I see a lot of faults with many of the methods described on the forums.
Let's see of I can stir the pot more without hurting anybody...
And the below is extreme visualization so it can be understood and seen.
We put a level on a the picatinny rail and level it, so it is assumed that the barrel centerline is perpendicular to the pic rail.
With this leveled you look thru your scope and see the plumb Bob line.
You rotate the scope in the rings until the retical's center vertical lines exactly to the plumb Bob line.
So the scopes retical is perpendicular and square to the level on the pic rail right?
So this must mean if you draw a straight line from the scope center to the barrel center the retical, and its center vertical line is perfectly lined up with this centerline right?
Nope, not necessarily.
So the below picture is the best without photoshop but hopefully with this visual you can see that drawing a line between scope center and the barrel center, it does not match up with the scope's retical. Thus the scope/barrel centerline can be canted from using a plumb bob.
So even though the pic rail was leveled, and the scopes retical was lined up to the plumb Bob line.
This basically means it is not the same or a good as using the mirror method like in this picture below.
I hope that worked the way i try to convey it and you can see what I mean. The plumb Bob method assumes the scope's vertical centerline is already in line with the barrel's center and because of that it can easily be wrong. I'm not saying the plumb Bob line is not perfectly vertical.
I hope I'm not wrong in how i understand the different methods and how they are better than the mirror method. If I am wrong hopefully you can explain it in a way I and others can understand.
Thanks
Allen