Shims under two piece mounts?

Discussion in 'Gunsmithing' started by Ray B, Feb 4, 2019.

  1. Ray B

    Ray B AH Elite

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    I have a Win M70 375RUM that I would like to set-up for 1,000 yard gongs. It has Leupold 2 piece dual dove-tail rings/mounts. I got the knob that is calibrated for the load/velocity but with the parallel bases the scope (a Leupold VX6 4-24x runs out of adjustment at 450 yards. I sent the scope to Leupold and after checking, said it was fine, that I needed 20 MOA mounts. Problem is the hole spacing on the rear base is .33" and Leupold makes no tilted 20 MOA bases for that receiver. Using the bases that I have, I need to raise the rear ring .0233". If this were a one piece base it would not be a problem because raising the back would tilt both rings, keeping them in line. but with just the rear base being elevated is going to cause the rings to be slightly out of alignment.

    So I have two questions: Is the angle caused by raising the rear base .0233" sufficient to cause damage to the scope? and
    Are there any suggestions regarding how resolve this issue, short of seeing if I can find a Talley or Warne or other that has a tilt and fits?
     

  2. bruce moulds

    bruce moulds AH Elite

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    there is probably a 20 moa base somewhere.
    could try nightforce for example.
    you can also buy blank bases and have holes drilled to suit your action spacing.
    bruce.
     
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  3. Von Gruff

    Von Gruff AH ENABLER GOLD SUPPORTER AH Legend

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    Ray. if you shim the rear base and then lap the bottom of the ring you should take away any potential for scope tube damage
     

  4. bruce moulds

    bruce moulds AH Elite

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    you would need to lap both rings, and be damn sure you got it right.
    bruce.
     

  5. IvW

    IvW AH Legend

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    With that caliber I would rather find the right bases for the job...
     
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  6. PHOENIX PHIL

    PHOENIX PHIL AH ENABLER AH Ambassador

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    I agree lapping the ring would be wise to prevent the bind on the scope. But if he does so, would he not have just have defeated the purpose of the shim?
     

  7. Von Gruff

    Von Gruff AH ENABLER GOLD SUPPORTER AH Legend

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    No. Draw it out on a piece of paper (with exagerated differences)and you will see how lapping the bottom ring untill there is a straight line from the rear of the back ring to the front of the front ring has not touched the rear of either ring or the the height difference. The top ring sits on the scope body and does not need lapping. There is nearly always enough clearance between the two surfaces where the screws go that it should not need to be touched.
     

  8. 1dirthawker

    1dirthawker AH Enthusiast

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    find and buy 20 moa base. thats a lot of shimming to get right.
     

  9. sestoppelman

    sestoppelman AH ENABLER AH Ambassador

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    Shimming is a last resort. Love the old saying though, "Forgive me Lord, for I have shimmed".:eek::D:D LOL:LOL:
     

  10. Tokoloshe Safaris

    Tokoloshe Safaris SPONSOR Since 2017 AH Fanatic

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    Funny!
     

  11. Ray B

    Ray B AH Elite

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    The problem is that if two bases are used instead of a one piece base, raising the rear base will cause the rings to no longer be in-line. so lapping is used to produce a straight line through the rings. It seems to me that a better solution would be to reverse engineer it. Get a dowel the same diameter as the scope/rings and secure the rings and bases as they would be attached to the receiver. Then place the shim under the rear ring and put a small amount of epoxy under both bases. Screw both bases down forcing out any excess epoxy. the front of the front base should fit tightly against the receiver ring with virtually no epoxy under it, the back of the front base should have a very tiny amount, just sufficient to give it the angle to put it in-line with the rear base. Then the rear base should sit on a platform of epoxy that would give it the correct tilt angle.

    This seems to me to be a workable solution to the problem of gaining elevation adjustment and avoid the need to spend $300 for a new set of bases and rings.
     

  12. GuttormG

    GuttormG AH Enthusiast

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    Install the bases as normal, no shims.
    Put the shim under the scope in the rear ring, bed in the scope in the front ring using epoxy (some release agent og the scope is nice.. ).

    Full contact and strength between the action and bases.
     

  13. Hogpatrol

    Hogpatrol AH ENABLER SILVER SUPPORTER AH Legend

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    This is simple. Buy Burris Signature rings and bases. If the scope tube is 30mm, you can get 20 MOA without any stress on the scope. If the tube is 1", same deal, 40 MOA adjustment by just changing the inserts. Shimming without lapping does your scope no good. As for as recoil?
    Never had an issue with several heavy recoiling rifles.

    The system explained here.

     
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  14. GuttormG

    GuttormG AH Enthusiast

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    Yes, agree. Signature rings or a rail with with the correct elevation is the best solution. But Ray B do not want to buy new mounts.

    The scope on my 7mmRM is installed as I described, works just fine.
     
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  15. Hogpatrol

    Hogpatrol AH ENABLER SILVER SUPPORTER AH Legend

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    I hear you on the not wanting to buy new rings but then you get in to a why do you beat your head against the wall? Because if feels good when I stop" scenario. Burris makes the rings and bases. Another option is Kelbly's. They are line bored in pairs (so only go on a specific way) to give the desired MOA, so no stress on the scope. https://www.kelbly.com/scope-rings

    Another option, E.G.Ws.
    From this chair, trying to modify the existing setup is a wheel spinning operation. YMMV.

    Aside that, have you done a ballistic calculation to see if 20 MOA is enough. That big a bullet has to drop like a stone after 500 yards.
     

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  16. Ray B

    Ray B AH Elite

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    Some may want to review the definition of Ballistic Coefficient and compare .805 to other smaller bullets before they display the lack of understanding. As my dad used to say: engage brain before operating mouth.
     

  17. bruce moulds

    bruce moulds AH Elite

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    JBM ballistics is your friend in these matters.
    bruce.
     

  18. Hogpatrol

    Hogpatrol AH ENABLER SILVER SUPPORTER AH Legend

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    Ray, Not questioning your BC, just asking if 20 MOA was going to be enough to get you to 1000 yards. What weight bullet are you shooting at that distance?
     

  19. Ray B

    Ray B AH Elite

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    I'd be using a 350 gr SMK, BC .805 MV2600, Zero at 300yards. program shows the drop@1000 yards from line of sight 266", 25MOA. The scope is a Leupold VX6 4-24x 34mm tube. The base-ring combinations that I've found that combine a 20 - 40 MOA tilt with 34mm rings have run over $300. I may end up paying that, it just seems a bit much if the problem could be solve with a small piece of stainless steel and some JB Weld.
     
    Last edited: Feb 10, 2019

  20. Hogpatrol

    Hogpatrol AH ENABLER SILVER SUPPORTER AH Legend

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    I hear you, $300, OUCH! I ran the ballistics on Applied Ballistics app usng your data, G7 BC numbers for that bullet (fortunately it's in the app's library) and at 59F, sea level, zeroed at 300, I get 68 MOA path of bullet/drop to 1000 yards. How much elevation can you get out of your scope?

    Burris makes 34mm rings. I'd have to check how much you can gain with the inserts but yes, lifting the back and bedding the rings is starting to make sense.
     

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