View Full Version : Shim scope mount to equal 20MOA?

April 30, 2008, 09:58 PM
How many thousandths of an inch would I have to shim the rear of a scope base/mount to equal 20MOA of elevation?
I'm not sure how to do the math on this.
I appreciate any help.

May 1, 2008, 01:40 AM
You would have to shim the rear base approximately .100", but I would not recommend it. That much shimming would put the mounting screws at an odd angle and they would not seat correctly and hold the base tight. If you are deas set on a do-it-yourself tapered scope base, buy a gunsmith base blank and have it cut to fit your receiver, then drilled, that way the screws will be in line with the mounting holes.

Harry Bonar
May 1, 2008, 09:29 AM
Never shim anything.
Harry B.

May 1, 2008, 10:48 PM
Do you want to bring the elevation up or down? Wouldn't raising the rear base bring your elevation down? Most people I hear of wanting that kind of base are doing long range target shooting. If you are going to get into long range shooting you might as well do it right and just buy the proper bases.

May 2, 2008, 06:02 AM
2oMoa= 1/3*

tan(1/3)= 5.818x10^-3
Multiply this by the space (center-to-center) of your scope mounts.
ex. 5.818x10^-3x 5"= 29.1 thousandths of an inch.

I'll leave it to you to decide whether it's a good idea, but that's the math.

May 2, 2008, 10:19 AM
If you need more elevation and don't want to break the bank on expensive tapered bases, get yourself a set of burris signature rings along with some offset inserts. They come in both 1" and 30mm sizes and can fit standard or weaver type bases. The rings use plastic inserts that sit in kind of a spherical groove so they can pivot to align your scope perfectly without the need to lap. the plastic inserts also will not mar your scope at all. Good luck.

May 3, 2008, 05:54 AM
Wouldn't raising the rear base bring your elevation down?
It'd bring the groups up in relation to your point of aim. Just like with leaf sights; the group goes wherever the rear sight goes.

May 3, 2008, 06:29 AM
THANKS for the math, I need to study that a little. At a glance I don't quite understand it. You're way ahead of me!

I really appreciate all the sound advice, I've got the Burris Signature rings on order, BUT, the 1'' rings are on backorder nationally. I have a set of them in 30mm and like them. I need the 1'' rings for a Leupold 36X scope. I think I'm going to sell that scope and cancel the 1'' ring order. The 36X fixed power does not seem to be the best choice for my Savage 6.5 x 284 for 600 to 1,000 yard shooting.
Again, THANK YOU gents!!!

May 3, 2008, 08:11 AM
all the goofy symbols make it look like heiroglyphics. :D

The idea is that a degree is broken up into 60 minutes. the 20 minutes you wish to move your group is therefore 20/60 degrees, or 1/3 of a degree.

The tangent function is simply the slope of the angle (rise/run). No matter how large the triangle, the slope will be the same. In this case, a triangle 1/3 of a degree will rise 0.005818 units for every unit of run (regardless of what unit you're using).
So you plug into your calculator 20 (minutes of angle) / 60 (minutes in a degree) then hit "tan". It spits out 0.005818. So for every unit of run, a 20MoA angle will rise 0.005818 units.

In this case the run (base side of your triangle) is measured from your front ring to your rear ring. So you multiply that run by 0.005818 to find the rise, which is how much you shim.

In my example, if your scope mounts are 5" apart you'd shim it 30 thousandths.

Hope this clarifies things!

May 3, 2008, 10:40 AM
EGW has a base in 25 moa for a savage, for 40 bux..
Warne makes 20 moa bases
Farrell makes 20 & 30 moa bases
Seekins makes 20,30, & custom bases
Nightforce makes 20 & 40 moa bases
Ior has a 35 moa base

going through this myself..
changing out a warne 0 moa for a EGW 25 moa base on a savage F/TR
with a IOR 6x24x50 scope in 35mm tube..


May 3, 2008, 01:22 PM
If you don't have rings like the Burris that allow their alignment axis to adapt to the scope tube axis, then, as Harry said, don't shim it. You will wind up with the front and rear rings not coaxial. It may be challenging to lap them far enough to correct the problem without loosening their grasp on the tube. If you don't, the offset in their axes will then indent the scope tube (also called, ringing the tube). That can damage the scope by throwing the optics out of alignment. If you can't get the ball-joint rings, get the special ring bases which have that 20' taper angle machined into both the front and rear to keep the rings on the same plane and their borings aligned.

May 5, 2008, 06:39 PM
The math is correct. You may want to try "shimming" the easy way and it won't stress your rings and scope either. Simply make the shim of expoxy!

Clean the bottom of the base well. Coat the base sides, top surface and the rifle's top surfaces with a release agent or Kiwi Neutral Shoe Wax. Plug the rifle's middle mount holes with candle wax and use a Q-tip to coat the two end holes with release. Put the two end screws in the mount and run a length of tape along the top of the base to hold them there until you are ready to screw them down.

At that point, you are ready to fit the base to the rifle. Place a metal shim, a thin (but curved washer) will do nicely, 25-30 thousanths thick under the rear mounting screw and place a layer of epoxy along the full length of the bottom of the mount. Attach the base to the rifle and just snug up - DO NOT tighten past snug! - the two screws. After a couple of hours, more or less depending on your epoxy type, clean off any squeeze-out with a toothpick, then let it set over night to harden completely.

When you remove it, your base will have a custom molded, stress free "shim" that you can then attach normally. And, if you want to remove the epoxy later just heat it up enough to allow you to scrape it off.

May 6, 2008, 05:24 PM
That method is still not perfect. Yes it takes any bending stresses out of the scope base, but the screws still will not sit correctly since the holes will be at a slight angle now. When torqued down the screw heads will be forced to bend in order to seat correctly in the countersunk holes. This is less than ideal, I strongly recommend the burris signature rings. They are not on backorder everywhere, Midway looks to have about half of them in stock. If you check The Optic Zone or SWFA I bet they would have any rings you need.

October 13, 2008, 08:23 PM
Would a shim be ok in that instance?
I think I about 1/1000th is all I would need, but again we come to the idea about stability. This is a 25-06 Sendero I JSUT today got.

I have a Barska scope (6.5 to 20 x 50mm) that I am trying to mount - should I be getting higher rings and a better scope?


Harry Bonar
October 14, 2008, 08:07 AM
If, I say, IF, you're going to "shim" up a scope a very small amount, you can put some cut brass stock under the scope at BOTH bases - not just one.
Some bases and rings are far enough off to start with. Scope, or, iron sight mounting is done so that the sights are paralel to the bore line (the bases) so that the optical center of the scope is also there, then you can adjust the scope internally.
A good test is to remove the bolt - look through the bore and raise your eyes up and see if the "exit pupil" is centered in the middle of the eyepiece!.
Mausers with Wever bases and rings are particularly prone to misalignment, but I still do not shim them. Everything must be "kosher" and aligned in scope mounting. Redfield one piece bases are very good because the alignment can be adjusted at the read. Even as close as Redfield on piece bases are you will still see the rear sometimes need to be pulled down as you tighten that screw! Scopes cannot be mounted, "on a bind."
Harry B.

October 26, 2008, 03:10 PM
cut a square from the side of a coke can. slightly smaller than width of base.
See how much elevation it give you. see how many shims you need.
I never needed more than two.

October 26, 2008, 03:34 PM
I just went throught shimming a scope. 3 thicknesses of a coke can weren't enough. I wound up cutting the brass off of a shotgun shell and using 2 of those and a couple of coke can shims. So far, so good.

October 28, 2008, 01:06 AM
I too tried this, I needed about .009 shim to give myself more vertical elevation and less in the downward direction. I ended up shimming so much that I then needed new scope base screws. So as for shimming don't do it. Consider a canted scope mount from somewhere like midway. Also I recommend sticking with a one piece base. They are proven to be stronger and more rigid. I have shim stock in .003" pieces of alum. I'd sell you some real cheap. About enough to cover shipping. Just don't do it. Below is my 700 VSF in .308. What a joy it is to see varmint brains in a fine red mist.