How to mount a scope: Part 2

by Tom Gaylord
Writing as B.B. Pelletier

Part 1

This report covers:

  • Rest of the story
  • Why did it shoot high?
  • Today
  • One last remark
  • “Level” the scope
  • You cant
  • The bottom line
  • Other than springers
  • What’s next?

Rest of the story

In Part One we learned how to properly mount a scope on a spring-piston air rifle. Today I’ll start by telling you what happened with my friend’s Gamo Whisper that I scoped in that report. I shimmed the tube on the rear scope ring because my friend told me his rifle was shooting all over the place. To me that’s code for the scope is adjusted too high. The majority of them are. He had taken the scope off before bringing me the rifle so I was just guessing. Thinking I knew the problem,  I shimmed the new scope in the rear. Then I gave it back to my friend.

A week later he called and said he had shot it at a box 150 feet away and didn’t hit it. So I walked him through the 10-foot sight-in. He did it and called back — the gun shot 2-inches high at 10 feet — not two inches low like I said it would. Oh, oh!

To cut to the chase he had to shim under the front ring instead. When he did that he was on target at 10 feet again. And also at 30 feet — so I know his gun will be good.

Why did it shoot high?

If you are following this you already know why his Gamo shot high at 10-feet. Gamos don’t normally do that. Almost no spring rifles do. They almost always droop, which means to shoot low. Why is this one shooting high? Unless the bore is drilled off-center there is only one real cause for that — the barrel is bent upward. How did that happen? Either the trigger was pulled with the gun broken open or someone let go of the barrel before the gun was cocked. That results in a bent barrel every time. I’m not 100 percent certain of that, but at this time it’s another good guess.

bent barrel
When a breakbarrel is closed too fast, the barrel bends up like this.

What can be done?

If the upward bend is not too severe you can compensate for it by shimming the front ring of the scope mount. My friend has done this and says it’s okay. You can also bend the barrel straight again. I described how to do this and showed the results in a 5-part report

I hope to hear more about this rifle from my friend. If I do I will tell you what I learn.

Today

Today we are going to finish mounting the scope on our rifle. In Part 1 we got it on the rifle and slanted down, which works for about 90 percent of all breakbarrels. The one-piece scope rings were properly anchored to the Gamo’s scope base, so the mount should remain where we put it.

I positioned the scope as far to the rear as possible, because that was a complaint my friend had with his old scope — the one that came mounted on the rifle from the factory. The eyepiece of that scope was mounted so far forward that the image my friend saw wasn’t the full-sized image the scope was capable of producing. It bugged him so much that he stopped shooting his rifle. When I showed him my ASP20 breakbarrel with the scope properly mounted he saw the difference right away.

Most variable scopes you buy today have an eye-relief of 2.5 to 2.75-inches. You don’t measure this with a ruler. You mount the rifle to your shoulder until it is comfortable and then slide the scope back and forth in the loose rings until the image seen is ideal. There were some comments in Part 1 that my eye relief and my friend’s are probably not the same. That may be true but there is some tolerance either way for small differences. So I positioned the scope as far to the rear as it would go. It worked for me and I have since learned that it also works for him.

One last remark

Mount the scope so it is positioned right when the rifle is on your shoulder. It won’t be right when you shoot off a bench because you will be holding the rifle differently. But unless you plan to only shoot off a bench, position the scope for the shoulder. Now it’s time do do something that very few shooters understand.

“Level” the scope

Along with many others I used to jump through many hoops to “level” my scopes. I put quotes around the word level because there is no such thing. Leveling a scope is impossible — it can’t be done! Because what are you leveling it with? The receiver? And how do you level that?

I have explained this hundreds of times and some people never seem to catch on to what I’m saying. Let me ask you this — can you level a ball bearing? Some may think I’m asking if a ball bearing can be placed on a level surface so it doesn’t roll in any direction, but that’s not what I am asking. I’m asking if you can level the ball bearing itself.

It doesn’t make any sense when you think about it that way, does it? Of course not! A spherical object like a ball bearing has no relationship with the concept of being level. You can level a foundation. You can level a driveway. You can level a recreational vehicle. But you can’t level a scope — to a rifle.

Besides — level isn’t what you want. What you want is for the scope to look right to you when you mount the rifle to your shoulder to fire. For that to happen, only one thing is required. The vertical crosshair of the scope must appear to bisect the center of the rifle when you look through it.

align scope
Align the scope (rotate it in the rings) until the vertical reticle seems to bisect the rifle when you look through it.

When someone else does this you may find that the scope doesn’t look level to you. That’s because you don’t hold your head against the stock on the same angle as the other person. This is another reason that it is impossible to level a scope — because it depends on how you look through it, and each person is different.

You cant

“I can’t what?

Not you can’t —YOU cant — as in you tilt! Most people only cant (their heads) slightly when they shoulder a rifle, but some are very pronounced. How much you tilt your head when you shoulder a rifle determines whether the scope looks level to you.

The bottom line

I could have told you to hang a plumb bob at 50 yards and cover the line with the vertical reticle. I used to do that. Since the line is completely vertical I knew the scope had to be “level”. Ah, yes, but what about the rifle underneath? How do I level it? Well, with any luck the bottom of the forearm just in front of the trigger guard was flat and, when laid on a flat and level surface, it agreed with the scope aligned over a plumb line.

Other than springers

So far I have only been referring to spring-piston rifles. With other powerplants we encounter different challenges. Both the precharged and CO2 powerplants share the same things, because the principal feature they offer is repeatability. That brings in additional scope clearance issues.

S510 scoped
This Air Arms S510 illustrates the problem of scoping a repeater very well. The scope has to clear the rotary magazine that can be seen sticking up above the flat receiver. The dimensions of the scope put its turret exactly over the magazine. This forces two things — high scope rings and two-piece scope rings.

The good news is that, except for big bore airguns, PCPs and CO2 guns have no real recoil. So all the work we did in Part One to anchor the scope rings to the rifle is not necessary.

What’s next?

Next we eliminate cant. That is a subject of its own and one that deserves its own report.


B.B. takes a day off!

by B.B. Pelletier

Announcement: Guy Roush is this week’s winner of Pyramyd Air’s Big Shot of the Week on their facebook page. He’ll receive a $50 Pyramyd Air gift card. Congratulations!

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!”