Home Forums Hunting & Shooting Gunsmithing Scope base shim question? Need advice.

If I put a 0.10 inch thick shim under my rear scope base, how much will it raise the point of impact at 100 yards? I need a little more elevation than my scope is giving me.

Thanks for your help guys.

If you work out the proportion it will be an enormous rise - about 7 to 8 feet. What is more appropriate is 0.010 or a 10 mil shim. That will give about 9". The proportion is;

x = shim height in inches
4inches is the center to center on scope base and 100 yards = 3,600 inches
you need to think how much change you want - say 9".

x/4 = 9/3,600 x= 0.010inches

Regards, stv

This is based on what Cheaha posted a few days ago in a thread. He said that to move up 40" you need to shim .040" so thats .010" shim for every 10" of elevation. I will verify this on paper tommorrow. Mine was 40" low from from optical center. I shimmed up .03" last night. It looks great and I was very happy with the outcome. I put release agent on the action, applied a thin layer of Marine Tex to the action, laid one shim, then a thin layer of MT on top of the first shim, then laid the second shim. I then applied a layer of MT on the bottom of the base. I did not put release agent on the base as I wanted the MT to stick to it. I then laid the base in place and tightened the base screws. Cleaned it all up with QTips. It looks exellent. You would never notice that it has been shimmed and I'm guessing that the perfect fit of the bedding makes it more stout. When I removed the base this morning, the MT was hard to distinguish from the base.

So, a close WAG is; I get 10 inches of rise for each .010 inch shim I put under the base?

If I wanted 50 inches of rise at 100 yards, five .010 shims would get me close.

Do I grasp the concept?

Yep, right on. But .05" is a lot of shimming. You would definitely need to bed that, both to sure everything up and for cosmetic perceptions.

I hazzard to get into this but if you shimm the rear base upward, doesn't that lower your bullet impact on the target? I thought I read on one of the threads that it was just the opposite. Which way is up ? <img src="/ubbthreads/images/graemlins/confused.gif" alt="" /> <img src="/ubbthreads/images/graemlins/confused.gif" alt="" />

Imagine your rifle is locked in a vice and the point of impact is lower than the crosshairs. With the rifle locked, and without moving it, how would you adjust your scope? You would adjust the crosshairs down, to the point of impact. Thus to point the scope down, move the rear up.

Yeah. I can see that. I was just thinking about some long range shooting posibilities....

Rolly, up is down and down is up. Left is right and right is left. If you are moving the front sight base it's all backasswards. <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />

To be perfect it would be best to have a one piece base. There is a chance of having the scope out of alignment otherwise.

Another answer is to use rings like the Burris Signatures with the eccentric inserts.

Just bending a scope up and shooting at long range does not seem to go together.

I can tell you that .030 shim under a rear base can bend a scope.I dunnit!

Oh yea, now I remember. Brain cramp ! <img src="/ubbthreads/images/graemlins/blush.gif" alt="" />