Folks, this is a question about shimming a scope that ran out of internal adjustment.
I�m not a gun smith, I�m a simple guy trying to do a friend a favor.
My friend�s rifle shoots high at 100 yards. He got all the adjustment out of the scope he could, but it still shoots high.
He asked me for help.
It�s been long enough that he isn�t sure how high it was. Maybe 8� to 10� high at 100 yds he thinks.
Here�s two questions for you.
Should I shim the underneath of the rear base where it contacts the rifle receiver, or shim the underneath of the scope when it meets the base?
Also, is there any way to determine ahead of time what thickness shim I should use? I have some good stainless shims, either .015in (.381mm) or .020in (.508mm) available, or I can get other thicknesses.
Thanks in advance for your help.

On the one occasion that I needed to shim a scope, I shimmed between the ring base and the receiver.

When I've needed to shim, I put the shim between the base and the receiver. You are going to need a shim under the front base to lower point of impact. You need to be careful that the rings are not out of alignment after shimming or you can bend the scope tube body.

I know this is not what you asked, but there may be a better way, IMHO. Find some Burris Signature rings http://www.burrisoptics.com/sigrings.html that fit the bases on the rifle. Using eccentric inserts, you can adjust the scope up to .040 " without placing a strain on the scope tube. .001 equals roughly 1" @ 100 yards. An addition benifit is, the inserts leave no dents or damage on the scope tube.

One trick you may want to try before resorting to shiming is just switching the front and rear bases if they are the same, and also the front and rear rings -- also try switching them.

If the gun is shooting high, that means the scope is pointing down. You would want to shim the front base to get the scope pointing up to where the bullet is impacting.

One way to visualize it is that you want to move the scope so it's "looking" at where the bullet hit the target. Works for iron sights and bow sights too.

You can calculate the required shim thickness with this formula:

Shim thickness = Distance between centerline of scope bases X (target elevation change desired/distance to target)

All measurements including distance to target has to be in inches to give shim thickness in inches.

It would be a good idea to epoxy bed the rings if you have to shim them.

Epoxy Ring Bedding

Shim the front and every .001" of shim material,will move impact 1" at 100yds. If he's driving turrets,get the erector towards the bottom third for his initial zero,if drivingthe reticle,mechanically center same.

I'd wager that he has quite a bit of windage cranked in too,which much negates available vertical erector travel..............

Yeah, shim the FRONT base.
I've had to do several. I get KS brass shim stock, but a pop can will work, too. Easy to cut is good.
Then center the scope, right? Fire for error (if it is bad, you might need to be at 25 yards and calculate for 100). Run the calculator as Ron suggests, do everything in inches.
Once you have a basic shim thickness, you can cut a shim for under the base. I usually epoxy the shim to the base with release agent on the receiver.
Next: If you have a one-piece base, it's probably a smart idea to bed front and rear with epoxy and mold release.
If front and rear bases, you should lap the rings for alignment. Or you can cheat with one layer of blue tape in the lower half of the rings. That will get you shooting, but doing it "right" for the long term, you should lap.
Finally, the quickest and dirtiest way is to put the shim stock in the front ring, just about 45 degrees worth on the very bottom. Works great when you DROP YOUR RIFLE ON THE GROUND AND BEND THE SCOPE 300 MILES FROM HOME. Don't ask me how I know this, but it's amazing what you can do with a pop can.

You boys like to make easy stuff difficult............

NoooOo... That would be machine the base and lap the rings.

Thank you - to everyone!
The post by Ron AKA got me thinking. I measured the front & rear rings & bases and found them different. Then I checked for any kind of ID marks on them and called the company to confirm.
They aren�t even the correct rings & bases for his receiver!!!
Yup, he bought the rifle used for what he thought was a good price, and was never able to get it to shoot right.
Now we know why!
I�ve got a new (correct) set of rings & bases ordered and will proceed.
I�ll center the scope ahead of time by bringing the windage & elevation adjustments back to the middle of their course, then mount the bases, set the rings and bore sight it.
I�ll also keep my fingers crossed that the shooting goes smoothly. Darn thing is a .300 Win., and I don�t want to loosen my teeth shooting too much.
Thanks for all your help.

Unless it's a one piece base, don't use shims. Don't. The scope is a rigid tube and rings are just tubes that clamp to the scope. They're even more rigid. They both need to be on the same centerline and if they're not, the scope has to bend. Shim one base of a two-piece set, the rings will be out of alignment. Shim one ring, same thing. Clamp down on a ring that's not lined up with the other, you also get an edge cutting into the scope. Lapping rings is a minor help but the problems you're talking about are major. Just use Burris Signature rings and don't screw around with other stuff. The Burris website has a beautiful explanation of what I said above.

Sight or scope adjustment.....the rear sight/eyepiece/reticle is adjusted in the direction you want bullet impact to go. To move the point of impact down, adjust the elevation down.

"They're even more rigid. They both need to be on the same centerline and if they're not, the scope has to bend. Shim one base of a two-piece set, the rings will be out of alignment. Shim one ring, same thing. Clamp down on a ring that's not lined up with the other, you also get..."

There are other problems you can run into with 2 piece bases.

I have attached both bases and tightened the one of the rings to the base, leaving the other one free.

In some instances, with one ring tightened, there wil be a space between the other base ring. If you do not shim this base, the scope tube will bend when you tighten the ring tight into the base.

I have noticed it more often with the rear base. It probably happens because of manufacturing tolerences in both the rifle receiver and the bases and rings.

It occurs frequently when sporterizing Mauser 98s and other military actions if you take some metal off the rear receiver ring.