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Thread: Unexpected zeroing problem

  1. #16
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    Quote Originally Posted by rancidtom View Post
    Soft shims used correctly will not bend a scope tube, hard shims will unless the screws are left too slack to gip the scope properly and seeing as this is a recoiling rifle it can cause crimping and bending to the scope tube if done tight enough to prevent the scope moving in the mounts.
    I stroungly suspect a barrel misalignment problem or quite possibly the rear of the mount has been fitted OVER the recoil arrestor stop thus not allowingg it to clamp correctly on the scope rails. A one piece mount will need to be mounted in front of the arrestor stop.
    Or the mount itself has a recoil pin which is protruding so the mount cannot seat on the rail correctly, OP states the clamp is on the left which would put the recoil pin under the front scope strap, if the pin is proud it would be raising the scope objective thus raising the POA.

  2. #17
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    I removed the recoil pin before fitting the mount, and I also removed some time ago the riser thingy which had the torx stud attached to it. That said, the way I have originally fitted the mount does put the empty recoil pin hole under the front strap so maybe there is a specific way that the mount should go?

    As I intended, this afternoon I spun the mount round and refit the scope, I just need to square and level things then get out into the garden.

  3. #18
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    Quote Originally Posted by secretagentmole View Post
    Just how tight do you do up your scope clamps? Shimming works, does not bend tubes if a modicum of common sense is used.
    By definition and geometry a shim always distorts a scope tube.

  4. #19
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    Quote Originally Posted by RobF View Post
    By definition and geometry a shim always distorts a scope tube.
    Unless the shims are softer than the scope tube and applied correctly to prevent edges of mounts distorting the tube.
    BSA Super10 addict, other BSA's inc GoldstarSE, Original (Diana) Mod75's, Diana Mod5, HW80's, SAM 11K... All sorted!

  5. #20
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    Quote Originally Posted by RobF View Post
    By definition and geometry a shim always distorts a scope tube.
    Correct it's unavoidable because the two scope cradles are parallel to the rail & if one is shimmed higher than the other the step between them must be imparted in to the scope tube.

  6. #21
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    Fist off I'd check the barrel isn't banana or on the squiff to action.
    Second check rifle with another scope.

    Plenty of cheap scopes lose their tracking or just stick. Test your scope for tracking. Could be a rubbish scope, plenty out there.

    Packing out scope mounts isn't that terrible if the scope isn't expensive. Most scopes only hold some value for a few years, so does it really matter?? Adjustable mounts worth having don't come cheap and bring their own issues; I try not to go there.

  7. #22
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    Quote Originally Posted by rancidtom View Post
    Unless the shims are softer than the scope tube and applied correctly to prevent edges of mounts distorting the tube.
    Hi Tom,

    It's not really a question of the softness, the rear mount will be higher than the front, which will have a clamp with a surface parallel to the rail (as Angrybear points out) and the rear mount surface will be higher and parallel. It doesn't have to dent the tube or bend it permanently but it will always slightly bend the tube when it's clamped up even lightly. If a mount is lightly tightend up then the only place it can touch is the top rear of the back and front lower of the back clamp, and the same on the front mount. Any tightness beyond touching will force the front mount to become parallel and try and take the back with it, which will be higher. The only way to avoid that is either the shim is squashed so it's not doing anything, or so the shims are made to be perfectly in line with the scope tube on the front and rear mount, above and below the scope, at the angle you want and angled and rotated so they align with the clamps and scope as well.

    That's a lot of work, I've never seen anyone doing it, and adjustables are a lot easier to do it with.

    As soon as you shim one mount, it's over, there is a bend stress in the tube. It might not take on a permanent set, but if you have a parallax or zoom mechanism it's sometimes possible to feel the stress as the mechanisms can sit right on the inner surface of the scope's tube and the slight bend that's imperceptable to the eye can make them stiffer because a straight thing is trying to move back and forth down a slight curve.

    Everyone used to shim to avoid this years ago because adjustables weren't common place items. But they are now. Little bit more expensive, but why skimp on £20 for an item normally costing 10x more than a basic set of mounts?

  8. #23
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    I have adjustable mounts and only use them for doing long distance groups with FAC guns to keep my scope crosshairs near the mechanical (and hopefully the optical) centre of the scope.
    As I said above soft shims can work allowing the scope tube to cant front to back in the mounts but they have to be used accordingly both mounts, front and rear, also top and bottom to prevent distortion from the top half of the ring as well as elimination of the edge contact and possible crimping issue. Plastic shims are easy to make and if they are soft enough will allow a scope tube to stay straight with some elevation/declination between mounts. The down side to soft plastic shims is that they can't be considered a permanent solution especially not on a recoiling rifle.
    BSA Super10 addict, other BSA's inc GoldstarSE, Original (Diana) Mod75's, Diana Mod5, HW80's, SAM 11K... All sorted!

  9. #24
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    Quote Originally Posted by secretagentmole View Post
    Just how tight do you do up your scope clamps? Shimming works, does not bend tubes if a modicum of common sense is used.
    IT IS A SHIT SANDWICH....it doesn't matter if you have to shim, its a compromise and your using a precision optical device and bending it end of.

    If there is a need to shim, there is a need to get a better solution or understand why the problem is there.

    Its either the gun, the mounts or a faulty scope or possibly combos or all three of the above.

    Swap scopes with one that works and see if the problem switches rifles which ID's it as a scope issue.

    If the problem persists, it points to a mount issue - so replace those or a rail issue etc.

    Shimming a scope means you are bodging and putting stress on a device that is not designed for it.

    If its mono tube then maybe its tougher but there are already sod off big hole cut into the side walls so they are dealing wth that load how.

    If its 2 tubes and a saddle, well that's just bad news as there are two points under stress.

    Never accept a scope set up that needs to be shimmed. Its just lazy and means you cant be arsed to get the right solution sorted.
    In a battle of wits I refuse to engage with an unarmed person.
    To one shot one kill, you need to seek the S. Kill only comes from Skill

  10. #25
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    Quote Originally Posted by rancidtom View Post
    Soft shims used correctly will not bend a scope tube, hard shims will unless the screws are left too slack to gip the scope properly and seeing as this is a recoiling rifle it can cause crimping and bending to the scope tube if done tight enough to prevent the scope moving in the mounts.
    I stroungly suspect a barrel misalignment problem or quite possibly the rear of the mount has been fitted OVER the recoil arrestor stop thus not allowingg it to clamp correctly on the scope rails. A one piece mount will need to be mounted in front of the arrestor stop.
    How so....one end is at zero and the shimmed end is at (x) difference. Unless both points are level, something is getting bent.
    In a battle of wits I refuse to engage with an unarmed person.
    To one shot one kill, you need to seek the S. Kill only comes from Skill

  11. #26
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    Quote Originally Posted by NigelG View Post
    Have you got a long straight edge, you can use a length of string pulled tight. lay rifle down and place straight edge along centre of barrel. If it starts to work its way to the top of the action, your barrel is drooping. In a gun shop years ago and a dad and son came in and complained they could zero it. The droop was so obvious just by being further away from it and looking at it
    You can also use a straight edge on the internal bottom half of the mounts to see if the steel edge runs flat or if it steps or if it pitches away. Both should be flat and no gaps on either ring.

    Banana barrels should be easier to spot
    In a battle of wits I refuse to engage with an unarmed person.
    To one shot one kill, you need to seek the S. Kill only comes from Skill

  12. #27
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    Quote Originally Posted by rancidtom View Post
    Unless the shims are softer than the scope tube and applied correctly to prevent edges of mounts distorting the tube.
    Still better to have no step or fall off at all? If a material compresses - it will change over time.

    No shims needed means no issue in the first place. Shims are a poor solution whether hard or soft.
    In a battle of wits I refuse to engage with an unarmed person.
    To one shot one kill, you need to seek the S. Kill only comes from Skill

  13. #28
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    Quote Originally Posted by Steyr View Post
    Still better to have no step or fall off at all? If a material compresses - it will change over time.

    No shims needed means no issue in the first place. Shims are a poor solution whether hard or soft.
    I only said shims (if used correctly) are a possible solution not an ideal one. Hard shims may make the problem significantly worse espcially if applied incorrectly. Tapered shims could be used but the easy option is soft shims which can deform before applying significant bending moments to the tube.
    Adjustable mounts are better, yes I own adjustable mounts for this very reason.
    The above solutions cure the symptoms not the fault, as has been stated the barrel and scope rail alignment needs to be checked, if a fault is found and cured the symtoms vanish.
    BSA Super10 addict, other BSA's inc GoldstarSE, Original (Diana) Mod75's, Diana Mod5, HW80's, SAM 11K... All sorted!

  14. #29
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  15. #30
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    Quote Originally Posted by rancidtom View Post
    The above solutions cure the symptoms not the fault, as has been stated the barrel and scope rail alignment needs to be checked, if a fault is found and cured the symtoms vanish.
    It's not always a fault.

    Lots of shooters don't realise they are shooting at quite a short distance with scopes with limited travel, on high mounts. Lots of people run for the supposed barrel droop issue but quite a few times it's just a combination of the setup and the range.

    If the target is at 25yds then it should be able to adjust to that... but if it's closer it might not.

    And I never discount the wonderful effects the beloved silencer has, or a gun running at 4ft-lbs... I reckon out of all the issues of guns not hitting what they should at in sterile conditions at 25 yds a bent barrel has never arisen.

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