PU scope calculations
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Thread: PU scope calculations

  1. #1
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    Default PU scope calculations

    I used to have a formula that would calculate how much to shim or grind a PU mount. Like I'm shooting X distance too far right, I need to shim the mount X number thousands of an inch. anyone know off hand what that is? I could just do trial and error, but its like 17 degrees out and almost two feet of snow on the ground.
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  2. #2
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    It's way past my bedtime, so I can't do the math now...

    But here is what you can do: Shoot the rifle with your iron sights at fifty yards. It would be real good if your windage is "dead on" and your POI is 3cm above your POA. If your windage is off, moving the front sight 0.3 mm will move your POI about an inch, at fifty yards.

    Once you are printing on paper where you want to at fifty yards with your irons you can use your iron sights to line up your scope.

    Prop the rifle on something steady, then look down the irons at something 100 yards out. Then shift your eye up to the scope and see what it is looking at compared to your irons.

    If, for example, the scope is "looking" at something to the left of the irons, you need to shim. If it's far to the right you need to file.

    I have in the past used an ordinary steel washer as a shim and sanded it down by hand with fine sandpaper until I get agreement between the irons and the scope.

    Good Luck!

  3. #3
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    I had the rifle all adjusted 5 years ago, but took the scope off to fit in the safe and I lost the shims I had for the sight. it shoots dead on with irons at 50 and about 3.5" right with the scope centered. I just don't remember how much I shimmed it before and don't want to go through all the trial and error again until its correct.
    As of 9/17/2011 new member of the 98k Sniper Club, bcd 45 lsr
    11/2/12 SSZZA4 bnz 43 #8301 added to the family
    1/2/13 Joined the Classic Arms 91/30 PU family


    Bitterly clinging to my guns and religion since 11/6/2012

    ΜΟΛΩΝ ΛΑΒΕ

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  5. #4
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    a/y = b/x = c/z equal angle triangles. a = the distance you need to move the POI = 3.5 inches = 8.9cm.

    c = 4950 cm = 49.5 meters = the range to target.

    z = 8cm = the length of the PU mount

    solve for y = thickness of the shim


    That's the best I can do this time of day. For all I know at the moment, it may be complete jibbersh. But I'm really trying to help.

  6. #5
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    I'm guessing that your shims will need to move the centerline of the mount a little over .1 mm outward. In the simplest scenario, both shims would be the same (for instance .115 mm), but it's not quite that simple: the relative thicknesses of the two shims will also have to be right, or the mount will be canted and the scope won't be over the bore centerline. Both pads (or shims) have to be exactly right to end up with a scope that's oriented correctly over the bore with the rifle held vertical, and still parallel to the bore line in the windage plane.

    In the end, you'd be very fortunate indeed to come up with an exact theoretical shim thickness that would immediately work with no trial and error fitting involved, unfortunately. Theory is fine, but when setting up firearms, it often goes out the window to some degree when the rubber meets the road. You can probably get in the ballpark by using your known 50 yard iron sights results to correlate the scope somewhat by putting the rifle on a gun vise, etc. aimed at a convenient object 50 yards away through a window (providing you don't have close neighbors who are going to think you're an "active shooter" when they see you through the window).







  7. #6
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    Yes, Ol'Relic is correct of course. By my calculations (8.9cm/y = 4950cm/8cm) then cross multiply, (71.2cm = (4950cm)(Y) and I get Y= 0.014cm which would be 0.14 mm.

    I don't know about you 8 x 57 Mauser, but I can't even see 0.14mm! Not to measure any way...

    However I can see if my irons and scope agree.

    So I think you're stuck doing it Ol'Relic's way.

  8. #7
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    I found my original calculations from back in 2011, i had it at .015" went back to the range today that put me dead center. I use stainless steel feeler gauge from autozone, comes in varying thickness from .001"-.400" its the perfect width to cover both pads with a single piece. I've never found the need to shim each pad differently on any of the 6 i have.

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    As of 9/17/2011 new member of the 98k Sniper Club, bcd 45 lsr
    11/2/12 SSZZA4 bnz 43 #8301 added to the family
    1/2/13 Joined the Classic Arms 91/30 PU family


    Bitterly clinging to my guns and religion since 11/6/2012

    ΜΟΛΩΝ ΛΑΒΕ

  9. #8
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    Quote Originally Posted by 8x57mauser View Post
    I found my original calculations from back in 2011, i had it at .015" went back to the range today that put me dead center. I use stainless steel feeler gauge from autozone, comes in varying thickness from .001"-.400" its the perfect width to cover both pads with a single piece. I've never found the need to shim each pad differently on any of the 6 i have.

    Click image for larger version. 

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    You've needed to shim all 6? That's pretty unusual.







  10. #9
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    Every one needed to be shimmed to bring the windage to zero. 4 are Molots, two are Rguns. all .012-.018

    haven't bought one yet that short left, all have shot right

    Quote Originally Posted by Ol' Relic View Post
    You've needed to shim all 6? That's pretty unusual.
    As of 9/17/2011 new member of the 98k Sniper Club, bcd 45 lsr
    11/2/12 SSZZA4 bnz 43 #8301 added to the family
    1/2/13 Joined the Classic Arms 91/30 PU family


    Bitterly clinging to my guns and religion since 11/6/2012

    ΜΟΛΩΝ ΛΑΒΕ

  11. #10
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    Quote Originally Posted by 8x57mauser View Post
    Every one needed to be shimmed to bring the windage to zero. 4 are Molots, two are Rguns. all .012-.018

    haven't bought one yet that short left, all have shot right
    That is really weird. Most of mine are pretty close in windage out of the box (of course, that's somewhat ammo dependent, though). The only Molot I've had that was significantly off shot WAY left.







  12. #11
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    Could be my ammo, they are my hand loads. I'm using 182gr FMJ BT, 46gr IMR 4895, running around 2,650 FPS. shoots really tight groups, one of my Rguns rifles likes it down at 44.3gr but the others do better with the 46

    Quote Originally Posted by Ol' Relic View Post
    That is really weird. Most of mine are pretty close in windage out of the box (of course, that's somewhat ammo dependent, though). The only Molot I've had that was significantly off shot WAY left.
    As of 9/17/2011 new member of the 98k Sniper Club, bcd 45 lsr
    11/2/12 SSZZA4 bnz 43 #8301 added to the family
    1/2/13 Joined the Classic Arms 91/30 PU family


    Bitterly clinging to my guns and religion since 11/6/2012

    ΜΟΛΩΝ ΛΑΒΕ

  13. #12
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    When I needed to shim mine (I was lucky. I was off to the right and could shim the entire bracket for that), I just bought some thin pieces of sheet brass down at the local hobby store, cut them in a "Fat H" pattern (the 'crossbar on the 'H' is the full height of the pad area, and the 'H' arms on the shim fit just in front and to the rear of the base of the coarse elevation screw lugs) and stacked shims by trial and error until I got the thickness I required.

    I could have left it that way, but I am a tinkerer. Another two (sturdier) 'H' shims cut from thicker material once I had the correct thickness figured out solved all my problems (I couldn't do it with one shim because I couldn't get the right material thickness).

    The 'H' shaped shims are pretty hard to lose once installed because they are pretty well held in the base by the 'H' lugs on the shim and by the elevation screws (one of which (the top one, of course) I had to back out some to get the shims in). The shims aren't going anywhere soon.



    I don't buy the story about having to cut the top and bottom of a shim differently to perform some "special alignment voodoo" with the scope. This statement about the scope needing to be 'precisely above the boreline' mystified me for quite a while until I realized that it doesn't actually MEAN anything.

    You actually need the scope optical centerline to be 'parallel in the vertical plane' to the bullet's path (bear with me here), and only REASONABLY CLOSE to being directly above the bore centerline.

    Claiming some special alignment benefits from getting the scope PRECISELY over the boreline just ends up confusing people about what is really quite a simple process. You could drive a machinist slowly crazy trying to actually DO this by just filing the pads.

    The two pads on the mount basically exist to match up a POI to the right by filing on (shortening) the bottom pad to make the scope 'look at the POI', and matching up a POI to the left by filing on the top pad. Once you have the POI where you want it with the optics centered, the scope is for all practical purposes parallel to the bullet's flight path, which is THE important thing.

    Who really cares at that point if the scope is .010" (or whatever) off of being DIRECTLY above the bore centerline? Your groups won't notice it, and your target certainly won't either.

  14. #13
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    No "voodoo" involved, just simple geometry. IMO, if one is going to the trouble of shimming a mount to center the reticle, one might as well do the job right and center the scope above the bore (without needing to turn the scope in the mount and consequently hold the rifle canted as per the Soviets' solution).

    The correct placement of the scope is completely dependent on the combination of base placement and however the mount pads have been ground in the past.

    But I'm sure a "tinkerer" like yourself is clever enough to understand that.







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