An observation on "scope shift"

by Tom Gaylord

The title suggests I don't believe in scope shift, but that's not true. I do believe there is such a thing - I just don't think it happens as often as some shooters believe.

In fact, I know a number of reasons you'll get a blown group and they may all look like scope shift, but none of them are. AND - they are the most common things that go wrong with scoped airguns. One of them, however, is head and shoulders above the rest as the leading cause for blown groups.

Elevation or windage knob adjusted too far!
This occurs in more than 90 percent of the cases I troubleshoot at AirForce. I've also had it happen to me. Here's what happens. The adjustment knobs work by pushing against a metal tube inside the scope. That tube, called the erector tube, houses the fixed reticle. On one side the adjustment knob presses against the tube. A coiled spring pushes against the other side of the tube.

Let's take the elevation knob as an example. When you want more elevation, you screw the knob in the "UP" direction. The knob moves outward and the coiled spring pushes the erector tube up. To go down you do the opposite. You can actually feel the spring getting tight when you adjust the elevation as low as it will go. When you adjust it up as high as it will go, you can feel the clicks get mushy and indefinite. And THAT is where the trouble lies!

When the spring is fully extended it doesn't press against the erector tube as tightly and the tube can move more easily. The result - you'll shoot to different places with the same sight picture. Because the reticle always looks centered to our eyes, we blame the scope for "shifting." It actually IS shifting, but not in any mysterious way. Because you can feel this sloppiness in the adjustment knobs, you can avoid it.

The best solution
The BEST way out of this problem is to use an adjustable scope mount, so the scope can have its adjustment knobs in the center of their range, where they are most accurate. The best (and one of the only) adjustable scope mounts if the B-Square AA. It's harder to install than a fixed mount, but the payback is the range of adjustability it returns to you scope.

Shimming is another way to fix this problem. I don't like to shim unless I can do nothing else because the scope tube gets out of alignment with the rings and can be bent during clamping.

If you are having problems with a blown or open group, examine your scope knobs and see if this is happening to you.