What's wrong with scope adjustments?
by B.B. Pelletier
This question came in from Pestbgone, who asked why a scope with 1/4" adjustments at 100 yards did not respond with 10 times the number of clicks at 10 yards. In other words, why don't 10 clicks move the pellet a 1/4" at 10 yards, if one click moves it 1/4" at 100 yards. Great question. Simple answer.
Nothing is for certain
How do you think scope makers get the reticle to move 1/4" at 100 yards? They do it by using a screw with a very fine thread. This screw moves the erector tube by a small amount. Attached to the screw is a ratchet or click detent that produces an audible and tactile signal that one "increment" of distance has been covered. For example, one click equals 1/4" at 100 yards.
The truth is that one click never does equal 1/4" at 100 yards. It's close, but not exact. That's because screw threads are not cut that way. While it's possible to get close, it's next to impossible to get exact movements that equal the measurements scope manufacturers put on the adjustment knobs.
Does that bother you? And, why do I know all this? Well, about 10 years ago, several field target shooters came up with a brilliant idea. They reasoned that if software programs could calculate pellet drop, they would never need to sight-in their guns at more than one distance. They could simply adjust their scopes for all other distances once they had been "sighted-in."
Only, it didn't work that way. When they actually tested their theory, they discovered that click adjustments came in all sizes. They found scopes with 1/7" adjustments, 1/5" adjustments and many others. What they never found were scopes with adjustments that were identical to the inch value given on the adjustment knobs or in the scope's instructions.
You would think that this would cause a tremendous uproar, but it never did. In fact, very few people today even know about this phenomenon. The truth is that when someone goes to the range, they adjust their scope til the gun hits where they want it to, and they pay little attention to the actual click value.
Airgunners, however, have a greater need than firearms shooters to know the click values. Because our adjustments are all at much shorter ranges, we have to apply many more clicks to move the strike of the pellet. Therefore, we notice when things don't seem to be working out the way we think they should. That's why Pestbgone noticed that his scope was not adjusting as expected.
Do the work
There's not substitute for actually sighting-in a scope at all ranges you wish to shoot. That's what the experts do, and that's what you'll have to do if you expect to hit what you're shooting at. This is the reason that my explanation of the two points of intersection (where the pellet is right on the crosshairs) is so important. No one has time to sight-in at every possible distance they will shoot. You need to find a good medium range to sight-in, then test your rifle at different distance throughout your intended range of use. Don't count on click values being correct. Even on precision target scopes, they're going to be off a little.