.
Whether the reticle moves relative to the tube, or one is moving an internal tube itself, the end result is to put the reticle cross where the bullets are impacting.
Ouch… so far as I know, most scopes (and probably all older ones) are 1/4″ clicks at 100 yards. More precise modern scopes are 1/8″ clicks at 100 yards. Even with the latter, a three inch adjustment would only be 24 clicks. About one rotation of the dial (I suspect one will not find a 12-click dial; 24 or 36 clicks may be more likely — if not even finer…)
BB,
I’m not sure I understand what you are trying to say in this article. I always do a rough optical centering of the scope by one of the various methods (count clicks, mirror, rotate) before mounting (to get maximum usable adjustment range) and then get the mounting as close to alignment with the center of the boreline as possible, trying each and every combination of ring orientation if necessary to get the best possible alignment with the bore in both axes; or I use the Accushot one piece which is very straight :). I am also fairly careful to use the lowest rings that work, assuming I have them on hand or can get them easily. None of this takes excessive amount of energy and it pays off big in the lack of trouble down the road. “Most airgunners” probably shoot at only one range regularly, say 25 yards, and click for maximum precision on the target. That scenario works just fine most of the time if you slap the scope on, but you might see problems if you try to shoot at a variety of ranges — most likely the pellet will be off to one side or the other outside your “zero” range or vertical error will mess up the usable trajectory.
Once the scope is setup, then pay attention to the AO — it is more than just a “focus knob” and needs to be calibrated to the shooter and setup on installation, then fine-tuned at each range — ignoring that by itself will swell groups 2x at times. The “head-bobbing” technique can easily eliminate most of the parallax error in a few seconds and even if you know the scope is dead on in terms of focus/parallax corrrection, the depth of focus makes it worthwhile to check regularly.
B.B.
As the scope tube is rotated (i.e. rolled), the crosshair path on the target is always circular in my experience and cannot be otherwise, imo, because the erector tube is not (typically) coaxial with the scope tube. Hence, I do not understand the triangular path indicated in your top diagram. Am I nitpicking or misunderstanding the geometry?
Also, regarding the so-called mirror method of optical alignment, its validity depends on the face of the objective end of the scope being perpendicular to the scope tube so that when the scope is on the mirror, the tube axis is perpendicular the to the mirror surface…no reason to believe that is true. In a serendipitous scenario, however, one error might help cancel another. 😉 Btw, both methods are done with AO (if exist) at infinity. Further, if the objective lens is adjustable (as in, AO), one must hope that the lens does not wobble when turned. Again nitpicking.
Out of curiosity, I tried both optical alignment methods (Vee blocks & mirror) on my two different airgun scopes (Centerpoint & Hawke). In all cases, the windage & elevation were WAY off their mechanical centers by the time crosshair orbiting was eliminated…way more than 3/8″ at 20yd…and the knobs must never be touched, else the optical centering is undone…and remember, you’re optically aligned but not yet sighted in, so one is condemned to the labor-intensive task of maneuvering the scope tube by tweaking mount base and/or honing/shimming rings until poi merges with poa…very discouraging. So I went back to keeping the subject of “optical alignment” on “IGNORE.” ;0
john
John,
Have you ever done this? This is how the crosshairs always move. They never move in a circular path. Maybe when all the errors are taken out they do describe a path that is more circular than this, but what I have shown here is pretty typical of what you see.
I have probably done about 20 scopes this way and this has always been what has happened. Plus, I talked with the Apelles A Team and this was their experience, as well.
B.B.
Or old type target scopes that did not have built-in adjustments — the micrometer dials were part of the rear mount ring.
Wulfraed,
Yes, and as I have mentioned in the past, those old scopes function just like the erector tube inside a modern scope.
B.B.
BB
The second drawing above is basically like the tool I made. It kind of looks like a 12″ AirForce barrel with the 2 aluminum bushings.
I took if I remember right a barrel from a 1377 pistol and checked it for straightness on a lathe with a indicator. Then I made two 1″ diameter bushings out of aluminum then drilled a hole in each for the barrel to fit in.
I attached the scope rings on the gun where they will be at when the scope is mounted then I put a bushing in each ring and used the straps that came with the rings and mounted the bushings with the 1377 barrel through the bushings.
The barrel extends out about 7″ in front of the scope ring. You can look over the top of the gun and see if the barrel in the rings is parallel to the gun barrel and equal side to side.
You can also look at the gun from the side and see if the barrel in the scope ring is parallel with the gun barrel that way.
I don’t know if this would be called optical centering. But if I put a indicator on the barrel that is in the bushings mounted in the scope rings and touch the indicator tip to the gun barrel I can tell real easy if the scope will be in line with the barrel.
No telling what happens inside the scope after I mount it in the scope rings that I just worked at getting parallel to the gun barrel. So maybe the mirror centering trick may also help after doing the above step.
Either way there is always going to be some error in the alignment.
John brought up a good point.
You have to learn the way your gun shoots.
If you zero different guns at the same distance and shoot at the same target at different locations you can tell pretty good if the sights of one gun is centered better than another. Of course if the performance of each gun is equal.
So again learn the gun you are shooting. That will give the best results. I use to keep all kinds of notes. It will drive you crazy if you try to get everything correct.
Again here we go. So many different variables to play into the equation.
I believe this blog has settled a problem i have been having with my SLR 98. It is dead on at 30 yards, .5 inches right at 10 yards, and 1.5 inches left at 50. Sounds like my scope is out of line with my barrel axis. Theoben uses the dampa mount which i supposed would make this unlikely. I was wrong, looks like i must begin to fiddle. Any suggestions? Thanks for the great blog.
Pop’s.
it seems your scope or rifle is shooting from right to left. Did you ever try to zero the scope at the 50 yard distance? If so, what did it do at 30 and 10 yards? Did it still shoot right to left? It could be that zeroing at 50 yards might result in being dead on at 30 and 10 yards but I’m not sure here. My suggestion would be to write down the windage settings needed for zero at 10, 30 and 50 yards (in addition to elevation setting) and click to those settings when you need to shoot at that distance. One other thing to rule out would be to determine if your pellet is corkscrewing. What is the POI at say 20 and 40 yards? Are you still shooting right to left or is there a reverse POI from what you’d expect – pellet trajectory now seeming to be from left to right? That would indicate to me the pellet is corkscrewing through the air as it travels down range.
Fred DPRoNJ
Thanks for the reply! I believe i have it fixed! Thanks for the help.
Pop’s SLR,
This problem is very common, and not juyst with airguns. Firearms have it, too.
The next part of this report will tell you how to fix it.
Thanks for reading,
B.B.
Thank you so much for your help B.B. I realigned the scope with the axis of the bore. I now have one less variable to calculate! Thanks!
I hear the field target shooters are still optically centering their scopes, and then fine tuning the mounts to get alignment to the barrel bore. But the reason for the optical centering is to get the most resolution from the optics (to allow for more precise range estimation), rather than to eliminate left to right shift at different distances.
Do you believe this is beneficial ?
John,
Optical centering is not to get resolution from the scope. It is to correct the left-right error.
B.B.
I don’t understand… I hear the resolution of most scopes is better near the center than near the edges. Wouldn’t an optically centered scope get a little higher resolution than one that wasn’t centered ?
John,
Don’t confuse the optics in the scope, which means the lenses and their alignment, with optical centering, which is a mechanical alignment of the erector tube so as the scope is adjusted up and down it doesn’t also do left and right.
Optics and optical centering are two non-related things.
B.B.
I have used Burris signature mounts with off set plastic rings for many years. After centering the scope reticle, I use the off set rings to get the group as close to the center of the target as possible. Then I use the scope knobs to fine tune the zero. I would like your opinion of the Burris mounts and my method of adjustment.
zimbabwaeed,
Are you speaking of rings with inserts that have offset holes in a collar so they can be turned to vary the height? Like the ones Conetrol invented back in the 1960s?
Those will work. I’m not familiar with what Burris has, so I will have to look into it, but the Conetrol idea is a solid one.
Conetrol apparently thinks their idea has been ripped off by other mount makes, as their home page mentions:
http://www.conetrol.com
B.B.
Burris uses split rings, they can be used to adjust windage (in one ring) and elevation in the other ring. Prior to using Burris rings , I used Pachmayer tip off mounts. The latest ones can be adjusted for windage and elevation .They are a species of quick detachable mounts. I used them in Africa (3 safaris, RSA and Zimbabwae) as well as in Canada( moose). They worked well, never lost their zero(Including a 50′ fall down a kopjie ).It is a pity that they are no longer made. The introduction of variable power scopes would have required them to design new rings. That and the hand labor in putting them together made them too expensive to make. Ed