Posts Tagged ‘shimming’
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 B.B. Pelletier
Guy Roush is the BSOTW — for the second time.
Today, I was supposed to show you a thrilling “fix” for a recent problem I’ve been having with an unnamed but very popular airgun. That pins it down, doesn’t it?
I won’t tell you what I’m working on because the work is not yet done. I need more time. You see, a lot of the time, I guess right about something and it turns out well, so I can report it as I fix it. That didn’t happen today. Today, it all went south and, for the life of me, I could not figure out why. Oh, I’ll figure it out in time, but I can’t report an unsuccessful project while I’m in the middle of it, because many of you get anxious for me. Then, you start giving me suggestions and before we know it, some of you are inventing alternate universes in which everything turns out fine (as long as gravity is cancelled and we all walk on our hands!).
Well, I don’t want that. I don’t want it for you, and I sure don’t want it for me!
So, today I laid down my work and took a break because I was so stressed that I couldn’t hold a quarter-sized group with a gun that can usually hit Roosevelt’s head on a dime every time. I needed to back away from the problems I’m having with this gun and allow some time to pass before I tackle it again.
So, instead of me revealing yet another mystery that’s been solved, I would like to share some of my not-so-random thoughts with you.
First thought — bent barrels
For years, I’ve been wondering about bent airgun barrels. I’ve seen them and I know what can cause a barrel to bend, but also I know that there are several airgunners who are bending their barrels purposely to avoid the trouble of shimming their scopes or using adjustable scope mounts. Let’s call that “corrective” bending.
Bent barrels are such a problem that everyone talks about them. Over the years, I’ve seen several airgun barrels that were bent. The ones I’ve seen were all bent the same way. Someone broke open a breakbarrel and fired it with the barrel broken open. The barrel snaps up violently and bends upward at the point where it enters the baseblock. Shooters who have done this have told me it was an accident, but I believe they did it intentionally — just to see what it would look like.
At any rate, the barrel always bends the same way when this happens. And I know that many airgunners would like to know how to straighten (or correctively bend) their barrels. And they want to be able to do it without disassembling their guns — naturally!
I’ve read in books that you can see a bent barrel by how the light shines through it. Mac has told me that he’s straightened many airgun barrels and that it’s always possible to see the bend in the barrel by looking through the b0re at a stro0ng light source. I have certainly seen a bent barrel from the outside, but I have never seen what one looks like when looking through it like the books describe.
What can I do about all this? Well, how about I take a picture of the inside of a breakbarrel barrel before it is bent? And how about I shoot some groups with it and note where the point of impact is, relative to the aim point?
And then how about I intentionally fire the gun with the barrel broken open, and show what happens? More photos of the inside of the barrel, just to see what a bent barrel looks like from the inside. Then I try to shoot the same gun with the same sight setting and note where the pellet now impacts. This last part may be difficult, because the barrel can be bent up so far as to render the gun impossible to aim, so we may have to take this one as it comes.
Second thought — barrel straightening (and bending) jig
I’ve designed a very simple yet (I hope) effective barrel bending jig that can be made by anyone out of common stuff found at a hardware or home improvement store. And it should be able to work on an airgun with no more disassembly than taking the action out of the stock. So, after I bend the barrel of the test rifle, how about I use the homemade jig and (hopefully) bend the barrel straight, again?
Dave Schwesinger (of Air Rifle Specialists) said he used a picnic table to straighten his barrels. Kevin uses a willow tree, so my jig isn’t up against stiff competition for elegance.
If it works, I have a follow-on experiment. I own two breakbarrel springers that both shoot higher than the sights can compensate for, even though I can see no evidence of bent barrels on either one of them. If my jig works, how about I bend their barrels slightly down to get them to the point of aim again? And then conduct an accuracy test (before and after, of course) to see if a bent barrel can be accurate.
We know that bent firearm barrels are not accurate; because as they heat from firing, they “walk” their shots. But airguns don’t heat up as they shoot. Several years, ago Feinwerkbau actually made a 10-meter target pistol with a barrel that wrapped around the CO2 cylinder as a sort of joke. The joke was that pistol is just as accurate as a regular 10-meter pistol — or so says Robert Beeman. Well, if you can do that with a barrel, I think a slight bend won’t hurt. But I’ll test it, of course.
Even twisted in a spiral like this, this FWB C20 barrel still shoots straight. Image courtesy Blue Book Publications, Inc.
Third thought — reflections on the first two
This idea about reporting on airgun barrel bending has been brewing inside me for awhile. But I have some misgivings at the same time. What I don’t want to do with this, or any unusual procedure that I report in this blog, is to start a herd of lemmings all racing to bend their airgun barrels. Maybe 99 percent of you will take what I say with the grain of salt it deserves, but there’s always that lunatic fringe that likes to seize these concepts and race over the edge of the cliff with every new and unnatural thought that comes along.
Back in the 1990s, there was a trend of cutting off springer barrels in the hopes of making them shoot faster. The Cardew book demonstrated that a short barrel is all that’s needed to achieve top velocity in a springers, and the experts went to work quickly to say that any barrel length after the optimum velocity was reached just slowed the pellet back down again. Thousands of great barrels were ruined this way, and I think this is where some of the interest in the hillbilly crown job came about. I don’t want to start another trend like that one!
I don’t want to get the following message a year after I publish my report:
“I want to purchase a Frauhocken 500 breakbarrel air rifle, but I don’t want to build a barrel-bending jig. Can you recommend someone who can bend the barrel for me? It would be nice if the gun could just be shipped to them so they can bend the barrel before I get it. Also, have you ever thought of installing a Nitro Piston made for a Walther Talon Magnum in a Bronco? It seems to me that the Nitro Piston will speed up the Bronco just enough to make it interesting; and with the Bronco’s easy cocking effort, this would be the ideal plinker! Just a thought!”