‘Scope shimming

I was recently asked why I shim ‘scope rings and how I fit them to ‘scope rings.

The easier answer to why shim, is to get the ‘scope as close to mechanical zero as possible. By mechanical zero I mean when the ‘scope elevation is wound as far down as possible, the rifle POI (Point of Impact) should be just below the aiming point for your chosen zero distance. So if you zero at 100m and wind the ‘scope right down to the bottom it will hit just below the target. The reason for this is two fold, one it utilises the full elevation range of the ‘scope and two, if you do have to return the rifle to zero you can just wind the ‘scope down, feel it bottom out and know you are maybe one click below zero, so much easier than having 13 clicks or whatever of redundant elevation.

How do I calculate the shim thickness? Some basic math and an xls spreadsheet, mine calculates in both metric and imperial and in MOA and mils however in simple terms one way you can calculate your own shims is as follows:

(The thickness of the shim) = (distance between rings) X (distance you want to move the point of impact at 100 yards) divided by 3600. The resulting number will be in inches.

Example:

Distance between rings = 4.6″

Distance to move the POI at 100 yards = 13″

Shim = 0.166″

4.6×13/3600 = 0.166

Great, so now we know one way of calculating shim thickness however producing and measuring a shim to three decimal places is going to be awkward at the best of times, a vernier certainly is not going to be able to measure the shim thickness accurately.  I tend to aim for three decimal places eg .134″ for the last one I did, the shim should if at all possible be in one piece and a softer material such as aluminium works best for me. I drill mine  (Dependant on ring type) cut or machine to size and then adjust the final thickness with a piece of wet and dry paper,  pushing down on the shim with a finger and rubbing it in circles on the wet and dry on a flat surface. I use a calibrated micrometer to measure.

For my own rifles I build and fit a temporary shim that puts me within an inch or two of where I need to be, shoot and then measure how many minutes/mils below zero it is, then calculate the required increase in thickness to get to just below where I need to be. As an example a couple of my rifles have one click below true mechanical zero so I can wind the drum down until it bottoms and know I am one click below my chosen zero. It takes a couple of outings to get it right but is well worth the time if you need the full elevation range of the ‘scope.

Why don’t I try and get it right first time? The easy answer is I am trying to get to 1 click below zero which is around .004″ and given the compression of the material and other variables I find it easier to do the job in two gos.

One thing to consider is the potential stress placed on the ‘scope tube if the shim is of a significant thickness and you may wish to lap the rings or even bed the tube itself in the lower half of the rings. Alternatively you could use a set of rings with self aligning inserts.

13 Replies to “‘Scope shimming”

  1. When measuring the distance between the rings where do you take the measurement from and is the formula valid for meters?

    Measuring from the middle of the rings on my 22 Hornet it seems I need a shim 0.009 thick to take back to the mechanical zero.

  2. Sandgroper, I measure from either the fronts or the backs of each ring with a vernier as it is more accurate than trying to judge the centre line of the rings. You can convert metres to yards and the equation will still work.

    /d

    1. You almost got me with the meters to yards bit until I realised that 3600 was the number of inches in 100yds and I had to change it to the number of mm in 100m if I wanted to work in metric!

  3. Maybe this is a stupid question but can you harm the internal parts of the scope being that close to bottomed out? i have been trying to get this formula figured out on my own and i basically came up with in a long drawn out sort of way. thanks for the information.

    1. I fit a mechanical stop ring tp prevent winding down a complete turn below zero and I usually build it so it is a couple of clicks or so below zero when bottomed out. In every day use it should be fine however I certainly would not recommend trying to force the drum to go below the range limited by the stop ring.

      /d

      1. Ok well then i should be in good shape. you said you like aluminum for your shims do you ever have problems with them failing due to recoil? i would have thought something harder like stainless would be better what’s your thoughts?

        1. Alvin, I use a reasonable aluminium such as 7075T6 however 6082T6 works just as well and without any failures due to recoil or compression. usually there is a reasonable surface area to spread the load and they are not torques up excessively so it should not be a problem.

          Stainless is fine as well, just a bit tougher to machine and work with. I have also used brass in the past which worked very nicely.

          /d

          1. Ok well that makes me feel better. you talked about shimming the rings, can i shim the base instead? i have a ruger m77 markII and i am going to replace the integral rings with a one piece weaver style base. i was thinking that by shimming the base the rings would stay more true to each other. what’s your thoughts on this?

            1. Alvin, When I say shim I always mean shimming the ring bases or the rail itself, packing a shim between the inside of the ring and the ‘scope tube would not work particularly well however you can bed ‘scope tubes to rings if needed. The way I shim is dependant on the type and make of rings being used and shimming the base certainly works OK.

              /d

  4. Ok thank you so much for the information. it has been a huge help now i just need the funds to buy the scope i want and i will be set.

Leave a Reply

Your email address will not be published. Required fields are marked *

HTML tags are not allowed.

349,895 Spambots Blocked by Simple Comments