acceptable zero, your work's not done. Now you must test your scope to see if it shifts when you change magnification or elevation or operate the Bullet Drop Compensator.
Since you zeroed a zoom scope at its highest magnification, now you'll check it at lower powers. Fire a 3.5-10x scope at 6x and 3.5x and carefully compare the point of impact to the initial lOx impact. If there's any variation, record it; if it's 1 MOA or more, replace the scope.
(I once used a collimator to test four different scope brands for drift from the grid's zero when they were zoomed. Several stayed right on dead center, but one wandered about 3 MOAs when I shifted from 9x to 6x, and it surprised the hell out of me. The excellent scopes were more costly.)
Next, test-fire the Bullet Drop Compensator at each 100-yard distance on its knob. Unless the BDC is expressly designed for shooting with your exact load, there will be at least a bit of variation between the indicated range and where you must set it to strike dead-on at that range. On some scopes, for example, I must click one or two increments up or down at most distances to be exactly on bull's-eye. This is the most time-consuming part of zeroing, but learning such exact fine-tuning brings tremendous confidence to your longrange shooting. Record these slight variations.
Should you have target knobs, test the comeup data we've provided for accuracy at appropriate ranges. Fire these at 100-yard increments, fine-tuning along the way so they're exactly on the mark. Record any variations, which will be slight, from the indicated values.
The final test I've dubbed "repeatability," which only means that your elevation or windage will go back exactly to where it was if you've changed it rather than back to the original setting. The simplest way to check it is to fire a three-round group at 100 yards, then click up a given number of clicks, fire another three-round group, then dial back down the same number of clicks and see if the next group places exactly where the first one did. Do not fire the test so quickly that you heat the barrel abnormally. Do the same thing for windage and see if there is any shift.
A shift of even 1 MOA would alarm me because what you're really testing is the consistency of the scope's internal gears, which means you could have a very significant problem at greater ranges. What's not commonly recognized is that scopes wear out due to their internal gears, not usually to scratches on the lenses or dings on the tube. A heavily used scope, especially having soft brass gears, eventually will wear enough that it no longer can be changed incrementally with consistency.
Should you experience repeatability problems, try the test again. If you have similar results, replace the scope.
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