Here's yet another way that works with the scope still on the rifle, and it can be done at less than 20 yards, easily, with one person. Securely bench the rifle on a steady rest aimed at a yardstick. Move the windage adjustment to the extreme left, and record the position measured on the yardstick. Then, move the reticle to the extreme right and record the second measurement. Take half the distance between the left and right extremes, then move the reticle to that spot. Rotate the yardstick 90 degrees and repeat the same drill to find the vertical center of the range of elevation adjustment, and set the elevation at the center Measure the vertical distance between the center of the rifle bore and the center of the optical path of the scope. This is typically about 1.5 to 2.5 inches, depending on the diameter of the objective lens and the height of the mounts. Now, replace the yardstick with a target on a good pellet trap. Aim at a clearly marked point of aim and shoot a round or two. If the scope is exactly aligned with the bore, the point of impact should be the same distance, straight vertically below the point of aim as the distance the bore is below the center of the scopes optical path. If the point of impact is left/right or up/down from where it is supposed to be, that's how far off center the scope is mounted. The range doesn't really matter, and actually, closer is probably better, as you will be in the flat part of the pellet's trajectory. You can do it a 5 yards, indoors if you want.
Remember that if you have recorded the measured left/right and up/down traverse between the two extremes of the scopes adjustments at the same range, you can quickly measure the variance in the point of impact and see if the misalignment of the scope is greater in magnitude for which simple adjustment of the reticles can compensate. Remember also, whether you have an adjustable scope mount, or whether you shim a non-adjustable scope mount that you will have to move the axis of the scope mounts to the correct the alignment of the optically centered scope, moving it toward the point of impact.
And finally, remember that the optimal alignment of the optical axis of the scope is not along the same vertical axis of the bore. If it were, the pellet would never "rise" into the zero of the scope's cross hairs. It would simply start out 1.5 to 2.0 inches below the optical axis of the scope and then drop at a rate dictated by gravity, out of the field of view. Empirically, I find that if I optically center the scope, and measure at 5 yards, then adjust so that the point of impact is half the distance below the aim point that the bore is below the center of the optical axis of the scope, then at 10 yards, the point of impact is near the optical center of the scope. At 20 yards, the point of impact will be above the axis of the scope and somewhere out around 25 to 30 yards (just like a stopped clock is correct twice each day), the pellet has dropped back to the zero point of the scope. This works very well for me as most of my shooting is done from about 10 yards to 30 yards.
Obviously, if you have a higher velocity pellet with a flatter trajectory, or if you intend to do most of your shooting at 50 yards or more, then the optimum alignment of your scope may be different.
see:
http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=8162&posts=2&highlight=screen%20porch&highlightmode=1#M59227
