How to mount a scope: Part 4

by Tom Gaylord
Writing as B.B. Pelletier

Part 1
Part 2
Part 3

This report covers:

  • What optically centering DOES NOT mean
  • What optically centering really is
  • How to optically center a scope
  • Why do we do it?
  • Field target
  • Counting clicks — mechanical centering
  • Erector tube return spring
  • A better way
  • What about left and right?
  • Why so anal?
  • Pragmatic approach
  • Summary

Today we are going to discuss optically centering a scope. It’s going to be a difficult report for me to write, because the subject does not have much merit for airgunners. So I will compensate by adding some things that do have merit. Let’s go!

What optically centering DOES NOT mean

Let’s start with what optical centering DOESN’T mean. The optical center of the scope is not the place at which there are an equal number of clicks up and down and side to side. I say that and some of you already know it and yet the website “RifleOpticsWorld” has an online article written by “Rifle Optics Team” that says that setting a scope to the optical center is simply returning it to the factory setting. Excuse me????? 

Who in their right mind believes that a rifle scope comes from the factory set to its optical center? The factory assembles each scope as quickly as possible, checks it at certain points for quality and ships it. They don’t spend 45 minutes or more optically centering each scope they make!

This article then proceeds to tell the reader that optically centering is a solution to scope shift! No, it’s not! Optical centering has nothing to do with scope shift and it doesn’t fix it. I will tell you today what really does affect scope shift and how to correct it.

After reading this online article it is obvious to me that it was written by someone (or a team of someones) who was assigned to write it and they made stuff up as they went. If you understand what optically centering is, I invite you to read the article and see how far off the mark it is.

What optically centering really is

The optical center refers to the reticle and the field of view. An optically-centered scope shows zero reticle movement against a distant backdrop when the scope tube is rotated in a complete circle. Theoretically, it’s possible to achieve, but I’ve never seen it. The best I’ve seen is a reticle that moves about a quarter inch against a target 20 yards away when the scope tube is rotated in a complete circle.

How to optically center a scope

There are two ways to optically center a scope. The first way is to set the scope tube in Vee blocks and rotate the scope while watching the reticle against a distant target. I have done this with a quarter-inch dot as the target — set 20 yards from my position. Believe me — it can take a long time to get the scope pointing exactly at that dot — even with two people working at it! The easiest way I have found is to put a large white sheet of paper at 20 yards distance and talk a friend into drawing the dot, while you look through the scope that’s sitting in the Vee blocks and direct him. I initially thought the precision of the Vee blocks mattered, but I’ve since recognized that you can use a cardboard box with two Vee grooves cut in the right place to support the scope tube. It’s not the blocks that give the precision; it’s the fact that, other than rotating on its axis, the scope never moves..

Now, rotate the scope tube in the Vees and adjust the reticles until both lines remain centered on the dot. On a good day with some luck this takes about 45 minutes to get as close as you are going to get and the reticle will still move off the dot in a few places in its rotation, i.e. the intersection will move in and out of the dot as the scope rotates.

The second way to optically center the scope is to stand the objective lens on a mirror in a well-lit room or even outdoors in bright sunlight. Look down through the scope and what do you see? If the reticle appears blurry or doubled, adjust it until all you see is one sharp reticle. In other words, the real reticle is on top of its reflection. This way sounds easier than the first method because it is. But it doesn’t give results that are any better than the first method and maybe not as good. You see, the glass on the mirror is not parallel to the reflective surface on the back of the mirror, and you will always be off by some small amount.

mirror technique
With the scope’s objective resting on a mirror, adjust the horizontal and vertical reticles until the heavy reticles are covering the shadow lines — as best you can.

Why do we do it?

The belief is that once the scope is adjusted to its optical center it can then be mounted in an adjustable scope mount and, without changing the elevation or windage knobs, zero the scope at an ideal distance by adjusting the mount, only. You will have to use a scope mount that adjusts in both directions to do this. Having done this several times I can tell you that it is absolutely impossible to do. But, for the sake of argument, let’s assume it can be.

Once the scope is optically centered and also zeroed, you can then adjust the scope’s elevation reticle for range changes to your target. Because the scope is optically centered, the pellet will stay on the vertical reticle at all practical distances. Let me give an example.

Field target

You have optically centered your scope and then zeroed it at 20 yards by adjusting the scope mount. You are shooting a .177-caliber pellet at 900 f.p.s. You will be on target from 20 yards to about 24 yards and then your pellet will start to rise above the center of the crosshairs. The pellet is not really rising, of course; it just appears that way to you.

From 24 yards to 28 yards the pellet rises until it is one pellet’s diameter above the center of the crosshairs but exactly in line with the vertical reticle. Starting at 29 yards until 32 yards the pellet will descend on the vertical reticle line but still be on the line, left and right. At 33 yards the pellet will start to descend below the center of the crosshair and by yard 35 it will be one pellet’s diameter below the center of the crosshairs.

dime and sight-in
This is the level of accuracy a field target competitor is seeking.

What you have just done is sight in an air rifle that is on target without any scope adjustment from 20 yards to 35 yards. You will be theoretically able to hold on the center of a kill zone on a field target within that range span and hit the paddle without touching the side of the kill zone — on all targets that have a 3/4-inch kill zone or larger.

All that I have just said is theory and it doesn’t work that way in the real world. In the real world the following is true.

1. It is impossible to optically center any scope. There will always be some slight movement of the reticle against a distant target as the scope is rotated through 360 degrees.

2. It is impossible to zero a scope with an adjustable scope mount. You can get close, but never exactly on.

Before we continue, let me define what I mean by exactly on. I mean the pellet is striking the point where the reticle lines intersect, with an equal amount of the pellet on either side of each line. This sounds anal, but the sport of field target makes shooters anal pretty quick. You see, there are kill zones that are smaller than 3/4-inches — some as small as 15 mm. When I competed in the late ’90s they were even smaller than that — down to 3/8-inch (9.44 mm). 

caliper and dime
This is the size (15 mm) of the smallest field target kill zone today.

If your pellet touches the side of the kill zone while passing through it can push the target “face” backwards hard enough to lock it upright, even though the paddle has been hit. The target won’t fall and you won’t get a point. This is the reason field target shooters are so concerned with accuracy.

Once when I was competing, one of the shooters brought his friend to the match to try it. He was a SWAT sniper and was confident he would do well. I think he thought he would teach us all a lesson in how to shoot. He finished in the middle of the pack of about 20 shooters and when it was over he told us that he was trained to shoot someone in the middle of their head. He reckoned that field target shooters would aim for one particular hair on the head — which is just about the case.

Counting clicks — mechanical centering

I hope I have made my point why optically centering a scope is impossible. Later on I’m going to tell you a very practical way to get the result that people desire from centering, but right now let’s discuss mechanical centering.

When you center a scope’s adjustments mechanically you are finding the spot in both adjustment where there are an equal number of clicks in all directions. If there are 123 clicks down there have to be 123 clicks up. Same for left and right, though they may not be the same number of clicks left and right as up and down. But don’t worry — it doesn’t matter, and here’s why.

Erector tube return spring

The reticle lives in the erector tube and the lines never appear to move when adjusted. That’s because they don’t. The entire tube moves while the lines remain stationary. Yes, there are European and Russian scopes whose reticle line actually do move, but they are an exception and not a part of this discussion.

There is a spring that’s mounted on a 45 degree angle to the erector tube and across from bothe adjustment knobs. It pushes back against both the vertical and right adjustments to keep the erector tube in whatever position the adjustments have put it. To the shooter it looks like nothing has moved.

At some point this spring gets relaxed and doesn’t push as hard. Then the tube can move without being adjusted — as in when it is jostled or bumped. That is when the scope starts to lose its zero and shifts randomly. So, centering the reticle (erector tube) mechanically doesn’t make much sense. Yes there may be 123 clicks on upward adjustment but the last 63 of them may be with the spring relaxed, so they are worthless. You don’t want to adjust the elevation there. Now that the scope is mechanically centered you have a lot of good downward adjustment that is useless (because you never adjust the scope that way) with very little upward adjustment before you start experiencing scope shift.

erector tube return spring
When the erector tube return spring relaxes, the erector tube starts moving on its own from vibration. Goobye zero!

A better way

Now I will tell you what really works and what top shooters around the world have discovered. Forget optical centering. Forget mechanical centering. Instead, adjust your scope until there is very little downward adjustment left. Once you zero the scope (with the adjustable scope mount) you will never use any downward adjustment. But you will use the upward adjustment, and this procedure has left a lot of it in the scope.

Everything I just said also applies to left and right adjustment, though it is not as critical. Gravity pulls pellets down; it doesn’t move them left and right.

What about left and right?

Okay, you understand how up and down works. What about left and right? Let’s assume that when you adjust the scope up the pellet stays glued to the vertical reticle. It never moves off the vertical line. It never does, but let’s pretend for a moment that it does. If you haven’t optically centered your scope, what happens when the farther out you shoot the farther the pellet strays to the left? 

Let’s also assume you are using a scope level for every shot, because none of this works if you aren’t. You notice that at 35 yards the pellet is half a diameter off to the left and at 45 yards it’s more than a full diameter off. What do you do?

What you do is check your zero at every 5-yard distance from 10 yards to 55 yards, because that is the distance at which you compete. Yes, I am aware the rules have changed and those distances are now stated in decimal fractions, but let’s keep this simple. And I said you check your zero every 5 yards and keep making small adjustments, but champions will then refine that to every yard — from 10 to 55 yards, or every meter from 10 to 50 meters.

What you do is adjust the left-right setting on your scope to get it as close to the centerline as possible at all distances. You never will get it perfect, but let’s say with careful work you get it to the place where your pellet is one diameter off to the left at 51 to 55 yards and one pellet diameter off to the right at 10 to 14 yards. It’s off by a lesser amount at the intermediate distances. Most field target competitors would be thrilled to have a scope that was that dialed-in.

Why so anal?

Do you really have to do any of this? Of course not. I don’t. You can go right on with your life, just as before. Nothing has changed. I wrote today’s report for those readers who were asking about optical centering. 

Pragmatic approach

The steps I have just given you (the last ones — the ones that really work) can take DAYS to complete! If you always want to see the bullseye get hit, watch a movie. This level of commitment to perfection is why some scopes cost more than $3,000 and some mounts cost over $500. You would be fooling yourself to think that serious competition can be done without a serious investment. Yes, you can follow these steps and do quite well on a budget, but remember — nobody races real cars in NASCAR.

My way is hard work. All the theory is out the window. As Jedi master Yoda told us, “Do, or do not. There is no try.”


In this article I have given you lots of things to do. But at their core there is one piece of invaluable advice. That is — it is impossible to do any of these things exactly. That applies to all of the procedures and desired results I have presented. You may think you are a perfectionist, but also recognize that for human beings there is no such thing.

You still should do your best to get as close as you can — so close that your hard work becomes a humorous anecdote that you can tell for many years to come, as I have just done for you!

How to mount a scope: Part 3

by Tom Gaylord
Writing as B.B. Pelletier

Part 1
Part 2

This report covers:

  • The Design an Airgun contest
  • Air gun?
  • How to enter
  • I lost one entry
  • The Godfather’s Gold Gun drawing˜
  • On to today’s report on cant
  • Canting is not part of scope mounting
  • What is cant?
  • The cant test
  • What cant does
  • Things that affect cant
  • What canting can do
  • When precision is a must
  • Consistency
  • How to eliminate cant
  • High scopes
  • Where the level goes
  • Summary

The Design an Airgun contest

Apparently it took a while for many of you to realize this Design an Airgun contest was happening, so I’m extending the deadline to Friday, October 16. I’m challenging you to design an airgun that we readers can build!

I’m guessing it will be a BB gun, but it doesn’t have to be. It doesn’t even have to be a gun, as long as it shoots something at a target. If it is a gun I’m guessing it will be a smoothbore, but again, it doesn’t have to be.

Air gun?

When I say build an airgun, it doesn’t have to work with compressed air. The Daisy 179 pistol is considered an airgun, but in reality it is a catapult gun. And spring-piston guns don’t have compressed air until the instant they fire.

How to enter

You can enter by writing your design as a comment. Attaching pictures helps me a lot. One reader has already entered by submitting a You Tube video. Or you can email me at [email protected] I’m wanting to show all of you how the winning design works in a special report, so any help you can give me is appreciated.

I lost one entry

To the reader who submitted two designs with photos to me via email several weeks ago — please send them again. I have lost them. I remember making a file of your entire submission, but after three days of searching I cannot find it. You were the first person to submit an entry and you explained it well enough that people could follow along and build it themselves.

The prize for the winning entry is the odd gun I have shown you many times. I called it the American Zimmerstutzen. I have to limit the contest to US readers because of the problems of shipping the prize.

American Zimmerstutzen
This is the prize for the best airgun we readers can make. It’s an intricate homemade weapon that I think is the perfect prize for this contest!

The Godfather’s Gold Gun drawing

The other contest for the Goldfather’s Gold Gun starts today. That is an Ataman AP16 air pistol that I built up using the new Pyramyd Air “Build Your Own Custom Airgun” software.

One day in October I will choose a winner from all the readers who comment. Whether you comment once or many times makes no difference. Your comment enters you into the drawing for this air pistol. I will not announce the day I select until the month is over. Once again, this is limited to US readers.

Ataman AP16 Standard
The Godfather’s Gold Gun will belong to one lucky blog reader after October is over.

On to today’s report on cant

Today we look at cant, as in tipping the rifle and scope while shooting. I’m going to sum up today’s report in a single word — consistency. Because that is what we are after. The report will explain.

Canting is not part of scope mounting

We are concerned with canting on every shot we take. It’s not something that gets addressed while mounting a scope — BUT!!!! So many shooters associate canting with scope mounting that we need to discuss it now. Part 2 was the last part of scope mounting, per se. 

What is cant?

Cant means a prominent angle or tilting, as defined by But in shooting it means something far more devious. Yes, it is a tilt, but it is also an inconsistent tilt, and it’s that inconsistency that creates the problem.

The cant test

When I wrote The Airgun Letter I did a cant test with my subscribers. I asked them to shoot three groups of 10 shots at 50 yards. One group was to be a regular group where they were to zero their scopes to hit the center of the target. I sent out special targets that had bolded lines for the shooters to cover with their reticles. One set of lines was tilted 20 degrees to the left, one set was aligned correctly and one set was tilted 20 degree to the right. If they covered the lines with their reticle they would shoot one group straight, a second group canted 20 degrees to the left and a third group canted 20 degrees to the right.

The results of the test were dramatic and all were the same. Everyone was sighted to hit the center of the target at 50 yards. When they canted 20 degrees to the left their 10-shot group landed low and to the left. When they shot straight on their groups hit the center of the target. When they canted 20 degrees to the right their groups landed low and to the right. Let’s look at the results from four shooters.

cant test
These are the results of 4 different shooters shooting 10 shots at each of three targets at 50 yards. Targets on the left were shot with the scope canted 20 degrees to the left. Same on the right. Center targets were shot with the scope level. We used the heavy lines on each target to align our reticle.

What cant does

When a scope cants away from the orientation at which it was sighted-in (notice, I did not say from where it was level), the pellet impacts in the same direction as the cant. A right cant produces a pellet impact that’s more to the right than it should be. The pellet also drops away from level at the impact distance. The next drawing shows this very clearly.

cant effects
The heavy curved line represents where pellet will impact if the rifle is canted from its sight-in position. The farther the distance from the shooter to the target, the more pronounced the curve.

Things that affect cant

Distance — the farther out you go the more affect cant will have.

Height of scope — The higher the scope is (farther from the bore axis) the more affect cant has.

Velocity — the higher the velocity the less affect cant has. If you look at the picture of the cant test you’ll see that some pellets didn’t drop as far as others. That was due to their velocity.


I made this test exaggerated to show the effects of cant dramatically. Most shooters would never cant by as much as 5 degrees, let alone 20. There are, however, some things outside the shooter’s control that will also enter into the discussion.

When I worked at Frontier Village amusement park in the 1960s, we had an attraction called El Sito Mysterio. It was patterned after an attraction in the nearby Santa Cruz mountains called the Mystery Spot, where gravity was supposed to be all confused. El Sito at the Village was a small shack that had several interesting illusions. My favorite was the bowling ball that rolled uphill. I worked there and gave the pitch and even I was fooled! So terrain and visual cues can fool anyone into making a mistake.

What canting can do

Even a one degree cant has an effect on shot placement. Let’s now look at two groups that were fired with and without cant.

what cant does
This drawing is representative of how cant affects a group. This is why it isn’t that noticeable. It just looks like poor accuracy. It’s easier to see at 50 yards than at 10.

When precision is a must

Cant doesn’t always matter. To a hunter shooting in the woods, a fast rifle with a quick second shot is more important. The squirrel doesn’t care if the pellet lands 1/4-inch from the intended impact point. But to a varmint hunter busting prairie dogs at 300 yards, cant is very important.

Field target shooters are quite aware of the effects of cant because when they hit their target one pellet’s diameter off their intended impact point they may hit the side of the kill zone and lock the target in the upright position. Canting costs them points, and they are already fighting their own body’s stability. They don’t need a second problem.


I said at the start of this report that consistency was the solution to canting. Now I’ll explain why that is. When you shoot, the pellet leaves the muzzle for the target  — hopefully with the muzzle in the same place each time. By canting or tilting your gun, you move the scope off target and then you have to move the gun to get it back on. When you do, the muzzle is in a slightly different place than before. It could be less than one-tenth-inch or 2.54 mm, but it is different and, if the gun is accurate, the pellet will fly to a different place. And that does affect accuracy in a most insidious way, as you see in the group above.

How to eliminate cant

Cant is eliminated by leveling the rifle before each shot. This is done with an accessory called a scope level. It doesn’t matter where the reticle is — so long as it is always in the same place when the rifle fires. If the rifle is accurate and if it shoots to the same place every time, the group will be small and exactly where the shooter wants it.

Let me show you a short test I did that shows the importance of this. The test is straightforward. First, I seasoned the bore with several shots. Then, I fired a group of 10 shots at 25 yards with the Whiscombe rifle canted to the right for 5 shots and to the left for 5 shots. The cant was controlled by the position of the bubble in the level, and I stopped tilting the rifle the moment the bubble came to the end of its travel. Obviously, there’s some error in this, as the bubble level is not a precision instrument, but I think you’ll get the idea.

canted left
The Whiscombe was canted to the left until the bubble came to the end of its travel on the right side, as shown above. Five shots were fired at the target with the crosshairs on the center of the bull at 25 yards.

canted right
The Whiscombe was then canted to the right until the bubble came to a stop on the left side, as shown above. Five more shots were fired at the same target, just as before.

ten shots canted
And this is the result. It’s impossible to tell that canting opened this group.

ten shots level
Same rifle, same distance, same pellet, only this time the rifle was leveled for each shot.

High scopes

Scopes that are mounted high above the boreline are influenced by cant the most. This is why many shooters want their scopes mounted as low as possible. Repeaters with magazines that stick out above the receiver mitigate against this, of course.

Where the level goes

Mount the level on the air rifle where it best fits how you shoot. When I use a separate bubble level I mount it sticking out to the left side of the rifle. Before each shot I look at the bubble and make any corrections that are necessary. But best of all is when the bubble is built right into the scope — like it is on the UTG Bubble Leveler scope! Then I don’t have to take my eye off the target.


Canting isn’t a part of mounting a scope, but if the scope is mounted correctly then the rifle can be leveled for each shot afterward with assurance that it will do its best.

We are still not finished with this series. The next report will be about optically centering the scope and centering the reticle mechanically. I hope you are getting something out of this.