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Maintenance How to mount a scope: Part 2

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 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.

Hunting Guide

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.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

83 thoughts on “How to mount a scope: Part 2”

  1. B.B.,

    Wait a minute first you say “You cant” and then you eliminate it? Going to have to stew on this to grok what you mean.


    PS: Section You cant 2nd paragraph last sentence, “How much you tilt your head when you shoulder a rifle determines whetherthe (whether the) scope looks level to you.

  2. BB
    I probably shouldn’t be posting this yet because I really need to think about it more.

    But raising the front of the scope won’t help the gun shoot lower the scope needs to be lower in the back ring. And obviously that can’t be done unless you do some work to the scope ring.

    Your not lowering the cross hair to the bore. Your just raising the front of the scope to the bore. All that will do is change your optical centering. You will just have to put more clicks in your elevation turret.

    Did I think right?

    • GF1,

      BB did not state such, but the first step would have been to remove the shim from the rear ring. That would change the relative angle of the scope view versus the rifle bore. I guess because of his experience, he recommended that his friend move the shim to the front ring. That would increase the angle further.

      Hold your hand parallel to the table surface. Raise your wrist, keeping your fingertips the same relative distance from the table. That is the result of shimming the rear ring.

      Now hold your hand parallel to the table and raise your fingertips while holding your wrist the same relative distance from the table. That is the result of shimming the front ring.

      See it?

      • RR
        Yep I know what you mean.

        What happens is the opposite scope ring of what you shim doesn’t let the scope tube sit in the ring saddle right.

        In other words the back scope ring cap would be at a angle. There would be a bigger gap in the front of the top cap to the bottom part of the ring saddle then in the rear.

        The opposite happens with the saddles and top cap when you shim the rear.

        I still say do it the right way and get a scope mount that already has the angle in it. And I guess you could flip it around and use it for rise instead of droop.

        Here is what I’m talking about.

        Or this. And scroll through the picrureson this mount. Its got some simple illustrations.

  3. BB,

    This is how I learned to mount scopes from my Dad. I guess I learned to hold the rifle the same way as I never had an issue with shooting his rifles.

    I never had to shim a scope mount on an air rifle, but that is because I have always used either one piece mounts with a bit of angle built in or I paid the extra for the Sports Match rings. I give them the coveted 3R rating. It is nice to be able to mostly zero your scope without taking it off optical center.

  4. Everyone,

    I threw all that we did to his scope into one sentence in this report, “To cut to the chase he had to shim under the front ring instead,” Obviously the rear shim had to be removed first, and when that didn’t solve the problem he had to raise the front of the scope with a shim. I’m sorry — I should have taken the time to explain everything.


  5. My brother uses two or three small bubble levels and it takes him about an hour or more to mount a scope. Me, I use my eye and line up the crosshair on what looks to be the centerline, just as your diagram shows. Takes 10 to 15 min. at most. The hardest part is tightening the screws so they don’t cause the scope to turn as they are being snugged up. Seeing how I’m the only one shooting the gun, it only needs to look right to me!

      • RR
        I should rephrase that.

        Usually what I do is get the cross hair as level as I can by a reference point like a door frame and kind of eye ball the breech.

        Then if its a gun that has a adjustable butt on the gun such as rotation. Then I shoulder the gun and see by eyeball that the breech and cross hair is still level. If not I adjust the stock.

        If the stock doesn’t adjust then you have no choice but to cant the gun stock. That I do not like but it is what it is with a non adjustable stock.

        And of course this all depends on what type of shooting your doing. So some cant for a fast action plinking gun like my Air Ordinance SMG doesn’t bother me as much when I’m shooting 15 to 25 yards. But my Condor SS at a 100 plus yards is another story.

    • Cloud
      Ok you do all that and I say good.

      But here is what we all know. That when a gun is shouldered it will move and locate in a natural position.

      To say what I do know is those field target guns can be adjusted to fit to high heaven.

      So I’m betting that after you do all that leveling of your scope you go the distance and put your time in the stock adjustments to keep your action and scope still level to your shouldering or holding the gun.

      And if you don’t. Why not if you went that far with the scope mounting?

        • Cloud
          So I know this is routine to you and you don’t even think about it anymore when your setting up your gun.

          So in the process your making sure the breech is staying level when you shoulder or hold the gun. If it’s a new gun and let’s say the gun stock cant’s to the left do you now adjust the butt of the gun in a way that its actually canted and the breech stays level?

          That’s how I have the butt stock that is attached to the bottle on my .25 Condor SS. Its actually off center to the right of the bottle and tilted into the left to fit my shouldering of the gun. That way my cross hairs and breech stay as level as I can get them.

          But that’s why I keep bringing up shouldering the gun and cant. Out at a 100 yards or more I got to repeat my hold consistently if I i want to hit what I shoot at. The longer distance really shows all my errors when I shoot. When I shoot out that far I need to be on my best game if you know what I mean.

          It really all does make a difference.

          • GF1,
            Yes I adjust the buttplate or butthook to shoulder properly with the breech and scope reticle level. I glance at the bubble when I am in the shooting position to ensure that I am still level, because FT targets aren’t always straight ahead, and the position of my body to point the rifle at the target can affect how I shoulder the rifle. A quick check of the level keeps me consistent.

            • Cloud
              Right you will always have to make minor cant moves when you shoulder your gun.

              Its all about the type of shooting a person is doing. Obviously the more time you put into scope set up and stock set up the easier it is to shoot consistently.

    • Cloud
      Seriuosly how do you feel about doing a gust blog on what you use to do scope set up and what you go through to get your stock set up.

      I think it would show how and why the tools you use do work for the shooting you do.

      Not much of this is actually discussed on the blog. The basics are good but so is the advanced set up.

      I really think you should do it.

  6. The UK forums are full of useless advice about all manner of convoluted ways to ‘level’ a scope. As long as the crosshairs are vertical with the rifle shouldered who cares how the scope is oriented compared to the action. It’s completely irrelevant!

  7. What about a scope that has the parallax on the right and the windage on the left? Depending on if you are using a magazine, which side it might load from and if you use a side wheel (and) if you shoot right or left. Just an idea.


  8. B.B. and Readership,

    Leveled, aligned, centered, and so many other possible things to do!

    So if you are the Gunner on a Battleship how do you do all those scope things when your platform is rocking, rolling, and yawing and still hit a bedsheet 20+ nautical miles away? That should give you a clue about what really matters. The Gun Director, not attached to the turret(s,) does it all. In shooting your gun you are the Gun Director;
    if you do your job you CAN hit the target!

    Similar thing apply in air to air gunnery.

    Just like the “trick shooters” all know. Don’t over think it!


    • Shootski,

      20 miles? Ok,.. you are talking the BIG guns. I would fully guess that computers take into (full) account of any “rocking and rolling” of the ship and compensate automatically/instantaneously. Maybe wrong? Probably am,…. 🙁

      😉 Chris

      • Chris USA,

        Computers! Like your laptop or tower? Not if you consider that the last Battleship was built before the end of WW2! Yes they had a mechanical computation engine in the Gun Director but it didn’t have any automated input capability. It was strictly a Balistics calculator and everything needed to be precalculated. The US Navy had the advantage of RADAR [RAdio (some say the A is for Azimuth) Detection And Ranging] during WW2. Most other Navies were still using some form of optical ranging.

        If you are really interested: https://en.m.wikipedia.org/wiki/Ship_gun_fire-control_system

        The use of gyro systems to level the platform (detecting x,y, and z axis) was also another upgrade feature. It has always surprised me that more rifles arent equiped with some form of gyro stabilization/orientation system! Just think no CANT, almost no wobble and potential recoil control! For a long time cameras were on tracks, booms, cranes, and dollies then Steadycam and other systems did away with much of that.


        • Shootski
          Gyro stabilizers are cool.

          I use one on the ailerons on my RC acrobatic plane. It helps keep the plane from rotating when I have the plane hovering pointed vertical in front of me.

          And very good story about the gun ships hitting their target.

          As the saying goes we need to know the gun we shoot. Just because I can set up my gun and shoot it and hit whatever I want for the most part. Then you try to shoot it and not hit or vise versa I can’t hit with your gun but you can.

          • Gunfun1,

            Yup! It is almost never the stuff we hang on our things but always what we have learned and remember RIGHT now to apply to the problem!

            How close to full throttle do you get the 1:1 thrust to weight ratio on your ducted jet to do the tail dance?


            • Shootski
              I should of said what kind of plane it is. It’s not my jet its basically a mid wing plane like a Extra 300.

              It’s got about 4 times more power to weight ratio. So it will hover vertical at just over a 1/4 throttle.

              My pusher prop YF22 will hover vertical at about 3/4 throttle and start tail sliding and it doesn’t have enough power when you go full throttle so you got to either pull back on the stick or push the stick and nose it over. I like pushing it over because then you can slow the jet up and fly on the stall with the nose at about a 45° angle. No tip stalling either. It’s very stable at slow flight. I just need a little more power to hover it vertical though. Fun stuff.

              • Gunfun1,

                Ah!!!! The Back Side of the Power Curve! We used to do short field landings with a big 4 engine turbo prop and turn off at the 1,000 foot mark. That aircraft had almost 70% of the wings blown by over 20,000 SHP (Shaft Horse Power) and another 450 Effective Horse Power from each of the four engine exhaust! Full reverse on the touchdown would hang you in your shoulderstraps! Fun stuff!

                You need to get yourself miniaturised and climb into one of your airplanes!


        • Shootski,

          A bit heavy for a morning read,.. but I did scroll the entire article. Pre WW2 end eh? Sounds like we could use some updates. Apparently though, we can still get the job when called upon to do so. I would take minute of bed sheet accuracy at 20 miles.

          Thanks,… Chris

          • Chris USA,

            King size flat sheets! Not fitted, Lol!

            US Navy ships really don’t have much in the way of guns anymore. Everything is really smart missiles, torpedoes, and mines. I worked on the first OTH Over The Horizon weapons in the 1980s
            during President Reagan’s SDI initiative (Star Wars) and had lots of fun shooting targets hundreds of miles away and scoring Minute of Twin Sheet hits.


            All 4 of the remaining BBs were decommissioned before this Millenium. I doubt any of them will ever see combat service again.


            • Shootski,

              Yea,… I am a big fan of the smart missiles that video the entire flight and show everything to the last second,…. fired from afar,… keeping our troops safer! No doubt a tad more costly than firing a large projectile from a ships cannon though.


              • Chris
                We made parts for the laser guided missles at the machine shop when I was younger. We made parts for alot of different missiles and other projectiles.

                I haven’t seen any military contracts in I bet 15 or more years now. They went somewhere else.

              • BB,

                Aliens?,… If you ever find yourself not sleeping at night,… find Coast to Coast on the radio,.. 12am – 5am. They will set you straight on the “good” ones and the “bad” ones. Real people call in live all of the time. They cover other stuff too,… like normal stuff.

                I used to catch the tail end of the show on the way to work in the early am.

                I would ask Shootski to weigh in on the topic,…. but I am afraid of what answer I might get!

                :0 Chris

                • Chris,

                  I flew thousands of hours in very sensitive aircraft and visited many shipboard and ground stations around this planet never had a far encounter! Remember that word ARTIFACT it happens!
                  My philosophy on Aliens is that if they come they won’t be in an Elon Musk vehicle so they will arrive and we will be toast or we wont have a clue they were here and left because we had nothing they want or need. I love SciFi but i always remember the Fi part is different in Semper fi.

                  Never say never…but!


                  • Shootski,

                    Awwwwe,… shucks! I was hoping that you would “spill the beans”. 🙁

                    Then again,.. that would mean that the Men in Black (and/or) the Aliens would have to “eliminate” you.

                    I would keep playing it dumb too! 😉 LOL! 🙂

                    Me? I have heard and seen enough on video and radio,.. and real people calling in,… to never say never. Real or not,.. I would bet that Government is keeping abreast of anything that is,…….. “questionable”. By the way, I do not do (any) on-line conspiracy sites. Talk about a DEEP hole! Ain’t goin’ there!

                    Thanks for the insight,… as always.


  9. Shootski
    Haha. Miniturised. A person can’t do the forces a RC plane would put on the body. I can snap roll my Extra 300 so hard that you better be watching to see it happen.

    Full right aileron and full right rudder and full up elevator as fast as you can move the controls. It happens so fast if you are not watching you will miss it. The the fun part is snapping off the controls and ending your snap rolls straight and level.

    And the planes will Lomcevak too. I can hold the 300 straight up in my hand with the prop pointed vertical at full throttle and let go and it will Lomcevak (full right aileron, full down elevator and full right rudder) 5 foot off the ground and relax the controls and go vertical. As you know it’s all about timing too.

    I always like when I get to go vertical with my planes no matter what direction its pointing. 🙂

    I think we should switch up your suggestion. I think you should srart flying RC planes. I really think you and your kids and grandkids would love it. It’s very fun and it’s not exspensive if you get the right planes.

    If you lived by me I would have you flying RC by yourself in less than a week. No joke. Especially since you understand wind speed and ground speed and all that other stuff already. It’s all the same and what is crazy is you can feel the control surfaces and the plane mushing and then flying out as the power comes in through the transmitter sticks. As we say it’s amazing what the eye’s and brain can compute.

    • Gunfun1,

      The A-4 or the USN Training Commands TA-4; 720° per second.



      WAS NOT Brit! It was Douglas!

      We called her: the Scooter and my favorite aerobatic manuver in the A-4 was a Squirrelcage!

      I have been researching the RC world. Just need to find a local club to get the 411 on local issues.


      • Shootski
        I seen RC plane fuselages break right behind the wing because they had big control surfaces and to much throw on the surfaces. The planes were violent to fly. Almost like a plane being to tail heavy. Very hard to control.

        Lucky for RC planes is you have dual rate switches for your control surfaces. They can be programmed to use only say 60% of their throw. And you can use exponential with the computer which will progressively add more and a faster move rate of the control surface. It helps with slow flight.

        In other words you can flip a switch and fly smooth at high speed like my pylon planes to reduce control surface movements. And then when you slow down to land you switch back to have full control movement of your control surfaces.

        I could go on and on. Everyone that I know that flys full size planes and RC planes say that RC planes are just like flying a full size plane. All principals apply but on a smaller scale. They say that a person that can fly RC could probably fly a full size plane no problem. But a person that flys a full size plane will have problems concieving looking at the plane and orientation verses being in the plane and picturing what the plane is doing on the outside of the plane outside of the cockpit.

        Get you a glider with a electric motor on the front and teach yourself RC flying. That’s what I did. Gliders are very forgiving. Then get you a slope soarer design glider with a motor on front. Get good with it then you can fly planes.

        That’s how I did it.

        Oh and what’s a squirrel cage? Never heard of that maneuver. Maybe your calling it something different than I know. How did you do the maneuver and what control surface movements? Or was it a flight pattern maneuver?

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