Shimming mathematics

Discussion in 'Equipment Reviews' started by Twang, Nov 17, 2010.

  1. Twang

    Twang New Member

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    Here's a poser ... I have a TX200 in .177" @ 11ft/lbs. using daystate select 4.51's. If I shim under the rear end of a longtitudinally supported one piece mount, giving me an extra 0.1mm lift, how much higher will the pellet strike at 40mtrs. I'd do it physically but it's bl**dy freezing outside... cheers ! Twang
     
  2. rich

    rich Active Member

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    OK this is rough as I have to go out now.

    It depends on the length base of the two mounts

    Let's say they are 80mm apart, so you have jacked up one of them by 0.1mm.

    The gradient is 0.1mm over 80mm length, and if you do the sums that is an angle of about 0.07 degrees. Multiply by 60 and that gives you 4.2 MOA, minutes of arc.

    If you have a 1/4 click scope then each click is near enough 1/4 MOA, so for that scope 4.2 moa is equal to 17 clicks, that is 4.2 MOA times 4 clicks per minute.

    For my rig, 17 clicks is about what I would give for 45 yards, zeroed at 25.

    If it's an eighth click scope it's just twice as many, 34 clicks.

    Wow that's maths.:D


    And about 40 to 45 mm higher I guess
     
    Last edited: Nov 17, 2010
  3. Twang

    Twang New Member

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    Wow !

    That's interesting Rich ! I'm afraid proof of your equation is going to have to wait till we have access to a local indoor range as the temperature has dropped to minus 10 - but hey...thanks for your lesson mate and thank heavens my homemade wine has now passed the main fermenting stage :rolleyes:
     
  4. Twang

    Twang New Member

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    Near enough ;o)

    Ok - here's a few facts, and a bit of rough maths, that gives an answer not far off Rich's. The length of the Sportsmatch one-piece Dampamount 11cm. The increase at the rear end is +0.1mm. Multiply both by 10 and we have an increase of 1mm at 1.1mtrs. Without going into MOA, the distance quoted was 40mtrs. Divide 40mtrs with 1.1mtrs gives 36mtrs (x 1mm = 36mm). So Rich's 40mm was give or take a pellet diameter, on the button.

    Having read some horror stories about shimming the rear or front ring with different kinds of materials, and subsequently deforming the scope tube, I've shimmed the underside of the mount. The dampamount I have, has a natural gradient. When the mount is bolted on without shims, and using the old type feeler gauges, the front gap between mount underside and action is 0.7mm, whereas the rear is 0.8mm. This, for me, was just not enough. I now have two shims. The front of the mount remains at 0.7mm. Half way along the mount base is a 0.8mm shim, with a 0.9mm shim under the rear- so the mount is supported all the way.

    Can't wait to try it now ;o)
     
    Last edited: Nov 20, 2010
  5. rich

    rich Active Member

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    With a longer mount base than I anticipated you have a smaller change in the angle, for the given shim thickness.

    So glad my sums were about right.:):)
     
  6. Jon

    Jon Member

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    Andy you will get crossover, might sound minimal plus the shallow depth of dovetail and not being same thickness over clamping area.

    Shot 2005 Worlds with you for two days.

    Any shimming under the scope tube.
     
  7. Twang

    Twang New Member

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    Hey ho Jon :eek:) Interesting theory young man. Just got an e-mail seconding your advice. Thing is - I have always zeroed at 50mtrs ( I cut out all that 35m gobbledegook years ago..), and always had to aim off to the right by 1cm at 35mtrs. Like you say...x-over. But, that was without shimming. So me reckons the American baggage handlers and/or officials bent the barrel as well as knackering my scope on the homeward trip from Pulaski a few years back. In +0.4 degrees yesterday I nipped up the range and had a blast. X-over is still there - around about 1 cm - but as long as I know ;) I have about 26 clicks of elevation left when zeroed at 50mtrs, which is just over half of a rotation on the turret. Question is now - shall I leave it like that, or attempt this shimming with epoxy caper you guys are on about :rolleyes: BTW - I'm REALLY impressed with the bushnell ..
     
  8. Jon

    Jon Member

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    Probably your mounts if a side clamp style like Sportsmatch or BSquare.
    For a Sportsmatch to fit the rail needs to be 3.65mm deep i forget the width off hand. Being on a 60 degree taper, the mount will rise up and to one side, though the TX are relatively shallow it will throw it quite a bit.
    Again from memory looking in the order of at least 3mm offset if sitting correctly to just about holding if shimming under that mount on a properly cut dovetail.

    I have always used Sportsmatch mounts but the dovetails have to be cut to suit. I can only think of one rifle made that is cut right and thats the Ripley AR5's.
     
  9. greyskullnz

    greyskullnz Non member

    Yes indeedy.........using high Weavers on the Steyr rail leaves me with 5mm offset from the boreline. The Steyr rail is just too damn wide for weavers to pull up evenly. Cannot get BKL or Warne stuff here, so dovetail is out.

    Im thinking rather than fight it, I might try setting the rifle on a cant.

    GS
     
    Last edited by a moderator: Nov 23, 2010
  10. Jon

    Jon Member

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    Your mounts are not proper Weaver or Picatinny then Adam.
    The only difference between the two is the slots in the top, Picatinny conform to MOD specs whereas Weaver can be anything.
    The widths are the same, so if made right it will be centred.

    Can be very pricey those Weaver/Picatinny rings but perhaps best seen so far are the March UK steel ones, bomb proof at £125+vat.
     
  11. berty177

    berty177 Judge Diva

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    Rings

    Can be very pricey those Weaver/Picatinny rings but perhaps best seen so far are the March UK steel ones, bomb proof at £125+vat.[/QUOTE]

    Jon,

    Ive ordered the March rings for when I pick up my scope.

    Are they that good in your opinion? Ive only seen the 30mm rings that were donated by March Scopes UK and they looked superb.

    I may as well go the full distance I mean Ive saved enough money for the scope as well haahaa...


    Berty
     
  12. Conor

    Conor Never been banned from sales Staff Member

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    The rings are that good they strongly advise NOT ot lapp them as they will be truer than the lapping bar!:eek:
     
  13. berty177

    berty177 Judge Diva

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    Fingers crossed

    Conor,

    I read that before on their site, but just wondered what Jon thought of them with his backgorund and knowledge etc...


    Berty
     
  14. greyskullnz

    greyskullnz Non member

    Yes I agree, you are most likely 100% right. What I have in various heights etc are "Weaver style" not genuine Weaver brand. The lower height examples do exhibit a better alignment though. Lets be honest they are ok for tactical firearms, just not for FT at shortish range.

    As with all things specialised, we cannot get them here, or have to find a mate with the engineering equipment in their garage to modify. Kiwis do a lot of that, either making or fixing stuff in their shed, its in the DNA.

    Imagine if a set of mounts cost you guys the equivalent of a weeks' income, just to import a quality set of mounts. That is what we are talking about down here at the bottom of the world.

    Still doesn't change the fact that the mounting system is the most critical link in the chain.........



    take care all.

    GS
     
    Last edited by a moderator: Nov 23, 2010
  15. Twang

    Twang New Member

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    Epoxy

    Let me get this right. When raising the rear of the scope, a shim would be inserted at the bottom of the rear ring, but epoxy would be used both under and above the scope in both rings to take up any gaps - correct ? :confused: How tight do the rings have to be until the epoxy sets ?
     
  16. Jon

    Jon Member

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    Never took much notice of the March picatinny mounts until the other day, just got rid of a Picatinny rail and have a Sportsmatch one spare.

    They are numbered, steel, plenty of grab, look robust and must have been tested on LR for the World champ to sell. Good enough for me, why pay more.
     
  17. Jon

    Jon Member

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    Dont think theres a right and wrong way without going adjustable mounts.

    I certainly wouldnt advise shimming under the mount dovetail for reasons above.

    Even when i had a BSA and a Niko with 3.6mm- You read correct, of packing under rear mount to scope tube, adjustable mounts not out then, screws barely long enough. Just need to pack inside mount so pretty cylindrical and tighten up like a cylinder head but get a feel for it stressing.
    Mount a Tasco CU and i have had many, Deben, any Leup, Burris, Meopta, Swarovski and more, need no packing.
     
  18. greyskullnz

    greyskullnz Non member

    As Jon said there's no wrong or right way,..........just the right result for what you need. If you can source adjustable mounts then that's probably a better way to go.

    For my own problem I packed with a shim to raise the front up..........then sat the scope tube on top of the epoxy with cling wrap between. The shims were two 1mm pieces shaped in a triangle 1cm wide at the very front of the ring with the point rearwards. (raising the rear of scope would mean doing in reverse in the rear ring)

    The contact points then are at the very front edge of the front ring, and the very back edge of the back ring leaving a void between where the tube is not supported. The epoxy takes up the space between so you don't want to do up the middle bolts causing the tube to bend into that void.....let it find its own level.

    I placed the top straps on but did not do them up tight, just nipped the very front bolts up, and the very back bolts up as normal, leaving the two middle sets out while the epoxy found its level. I did not use epoxy in the top straps as they find their own place flush with the scope tube anyway.

    Depending on the set time of the material you use for bedding, trimming whatever is squeezed out can take place when it takes on a semi hardness, but before it is fully set. I test the material still sitting on the mixing tray to give me an idea when to pull the scope tube off and start trimming. Leave it to fully set for its full cure time without scope tube in place.

    I've now got 2mm extra height at the front without bending the tube, leaving a 5 moa forward tilt, at 3" scope height above bore.

    That's a general run down of what I did,.........so really you should adapt for your own particular needs, shimming rear etc.


    For me it is an interim solution, until a more permanent custom one is sourced.


    GS
     
  19. Twang

    Twang New Member

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    Just when you though this was dead n'buried

    Many thanks to our contributors :D for solving the elevation problem. Ahem... now we go on to crossover :eek: ... and I hope our maths genius picks up on this thread ;)

    I've just optically centered the scope. Fired off a few indoors at 8mtrs at a permanent marker dot on white paper which is the diameter of a .177 pellet. Hits are 5mm to the left (c/c) and spot on horizontally.

    I have measured distances between scope and barrel, and looking down the barrel from the dangerous end the scope lies 1mm more to the right. I'm pretty sure my dovetails are accurately cut, which leaves a few possibilities.

    Either the barrel has taken a knock, the mounts need to move to the left 0.5mm, or the vertical hair needs rotating to disect the centre of the barrel - but then my level is u/s.

    Anyone experience something along these lines ? and what did you do to remedy the situation ?

    TVM
     
  20. Jon

    Jon Member

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    What mounts side clamp or self centring?
    If side clamping and not to suit the actual rail how its cut there will be crossover.

    Considering all manufacturers dovetails are cut differently in depth and width that poses a problem. Even the self centring mounts like BKL may not expand or contract evenly.
     

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