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Scopes shimming my scope

Discussion in 'Anything Airgun Related' started by sillygarry, May 9, 2014.

  1. sillygarry

    sillygarry Posting Addict

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    hi guys
    i have a problem with my scope the up turret has bottomed out i can not adjust the reticle down to meet my point of aim the pellet are hitting low of the crosshairs ,so do i shim the front or rear mount to raise it :confused:
     
  2. r10hunter

    r10hunter Honorary Member

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    Yes shim the rear of the scope with Ali foil or 35mm film ( always worth buying a roll)

    Remember your scope has to be pointed where you gun hits. So if you are hitting low your scope needs point more downwards.
    Cheers Andy
     
  3. neiled

    neiled Donator

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    If I remember correctly if your pellets are hitting low then you need to shim the back mount (2 piece mounts)
     
  4. Pelletpower

    Pelletpower Donator

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  5. Ichabod Armacost

    Ichabod Armacost Donator

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    Very useful!! Ta muchly!:cool::)
     
  6. Akita177

    Akita177 The Absolute State of Britian podcast

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    Shim the rear, and dont make the mistake of making the shim to long so it fills the whole of the bottom cup of the rear mount.
    As then your making a 25mm cup to i.e 24.5mm cup so the scope may never sit correctly in the cup making it easier to crimp the scope tube and have possible POI issues.
    You can make the shim as smal as 20mm in lenght and as long its central you shouldnt have any issues, then if you fit a second shim make it a bit smaller then the last.
     
  7. JD

    JD Donator

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    Its not the scope, the barrel is drooping down, buy a mount to compensate for this.

    I do not advocate shimming a mount, to get the scope shooting in line with the barrel.
     
  8. r10hunter

    r10hunter Honorary Member

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    The reason many scopes require shimming is that they are designed to be fitted to firearms at a Zero range of 100 yards, the gun having a flat trajectory still at this range.

    When fitted to airguns and rimfires the scope needs to tilt down more to cope with the shorter zero range and the loopy trajectory.
    You can prove it every time you swap scopes around. When you put a CF scope on a Rimmy or airrifle it nearly always needs shimming up at the rear.
    Cheers Andy
     
  9. sillygarry

    sillygarry Posting Addict

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    well i found some 35mm negative with some photos of me (. i was good looking then 30yrs ago)and will let you know if it works :up:
     
  10. JD

    JD Donator

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    Not what Ive experienced with scopes myself, shimming is not good for the scope tube, a droop compensating mount is the way to go.

    Atb

    JD
     
  11. terry1001

    terry1001 Major Poster

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    I would expect that the rifle makers cut the dovetails to allow for the trajectory and for the barrel alignment, anything else would be simply unacceptable. I can understand that some people want to have the scope zeroed with the adjustments centralised - well actually I can't understand it but I accept it but unless you're doing very long range shooting the scope should just work when fitted to decent mounts.
    If a scope actually runs out of sdjustment and still isn't zeroed then it suggests to me that something is wrong, the most likely culprit seems to be mounts but there is always a possibility that the scope itself has suffered some form of terminal trauma.
    It doesn't take long to check that the mounts are identical and that they are correctly fitted to the dovetails, I prefer to fit the lower half of the mounts before putting the scope itself in place as this helps with getting them right. Simple checks then involve swapping mounts front to rear and then turning them so that the clamping screws are on the opposite side of the rifle to where you first had them - both sets of clamping screws should be on the same side each time.
    When doing this check the rifle dovetails and the scope mounts for any signs of damage.
    If altering the mounts doesn't help then fit another scope, which is known to be good, to the mounts leaving the lower halves in place and check to see if that would zero - you probably don't need to adjust it and if you're careful and mark where it fitted on the other rifle, if that's what you did, you should be able to put it back very close to zero.
    When you've done these checks you will know if the mounts or scope are the problem, if not then the issue lies with the rifle as that's the only thing left. So if you've just bought a new one then take it back asap, if it's used then you can carry out some checks on it depending on what type of rifle it is.
    You can check the barrel with a straight edge for bends/kinks, if it's a pcp with a barrel clamp check that the clamp isn't causing distortion although that is much more likely to affect windage.
    Set the rifle up in a workmate or similar and level it along the scope mounts and then check to see if the barrel is also level.
    I would also look to trying another set of mounts or a scope and mounts from another rifle that is working properly. The amount of shimming required to get a scope centralised if it has run out of adjustment is a lot, somewhere about 15/20 thou for each complete turn of the adjusting screw and if you need 1.5/2.0 turns then the chances of causing damage to the scope when clamping it in place are extremely high. If nothing else can be done then a set of adjustable mounts is probably the way to go as they are usually self aligning and won't damage the scope tube.
     
  12. Bemused

    Bemused Engaging Member

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    Allthough as a novice I don't know what I'm doing for sure, I did find my little shifting zero problems went away after optically centering my scopes and fitting adjustable mounts to keep the scope optically centered when zeroed on the rifle. It does not take long in any case and leaves full clicks for future intermediate range changes, I found various threads which describe a scope being optically better when the errector is centered and also lets the errector springs sit in mid compression rather than nearly fully compressed or extended.
     
  13. edtwozeronine

    edtwozeronine Donator

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    I shimmed the rear mount on my Hatsan Mod 60S due to a minor bit of barrel droop. Was sweet as a nut after that. I used plastic from 2 litre fizzy drinks bottle.
     
  14. r10hunter

    r10hunter Honorary Member

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    Shimming your scope will not cause any harm unless you use a huge amount of shim or do the Allen bolts up crazy tight.
    I shim nearly every scope I own and haven't broken one yet.

    Scope manufactures often recommend shimming a scope to keep the scope near its optical centre. Many scopes will just not adjust very well or lose zero once they get away from optical centre. Many scopes are just not designed to point that low reliably.
    An 11.5 ftlb .22 pellet can drop 5 inches by your 30 yard zero.

    How hawk say to fit a scope:

    http://www.hawkeoptics.com/user/chairgun/help/scopeshims.htm
    Cheers Andy
     
    Last edited: May 9, 2014
  15. sillygarry

    sillygarry Posting Addict

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    well i have shimmed it with 1 piece of film negative and now it shoots spot on and the turret are set midway thanks for all the advice:up:
     
  16. terry1001

    terry1001 Major Poster

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    A single piece of film negative is about 0.005" and at a range of 30 yards should move the point of impact about 1" so something else must have changed to get from running out of adjustment before getting zeroed to being zeroed with the turret set near the centre.
    It's good news that you've got it sorted.
     
  17. JD

    JD Donator

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    sounds to me like abit of barrel bending took place ?
     
  18. r10hunter

    r10hunter Honorary Member

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    How much a shim moves the point of impact depends on how far apart the mounts are. That's why in full bore shooting we use a set of shim tables or an app that take into account how far apart the sight bases are and tells us what thickness of shim to use to enable the scope to cope with long range while keeping it reasonably optically centred so that one click is one click.


    The negative will have brought the scope near to is optical centre where a cheaper scope will adjust more accurately.
    Cheers Andy
     
  19. sillygarry

    sillygarry Posting Addict

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    JD what does barrel bending mean ?
     
  20. terry1001

    terry1001 Major Poster

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    There's a useful tool in Chairgun which does this although to be fair the maths is simple enough. The estimate I made is for scope rings 4" apart but if they are closer then less shimming is required. Going on the basis that even on max adjustment the poa was still above the poi it would have needed to be moved slightly more than half the total adjustment range to get zeroed which is a fair bit.
     

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