Posts Tagged ‘reticles’

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 Roberto Martinez. He’ll receive a $100 gift card. Congratulations!

Pyramyd Air Big Shot of the Month

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Some scope fundamentals: Part 2

by B.B. Pelletier

Part 1

This is one of the most popular reports I’ve done in a long time. That may be because scopes can be very cantankerous to deal with — hard to mount, difficult to zero, always seem to shift their zero, etc. Today, I’ll address some of the problems you can have and some ways to minimize them.

Scopes should work — no?
To the non-shooter, the telescopic sight seems like a guarantee of accuracy. We’ve all seen the movies. Put the crosshairs on the target, squeeze the trigger and you can’t miss.

Then, you try it for the first time, and you notice that you can’t keep the scope’s reticle (crosshairs) steady. As long as you hold the rifle, no matter what you do, the crosshairs move. Each beat of your heart makes them jump a little. Each breath you take in can move the scope or at least tilt it. You can minimize these movements through training, but nobody can eliminate them entirely. That’s why I shoot from a rest so often. But sometimes that doesn’t work — especially with spring-piston airguns. You have to learn the artillery hold; and since that technique goes well beyond what many people think, I’ll explain it more fully here.

Relax for a neutral hold
The artillery hold is really just a way to get you to follow through, but there’s more to it. An important part of the hold is how you are at the instant the gun fires. You have to be completely relaxed, so the gun doesn’t recoil back and encounter an off-center obstruction that shifts the muzzle in a certain direction.

Here’s how to achieve this relaxed state. After putting the crosshairs on your target, take a breath and expel most of it. Try to relax as you do this. The crosshairs will usually move off the target in a certain direction. If you had fired before relaxing, the pellet would have gone off target in the same direction the crosshairs just did. Maybe it wouldn’t have gone quite as far as the crosshairs seemed to, but it would have moved in the same direction. The result is a larger group.

Let’s try again. This time, after you relax, move the crosshairs back on target by shifting the gun or your hands slightly. It doesn’t take much.

Once you’re back on target, take a deep breath, close your eyes, let out most of the air and relax again. Now, open your eyes and see where the crosshairs are. They probably moved again, only this time they didn’t move so far. Shift things to get back on target again and repeat this procedure.

You may have to repeat this procedure several times before the crosshairs are still on target when you open your eyes. When they are, you can take the shot — making sure that you allow the gun to recoil and move as much as it wants to. This time, the shot should feel very different than it normally does. It should feel neutral — as though you’re no longer connected to the gun. That’s the feeling of a perfectly neutral shot and one that will group as tightly as the gun is capable of — if you can repeat the process several times.

What does this have to do with scopes? Everything! This is the only way to shoot a recoiling airgun with any accuracy; and until you can do that, you’ll never have much success with a scope.

With most firearms, except .22 rimfires, the hold isn’t nearly as important for accuracy because the bullet is out of the gun before all the movement takes place. But with airguns, and especially spring-piston airguns, the pellet hasn’t started to move before the gun does. Only a .22 rimfire is similar, and even they’re much more forgiving than most airguns.

However, you do need to know that all firearms are affected by hold, as well. Even centerfires that shoot in excess of 3,000 f.p.s. will benefit from the hold I’ve described here, but the amount of accuracy increase is so small that it’s only of interest to target shooters and long-range varmint hunters. The average shooter won’t normally notice the difference between a 1-inch group and a 1.25-inch group at 100 yards. Or if they do, they won’t care. I’ve heard that from so many shooters at my rifle range over the years that I know it’s true.

Now you’re ready
If you can learn how to neutralize your rested hold using the process I just described, you’ll see an immediate increase in accuracy from your scoped guns. Then, you’re ready to discuss scope fundamentals!

Temperature is critical
We don’t appreciate how sensitive a modern telescopic sight can be. I don’t mean fragile, either — I mean sensitive. Every change in temperature changes the point of impact of your scope a little. No scope is immune to this phenomenon, yet most shooters act as if once the scope is zeroed it stays zeroed.

Field target shooters know different. I’ve seen a field target scope with three different sets of click values on the elevation knob, each color-coded to a 20-degree temperature range. The shooter who owned that scope took the time to not only figure out all the elevation click values for every yard between 10 and 55 — he did it three separate times when the temperature was in three different ranges! That’s something Hollywood will never show you.

The optical elements inside a scope are refracting light to the millionths of an inch. When they move in relation to one another — because the metal tubes that hold them expand and contract from changes in temperature, the light beams do move. The movement is very slight, but it can and sometimes does change where the images appear. The point of aim changes.

There are many other reasons for a shift in the point of aim, but temperature is a constant one that must always be taken into account. If you’re looking for the way to prevent such changes, I’m sorry to disappoint you. There’s no solution to cancel the effect of temperature changes on a telescopic sight. But if you know it will happen you can at least anticipate it and adjust your scope when the time comes.

Reticles
There are so many different kinds of scope reticles that it would take a book to cover them all. And most of the highly specialized ones are for specific purposes, such as the ballistics of a single military round, so they have no place in a general discussion. I’ll address hree general types of reticles found on most scopes. If I miss something, you can bring it up in the comments.

The oldest type of reticle is the plain “crosshair,” which is two straight lines — one vertical and the other horizontal. In some scopes, these lines actually appear to move as the scope is adjusted, but that’s getting pretty rare today. More often, the crosshairs remain in place in the center of the image and the adjustments move the whole image, so you don’t notice anything.


The plain crosshair is the oldest type of reticle. This image shows thick reticle lines, but they can be much thinner for greater aiming precision.

Often a very thin reticle can be difficult to see against a background, so there will be a small dot at the center of the crosshairs that makes them stand out. This dot will be small, perhaps one or two minutes of angle (a minute of angle covers about one inch at 100 yards), but it doesn’t take much to be noticeable against anything but a dark woods background.


This dot looks large on the heavy reticle lines. But in many scopes, both the dot and reticle lines are very small and fine. This is just for illustration.

Plain crosshairs are best in open country and are therefore favored by long-range shooters. They’re fine for plinking, as long as the reticle lines aren’t too thin. They’re less useful in deep forests, where the reticle lines don’t stand out. For that terrain, probably nothing beats the duplex reticle.

The duplex reticle is a plain crosshair that has thicker lines near the edges of the field of view and thinner lines in the center. When I shot field target, I used a scope with a duplex reticle for two reasons. First, it was much easier to see in the deep woods where many matches are shot; and second, the duplex offers four additional aimpoints.

The duplex reticle uses crosshairs of two different sizes. The ends of the thick posts provide four additional aim points that can be used for things…like greater or lesser distance and wind.

Duplex reticles are the favorite of hunters, because they work well in deep foliage yet they permit precise aiming at the same time. Like plain crosshairs, duplex reticles come in different thicknesses.


In the mil-dot scope, the dots are an exact size (measured in mils) and are spaced apart an exact distance. On variable scopes, they must be used at one power setting to work as designed. Read the information that comes with the scope to discover how this works.

Mil-dot reticles are a more recent invention. They feature dots of a controlled size spaced along one or both reticles at regular spaces. Mil is short for milliradian, a measure of angle that, unfortunately, has never been standardized. Or perhaps it’s more correct to say that it has been standardized dozens of times — each with a different measurement. When I was a mortar platoon leader, our fire direction center and mortar sights used the old French measurement of 6,400 mils to a circle, but there are many other measurements that differ — some slightly and others in a more significant way.

One common use for the mil is rangefinding. Though it isn’t exact, we say that one mil subtends (covers) one meter at 1,000 meters. At 100 meters one mil subtends one-tenth of a meter or 3.9 inches. That’s so close to 4 inches that we round it up.

A whitetail deer is about 12 inches from the top of the shoulder to the bottom of the chest. A two-mil dot will just about cover the deer’s chest at 150 meters.

Is that too much for you? It is for many hunters who still use the mil-dot scope for aimoff when there’s wind. Or they use the vertical dots for aim points at distances other than the range for which the scope is sighted.

Focusing the reticle
The first thing a shooter must do with a scope is focus the reticle. The eyepiece should adjust to allow you to do this, and it does on all but the cheapest scopes. Focus by looking through the scope at the sky or a light-colored wall and turn the eyepiece until the reticle appears in sharp focus. I’ve read that this is supposed to be done incrementally; because if you stare at the reticle very long, your eyes will naturally focus on it. So do it in stages.

After you focus the reticle, some scopes have a locking ring to hold that focus. Others don’t have the locking feature, but the focus rings should be stiff enough to hold your focus without it.

Focusing the reticle is very important for scopes with adjustable objectives, because the scope’s designers assume the scope is in sharp focus when the objective ring or sidewheel is turned. Only when the reticle is in focus will the scope come close to the distances marked off on the parallax ring or knob, which is the adjustable objective we are discussing. And, of course, that will also depend on the temperature when the scope is used.

On the other hand, on lower-powered scopes that have a fixed parallax setting you can use the focus to bring close targets into better focus. This isn’t what the adjustment is for and it will blur the reticle somewhat, but sometimes it’s the best way to use a low-priced, fixed-focus scope at closer distances than it’s intended.

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