Determining Correct Scope Base Angle By Robert H. Thompson


Mar 6, 2008
During a recent rifle project I wanted to know exactly how much angle correction I needed in my scope base to center the elevation settings that I would be using, within the scope’s adjustment range. This was an economy long range rifle build and I was going to use shims under the rear scope base to get the angle correction needed. I was using Burris 30mm Signature Zee rings, which allow some adjustment with their offset inserts, but I had already used some of that feature to center the windage adjustment, and the 30mm inserts didn’t offer as much correction as I wanted. Read More...
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Great article! Great way of describing how to figure out how much shim to use. I've used the formula: TAN (angle of base) * length between centers. A little more complicated as it requires a scientific calculator and I had to relearn myself on high school trig, but it works. Remember that it requires you to keep everything in degrees so 30 MOA would be .5 degrees.

Thanks for the teaching, great article. This information should help many of us with future projects.
Couple of points to remember.

If you shim the rear base much more than 20 MOA then you may need longer rear base screws. You have to make sure they are not too long and hit the bolt. If you try to use too short of screws you risk stripping the threads when they pull loose.

Before you shim a scope check the receiver to see if it is level to begin with. Many are not square and you will begin with some positive or negative angle even with a flat base. This must be accounted for in determining the amount of shim needed.
Brass shim stock, I think it was 0.010 inches or 0.25 millimeters thick. Here in America it is readily available from industrial supply houses. I purchased a pack of assorted thicknesses many years ago.

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