Posts Tagged ‘optically centering a scope’
by Tom Gaylord, a.k.a. B.B. Pelletier
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.
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.
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.
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?
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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