Another cause of scope shift: over-adjusted scope knobs
By B.B. Pelletier
In my March 14 report, What causes scope shift?, I promised to return to some of the other causes of scope shift. One more way to acquire scope shift is when either one of your scope knobs is adjusted out too far, allowing the reticle to flop around.
How your knobs got over-adjusted
Inside the scope tube, the reticle is housed in a smaller tube called the erector tube, which rests on springs that are on the opposite side of the windage and elevation knobs. When you adjust the elevation knob "down," you are actually applying pressure to one side of the erector tube, which compresses the spring on the opposite side. If you adjust either knob as far as it will go, the spring either becomes bound up and refuses to move, or it becomes so relaxed that the adjustment knob starts feeling mushy and indistinct.
When the adjustments feel mushy, the spring that pushes against the erector tube is relaxed and not able to keep proper tension on the tube. A bump to the rifle or even regular recoil can push the erector tube to a different spot - and you end up with scope shift! Some scopes are designed to minimize this problem - but yours may not be, so keep scope knobs fairly well centered (up/down and left/right).
Get more out of your scope without risking scope shift
How do you get on target without using the scope's adjustment knobs? Use an adjustable scope mount! It lets you zero a scope without using its internal adjustments. When you want to make small changes, you'll have the scope's entire adjustability available.
Adjustable scope mounts are more difficult to set up initially. Once they've been properly adjusted, the scope is far easier to use - you put in time up front to save time later. If you don't want to remount all your scopes, follow my rule of thumb: use adjustable mounts on your most precise airguns and fixed mounts on guns used for more general shooting.