Re: Reticle Perpendicularity by Darrell Holland
SBruce: the ACD needs to be lined up with the scope, NOT the gun. If you follow this method the ACD, scope tracking axis, and bore will all be in a nice line, but getting the ACD set to the tracking axis is much more important, and should happen first and independently.
My method for ACD calibration is to set the gun in a vice so the crosshairs are lined up with a building edge or other vertical object. Then I crank the turret to ensure it tracks vertically, in-line with the crosshairs (all my Leupolds and Super Snipers always have). Now install the ACD with a centered bubble, ensuring you don't bump the scope off-level as you go.
After that exercise, you may want to line the tracking axis up with the bore. I eyeball it and it's good enough for me - this is why:
There are several things going on here:
1) Your scope may not track exactly along the lines described by the crosshairs. Thus, lining the crosshairs up perpendicular to gravity with an ACD and dialing a shot may result in error. On high-quality scopes I have never found one measurably out of alignment, but it's common on very cheap scopes.
2) There is some error induced by having the tracking axis of the scope not perpendicular to the bore. However, it is linear with distance, and very small. A 5 degree non-perpendicularity (very noticable to the eyeball) and 2" distance from scope axis to bore axis (very long) will only result in 0.875" error at 500 yards, assuming you sighted in at 100 yards.
3) The real issue is perpendicularity of the tracking axis to gravity, which is accomplished by using an ACD referenced to the tracking axis of the scope. A 5 degree error here (often hard to see against a tilted horizon) produces a 5" horizontal error at 500 yards. If you're using a drop reticle, then you need to ensure the reticle itself, not the tracking axis, is lined up with gravity.
So, lining up the tracking axis (or drop reticle) with gravity is crucial, as we all know. Lining up the tracking axis with the bore is of debatable value for hunting applications. Certainly it's worth getting right, but make sure you have an ACD installed correctly first, since that's a much bigger error.