The importance of scope and barrel alignment

Discussion in 'General Airgun Chat' started by athelas, Mar 1, 2019.

  1. athelas

    athelas New Member

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    I use either old fashioned camera negative film or translucent acetate sheet.
    The negative film is thin enough to provide a reasonably small incremental adjustment, if you require more than one layer. This however, is probably not ideal, as the film will chemically deteriorate over time, and the emulsion might start to physically change its size (!!)
    In my experience, it is important to cut a shims length so that it completely fills a half ring, and no more or less. This ensures that when adjusting elevation, the scope is lifted up at 90 degrees away from the contacting surface. If you use a shim that only partially covers the scope ring, then you risk the scope bieng lifted up, but away from the vertical - which will confuse you when tsting where your shots are then going!

    When using a shim to adjust windage, the same applies. I cut a shim to match the entire inside diameter of the half ring. Then I place the scope into the ring. On the shim, I mark a centre line, and then slide the shim down between the scope and the ring - whichever side of the ring it is required. When the half way line is positioned just above the inside top edge of the ring, then I know that the shim is symmetrically located, so that it will push the scope away from one side to the other at 90 degrees. Then I place the top ring into position, knowing that the weight of the scope will prevent the shim from moving.
    If you are using multiple layers of negative film (or similar) - I also apply a very little gun oil to one of the shims with a fine artists brush. Then put the other shim on top - the oil helping the two to stick together so that they are less likely to separate during placement.

    If anyone else would like to share their own techniques - I am always wanting to learn better ways to do this!

    Thanks
     
  2. C.Eaton

    C.Eaton Confirmed Anschutz Nut...

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    If you want to use material for a simple shim then just take a pair of scissors to an aluminium drinks can, think coke or budwiesser, has to be thin aluminium as this is the perfect material for this kind of shim as it doesn't corrode, expands the same as the mounts/scope and is easy to do.
    Negative film is ok until it gets wet, then it swells and goes all gloopy.
    Cut strips the width of the mounts and about 45mm long for a 30mm tube scope. They're already curved, so just push into place in the mount and mount the scope on top.
    If you need more height, just stack the strips, you should not need more than 3.
    Just be careful on the cut edges of the can as the metal will slice fingers.
     
  3. Phil Kennett

    Phil Kennett Active Member

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    Thanks for such a concise reply
     
  4. Phil Kennett

    Phil Kennett Active Member

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    Good tips Colin thanks
     
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  5. Scott Hull

    Scott Hull Member

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    "...if I optically centre the reticule in my scope, then stick the scope on the gun, and shoot at a vertical line 8 yards away, the pellet is striking about 5 - 10mm to one side of the line...."

    "If you do have an error and realise you need to fix it - what's the best way to go about this?"

    It is good pratice to keep the scope as close to opitcal/mechanical center as possible, but 5 - 10mm at 8 yards is only about 2.5-5 MOA. If that were the only correction needed, I would not have a problem with adjusting the windage turret to compensate. It's still close enough to "optical center".

    But if the 5 - 10mm at 8 yards is happening when the scope is zeroed perfectly at say 25-30 yards, than you also have the problem that some refer to as "crossover". If the scope is zeroed at 30yds, but the pellet is striking 5 - 10mm to the right at 8 yards, then loosen the scope and rotate it in the rings a few degrees CCW. Try 3 or 4 degrees, but the actual amount or rotation will depend on scope height and pellet velocity. If the pellet were hitting to the left at 8 yards, then you would use a CW rotation to correct the problem. The CW and CCW direction are when looking into the eyepiece. Re-tighten the scope, re-zero, and recheck for "crossover". It might take a bit of iteration as you adjust the direction and amount of scope rotation in the rings.

    Most misalignment problems can be handled with a few degrees of scope rotation and/or a few MOA of windage adjustment. But once in awhile a gun can have a barrel that is misaligned or bent a significant amount in relation to the scope rail. In those cases it might take a large amount of scope rotation and/or windage adjustment to gets things lined up. That can use up available windage adjustment and place it far from optical/mechanical center. I don't like to add shims to scope mounts. So for severe misalignment, I'll either straighten the barrel, or machine the scope mounts.
     
    Last edited: Mar 12, 2019
  6. Adam

    Adam Well-Known Member

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    Adjustable mounts are the way forward. Hate faffing around with shims and they stress the scope tube.

    I mechanically centre the scope on the turrets. It’s close enough and life’s too short to optically centre using V blocks. The mirror method never worked for me. If the end of the scope tube isn’t absolutely 100% square to the erector tube gimbal it doesn’t give a true result.

    Good quality adjustables will never shift even on a springer if you lock em up tight and they are set and forget for that rifle. I can zero a scope on the mount in about ten minutes, even with AOP55 the fiddly ones you have to take scope off to adjust elevation.

    I usually get it to inside 1” at about 25 yds then do the fine tuning on the turrets.
     
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  7. Andy P

    Andy P Gently does it......

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    A good alternative to negative camera film which does degrade is laminate sheets. These come in various thickness and once put through the laminate machine will last a life time through all weathers. Simples...
     
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  8. mod23

    mod23 --Sywell--

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    .... Sywell... Waresley .....
  9. Galli

    Galli New Member

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    Good point. This could possibly be the reason, why I keep missing.
     

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