Posts Tagged ‘scope out of adjustment’
by Tom Gaylord, a.k.a. B.B. Pelletier
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.
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.
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.
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).
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.
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.
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 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.
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.
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.
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 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.