Posts Tagged ‘scope adjustment’

Scope dope — I hope! Part 4

by Tom Gaylord, a.k.a. B.B. Pelletier

Part 1
Part 2
Part 3

This is an ongoing series about scope questions and issues. Blog reader David Enoch asked for it originally, but many other readers have jumped in since it began. Today, I’ll talk about adjustable scope mounts.

First things first
Why do we need adjustable scope mounts? Aren’t the scopes, themselves, supposed to adjust? Yes, they are, but 2 things quickly become a problem. First, the scopes don’t adjust as far as we need them to; and second, when a scope adjusts toward its upper and right limits, it loses its precision. I will address the second problem because it’s really the principal one.

When we look at a scope, we see that it has a range of adjustability and assume that it’ll work as it should throughout this range. But that’s not the case. Inside each modern scope there is a smaller tube called the erector tube. The erector tube often contains the reticle; and as the tube moves up, down, left and right, so does the reticle. So, moving the erector tube is what moves the reticle.

There is a spring or springs that press against the erector tube, making it press against the adjustment knobs, in turn. That spring has a range of movement it goes through as the tube moves. When the tube is up high or far to the right, then a spring or springs that press it against the adjustment knob or knobs are relaxed and can allow the erector tube to move when the gun vibrates, such as with a shot. This is one of the chief causes for “scope shift.” You fire the gun, and the erector tube moves slightly, taking the reticle along with it, of course. On the next shot, the scope will be aiming at a slightly different place. It’ll appear that your scope is wildly throwing the shots around.

scope knob adjustment range
This elevation knob is adjusted up to its maximum limit. I recommend not adjusting it higher than the number 3 line on a scope like this to avoid scope shift.

I tell folks that a good rule of thumb is to not adjust their scopes above the 3/4 mark on the elevation knob or past the 3/4 mark on the right windage adjustment. Some scopes can adjust farther than this without a problem; but the closer you stay toward the middle of the range, the better. If your scope doesn’t have knobs like these, you may have to count the actual clicks of adjustment to know where you are.

Is adjusting the scope in the opposite direction (i.e., down or to the left) a problem? No, it isn’t. You can adjust all the way until the adjustments run out in the down and left directions. It doesn’t hurt the scope, nor does it affect accuracy.

So, the scope that you thought had a huge adjustment range turns out not to have as much as you believed. Yet, your airgun (or firearm) needs more adjustment than you have. How do you compensate for the adjustment you no longer have but may need? With a scope mount that adjusts, of course.

Adjustable scope mounts
The purpose of an adjustable scope mount is to align the axis of the scope in a direction different than the scope base on the gun dictates. If all scope bases were aligned with the axis of the barrel, there wouldn’t be a problem, but they aren’t. Adjustable scope mounts can compensate for this, leaving the scope’s internal adjustments to serve the ballistic requirements of the gun in question.

Up and down, left and right
A barrel can point off from a gun’s scope base in any direction, but the most common direction is down. The barrel “looks” down, in relation to where the scope tries to look. The other 3 directions are also possible, with left being the second most common. After that, the other 2 directions happen pretty infrequently.

So, if you’re going to need extra adjustments, it will most likely be extra “up” that you need, followed by extra “right.” Adjustable scope mounts have to provide extra scope movement in all directions, with up and right being needed most often.

Scope tube integrity
The scope tube is a hollow, rigid tube that must maintain its integrity to keep the lenses in alignment. If the tube were to bend, it could seriously damage or even break the scope. Adjustable scope mounts must either move the scope as a whole without putting any stress on the tube, something that only a 1-piece mount can do; or they must adjust in such a way that when the rear mount moves, the front mount can relieve the stress on the scope tube. Only the B-Square AA adjustable scope mounts were able to do that; and when B-Square sold the company several years ago, the new owner moved the manufacture of the AA adjustable mount to China, where the quality control was soon lost. You cannot buy new AA adjustable scope mounts any more.

Benjamin Marauder old style magazine
When the rear mount is raised above the front mount, if the front mount doesn’t move to compensate, the scope tube will be strained. These B-Square 2-piece AA adjustable mounts have rings that pivot forward to allow the scope tube to remain straight.

Sports Match has 2-piece adjustable mounts on the market; but as far as I can see, they make no provisions for relieving the stress on the scope tube when the rings are adjusted separately for elevation. I guess I need to test them to learn their operational parameters. I don’t see how they can avoid stressing the scope tube when the front and rear mount are at different heights, but I’m willing to hold my opinion until I’ve examined them.

I’ve tested several 1-piece adjustable scope mounts and found all of them to work well in this regard. Most recently, I tested the BKL adjustable mount and found that it moved well in both directions.

What about precision?
To date, no one has made an adjustable scope mount that adjusts with precision for a modern scope. Such mounts do exist for vintage scopes that have no erector tubes because the entire scope has to be moved by the mount. I have shown you this kind of adjustable scope mount a couple times.

adjustable scope rings
This Unertl scope ring adjusts to move the entire scope. It has the same precision as the adjustments on a modern scope.

Slippage is common with adjustable scope mounts
The most common problem is the adjustable scope mount that does not hold its position. That’s why the Chinese-made B-Square adjustable mounts failed. Their screw holes had sloppy threads that tore out under stress, and the mounts couldn’t hold in position. So, whatever adjustable mount you get, it must hold its position once it’s been adjusted, or it won’t work.

And slippage happens soonest on spring guns because of their recoil and vibration. Ironically, spring guns are the very ones that need the adjustable mounts most often. There’s nothing that can be done about this, but you must understand that you don’t want a scope mount that can’t hold its position.

Firearms shooters need adjustable scope mounts more today than ever before. I think that’s because modern guns are being assembled faster and with less precision than they were in the past. The thing is that firearms shooters are not as aware of scope problems as airgunners, so they tend to have more of them; and when they do, the problems are harder for them to resolve. I’ve tried to help people who I knew were having some common problems such as adjusting too high in the scope’s range, but they just looked at me like I was crazy. Surely, no scope manufacturer would field a scope whose adjustments were not 100 percent useable?

That’s all I have for you today. How about telling me your other unresolved scope issues?

Scope dope — I hope! Part 3

by Tom Gaylord, a.k.a. B.B. Pelletier

Part 1
Part 2

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.

crosshair shift when scope is rotated
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.

scope alignment problems 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.


group movement over distance
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.

optical centering fixture
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.

Big Shot of the Month
Pyramyd Air’s Big Shot of the Month on Facebook is┬áRoberto Martinez. He’ll receive a $100 gift card. Congratulations!

Pyramyd Air Big Shot of the Month

Roberto Martinez is the Big Shot of the Month on Pyramyd Air’s airgun facebook page.

Scope dope — I hope! Part 2

by Tom Gaylord, a.k.a. B.B. Pelletier

Part 1

This report is for blog reader David Enoch and for several other readers who asked for it after he did. When I did the first report on the main cause of scope problems, which is the scope being adjusted either too high or too far to the right, David asked me to explain how to correct the situation.

There are several ways to correct this situation, and today I’ll explain the easiest one, which is also the one most often attempted by shooters: Shimming.

Important fact!
The problem we’re trying to correct here is that the scope does not adjust high enough to get the pellet to hit the aim point. Here’s a very important point about that. Many times, the scope will have been adjusted beyond its upper limit, and the pellet will still be striking low. So, if the shooter shims the scope like I’m about to show you, he may discover that the problem has not been fixed. That’s because the scope was adjusted way too high in the adjustment range. Even though he’s corrected the angle a little, he hasn’t corrected it far enough.

This frustrates those who are new to this sport, and they’re often put off by the fact that the fix doesn’t work. They think either their scope is trash or their gun is trash because they do not appreciate what’s really happening. I read comments all the time from people who experience this, and it’s clear to me that no one has ever explained everything to them.

I don’t know if this report will help them, either, because these people are not the type who research their problems. But if you have a friend who tells you about something like this that happened to them, you’ll now have the information to explain what’s happening.

Shimming helps
You all should know that on a gun with open sights, the rear sight has to be moved in the direction that you want the pellet to move. The rear of the scope is like the rear sight of an open-sighted gun. And the front of the scope is like the front sight, by the same logic.

If the rifle shoots too low, we want to move the rear of the scope up, and that can be done by shimming the rear scope ring. What we’re looking for is a downward angle (slanting downward toward the muzzle) to the scope, so the shim is placed under the rear scope ring to lift up the scope tube at that point. Nothing magical about it.

What is a shim?
A shim is a piece of material that causes something to move. Carpenters use shims all the time to square-up the frames of doors and windows. In their case, the shim is a wedge-shaped piece of wood they drive into a crack until the frame they’re straightening is true. Then, they break off the part of the shim that sticks out of the crack and the job is finished.

In our case, we will make shims from flat pieces of metal or plastic. We’ll cut them to the same size as the scope ring half they’re going into…or just a little smaller. Then, we’ll put them under the scope tube when we mount the scope in the rings. The shim will raise the scope tube by fractions of an inch; and if the problem isn’t too great, that’ll solve it.

cane and shears for shimming a scope
Aluminum soda cans make good shim material. I used metal shears, but good scissors (not your wife’s sewing scissors, though!) will also work well. I flatten the can in the middle to make the cut easier, and you only want one thickness of material for one shim (i.e., not both sides of the can).

shim cut and laying in scope ring
When the material is cut to size, lay it in the bottom ring. Metal will conform easily to the shape of the ring. Plastic from a soda bottle will not and will have to be flattened by the scope tube when you mount it.

Once the shim is in the bottom of the ring under the scope and the cap of the ring has been tightened, it’s very difficult to see the shim. If you can see it, it either isn’t in the ring all the way — which isn’t a huge problem — or it’s too thick, which is a problem.

How many shims?
The reason people don’t shim scopes is because too many shims will dent the scope tube when the cap screws are tightened. How many shims can safely be used depends on the thickness of the shim material. If you use metal cut from aluminum soda cans, you can use as many as two shims stacked on each other. If you use the thicker plastic cut from a 2-liter soda bottle, I would stop at just one. Any more and you risk the possibility of denting the scope tube when the cap screws are tightened.

What doesn’t work
If shimming under the rear ring will cause the rifle to shoot higher with the same sight setting, what about shimming above the rear ring (under the scope cap)? Will that push the pellet down? The answer is no. The impact point will not move if you shim above the scope.

To move the impact of the pellet down, put the shim under the front ring. That lifts the front of the scope, which is the same as moving the front sight up. Moving the front sight always moves the impact point in the opposite direction.

What about shimming left and right?
Can you shim the scope on its side to move the impact left or right? Yes, but be careful. If the shim extends down to the underside of the scope, it’ll also move the scope up or down, depending on which ring you’re shimming.

Do this first — before shimming
If you can, turn the rings or one-piece mount around and try them that way. Scope rings are not made with tremendous precision, and sometimes turning one or both of them around — and swapping the front ring with the rear ring, in the case where 2-piece mounts are being used — will move the impact point the way you want. Do this before you start shimming, as it puts less stress on the scope.

Sometimes, the design of the scope rings does not permit turning them around. This is particularly true in the case of 1-piece mounts that are also asymmetric. They can be mounted only one way and have a very limited range of positions for the scope. In this case, shimming may be the only recourse.

What about…?
I’m stopping this report here because I want to bound the information. I can talk about adjustable scope mounts, optical centering and other scope-related topics in later reports. In fact, I’ll be watching your comments to this report to see what’s needed.

Scope dope — I hope! Part 1

by Tom Gaylord, a.k.a. B.B. Pelletier

I recently said that, with all the new readers of this blog coming from the firearms world, I need to concentrate on the fundamentals. Today will be such a report.

I was at the range on Tuesday and watched a familiar problem play out. Only this time it happened to a firearm owner rather than an airgunner, and I believe that is why it was such a problem. Airgunners are conditioned early that their scopes don’t look where the barrel of the gun points. Not so for firearms shooters. They seem to take it for granted that the barrel is in alignment with the axis of the scope — which is almost never is.

This shooter was experiencing problems getting her .243 Thompson Center Encore sighted in. I was three benches away and here is what I saw. She kept adjusting the scope up and up, and it didn’t want to go as high as she needed it to. She finally got on target, but she was getting 5-shot groups with three close together and two about 4 inches away at 100 yards. This was a rifle that was known to be accurate, and she had already shot a Ruger .204 Encore several times this same day, getting inch-sized groups. So, I knew she could shoot.

I could diagnose this problem in my sleep because I’ve seen it hundreds of times. But do you know what was happening?

The problem
The scope was adjusted too high, and the erector tube spring was relaxed, so the erector tube that holds the crosshairs was bouncing around with every shot. Sometimes, it would stay put, and other times it moved. The result was a shifting point of impact.

I saw that her scope was a cheap one. It had two things going against it. First, it had the kind of adjustment knobs that provide no feedback about where you are in the adjustment range. When you adjust the reticle all the way up, you can’t see that you have. Second, because the scope was a cheap one, its erector tube spring was also cheap. That means the useful range of adjustment before the spring relaxes and allows the erector tube to move is quite limited. In short, these scopes get into trouble a lot sooner than scopes that are better made.

cheap scope adjustments
A scope adjustment like this gives you no feedback on where the elevation is set. It’s easy to adjust up out of the range where the erector spring holds.

AirForce 4-16X50 scope better scope adjustment
This adjustment knob tells you at a glance (by the vertical index marks) how much elevation is applied. This is a so-called target knob, but there are many scopes whose adjustment knobs have covers and work the same way.

How to check for this problem
My shooting buddy, Otho, was mentoring this lady, and he called me that evening to ask what was happening with her gun. I told him about rifle bores never aligning with receivers or scope bases, and he understood right away because he’s drilled and tapped dozens of vintage rifles to mount scope bases. He knows very well that a rifle bore will seldom line up with the top of the receiver or with the scope mounts.

I walked him through the problem with the scope that I mentioned above. It turned out that the lady’s husband had switched scopes on that rifle and, of course, nobody knew where the current scope was in its range of adjustment. If it had been on a drooper firearm before, it might already have been adjusted high in the elevation range; and what she had to do to get the Encore on target on this day might have pushed the vertical adjustment into oblivion.

I have to stop for a moment and answer a question that is bubbling up in someone’s mind. If the scope can’t be adjusted beyond a certain point without causing problems, why don’t the manufactures limit the adjustment range? Why, indeed?

Why do you suppose automobile speedometers go up to 120 MPH when the cars they are in will only hit 98 on a perfect day going downhill? Because it’s easier that way. They sell more cars that way. Because the companies who manufacture speedometers only make them certain ways, and the car companies have to buy what’s available. Etc.

Oh, don’t you dare tell me speedometers are all digital these days! I know that. I’m making a point, and you know very well what I’m saying.

There is an easy way to check this — to see if I’m right, or if something else has happened. All you have to do is dial the scope’s elevation back down 40-60 clicks and shoot again. Put the aim point on a large piece of target paper with lots of room below it and see where the shots land.

While you’re at it, run the windage adjustment to the left 40-60 clicks, too, because adjusting too far to the right is the same as adjusting too high; and on many scopes, there’s only a single erector tube spring set on a 45-degree angle to both adjustment knobs. If you get a tight group with these shots (regardless of the fact that it’s low and to the left), you know that the erector tube was floating before. The solution is easy.

Unertl scope adjustment knobs return spring
This shows how an internal erector tube is adjusted. Even though this is an externally adjusted scope, it works the same. The spring is at the 5 o’clock position inside a blued steel button. As either adjustment knob is turned, the spring inside the button compensates, keeping tension on the scope tube at all times.

Fixing the problem
It’s perfectly okay to own and use cheap scopes. I have several of them that work fine. As long as you keep the adjustments in the range where the erector tube spring can do its job, these scopes work fine. When there’s a sight-in problem, raise the rear of the scope enough that the vertical adjustments are closer to their center (from stop-to-stop according to their clicks — not optically centered), so there’s tension on the internal spring.

If you have a gun that shoots to the left, you may also need to adjust the rear of the scope to the right for the same reason. Adjustable scope mounts are the easiest way to do this; though, for small corrections on elevation, shimming under the rear scope ring will also work.

Why do guns always shoot low and to the left?
I don’t know. Why does the doorbell always ring when you’re about to get in the shower? But before some mathematician starts wondering why barrels don’t shoot high and low equally often, let me just say that they don’t. They tend to shoot low far more than random chance would allow. And they also tend to shoot to the left more than they do to the right, although left (and right) is far more unusual than down (or up).

The point
The point of this report is not to convince you to buy expensive scopes. I can’t afford to do that, and you don’t need to, either. What you need to do is understand this problem, which is by far the most common scope problem for both airguns and firearms, alike.

Once you understand it, you won’t condemn every scope, crying, “Scope shift!” when the problem is really one that can be easily solved. And maybe you’ll pay more attention to the adjustment knobs on those bargain scopes in the future and look for ones that give you feedback on where the adjustments are.

One last thought. Scopes that are less expensive will generally have less useful adjustment range in either direction. So keep a close eye on them. As long as the return spring is kept under tension, there’s no reason these scopes shouldn’t work very well.

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