How much shim to use under the Scope mount?

B

Bigdog2

New member
My rifle, 25-06, has been sighted in for a 250 yd zero. I've started to use it as a beginning rifle for F-Class. Yes I know that it's not competitive but it's still fun shooting at 300, 500 and 600 yards and I can punch paper on a more regular basis than I can go hunting.

My scope is a Leupold 3x9 about 30 years old with a Dot reticle. I don't want to go out and buy a newer, more adjustable scope for awhile, so I'm happy just holding under and over the target, as needs be (a little Kentucky Windage). I decided that I'd like to zero it in for 500 yards but when I tried, I only had about 1/4 turn left on the adjustment, and that only gets me to a 300 yard zero.

I figured I'd shim under the front of the mount to raise the scope up, and I was thinking of just cutting up an old COKE can and using it for shim material---it's about 0.005" thick.

I'd like the scope to be set up so that I zero to 500 yds and yet still run the adjustment all the way to the bottom and still zero in at 200 yd zero, if needed. Top to Bottom the scope has 4.7 complete revolutions of adjustment.

Any recommendations as to how much shim thickness to install?
 
It's not how many turns you have, it's how many clicks you have.... so what is 4.7 turns equal to in 1/4 clicks ( or lines).

You don't want to shim the front - that will make it worse.

You will need to shim the back.

So, how many MOA is equal to 4.7 turns - and haw far apart are the rings, and on what rifle, with what bases?



.
 
No Clicks, it just turns smoothly and the markings around the adjustment screws are evenly spaced so that a full revolution would goes from 0 to 6.
The mount is a single Redfield base with 4" of distance between the c/l of the rings. The front ring is dovetailed into the base, the back one fits between the Windage Adjustment screws on the back of the base.

It a '98 Mauser action.

Jack---I guess I have. Care to enlighten me?
 
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Use the ratio 4 inches/100 yards (3600 in.) to get .0011 inches/MOA. Figure how many MOA change you need, multiply by .0011 and shim the rear of the scope upward that amount.

There's a real danger that, since the scope is now not going to be setting flat in the rings, you'll bind and possibly bend your scope tube.

But there's the math for you.
 
A good rule of thumb is that you get 1 MOA more elevation for each .001 of shim material you add under the rear of the mount between the screws. Most gunsmiths have shims for this purpose. An old tape measure is a good source for nice curved shim material. You still should drill the holes for the rear mount screws. For example, a .020 shim would give 20 MOA, or
20 inches at 100 yards
40 inches at 200 yards
60 inches at 300 yards
80 inches at 400 yards
100 inches at 500 yards

If your scope has 4.7 revolutions X 6 MOA per revolution, thats a total of 28.2 MOA total elevation adjustment. This does not seem right to me since most 3-9 Leupolds have twice that amount of elevation.

There is no danger of bending the scope tube when you have a one-piece base as you stated.
 
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There is no danger of bending the scope tube when you have a one-piece base as you stated.



That's true, Quote:
The front ring is dovetailed into the base

threw me, I was thinking you were going to shim between the ring and the tube. Avoid that if possible. I was forced to do it on my .308 due to the type of mount I use and it worked, but it's risky.

I used brass shim stock I got from the hobby store (.015), but you can get it in thinner and thicker increments. It's been working fine for 25 years and the zero has never changed.
 
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Thanks guys for the help, I knew you'd come through.

I think I've seen the "1 MOA more elevation for each .001 in." rule before, maybe on the Leupold site. But I couldn't find it on their newer web site.

Thanks again
 
(The thickness of the shim) = (distance between rings) X (distance you want to move the impact at 100 yards) divided by 3600. All dimensions are in inches.

This way it is a simple proportion, you don't even have to think angles.

Jack
 
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