ok i think i see where mkollman74
is getting messed up. lets say we take a rifle and level it with a level on the action somewhere. then we mount a scope on it and set the normally horizontal xhair as verticle. so now we have a scope that is at 90 degrees to the rifle. lets say the scope is 1.5 inchs above the center of the bore. then we cant the gun to the right 90 degrees so the x hairs in the gun ( the usual verticle one) is verticle. assuming that the bullet path is straignt for the moment and the line of sight is parallel. then the bullet will hit 1.5inchs left of line of sight at any range.
now we all know that bullets dont travel in straight lines other wise id be shooting bunnies at 1km. if the bullet path was horizontal with the line of sight at 100m then the impact point would be 1.5inch left and 0 up and down. then go to 200m and the impact point would be 1,5 left and lets say 2 inchs down. then at 300m impact would be 1.5 left and lets say 6inchs down. and so on. the up and down could be corrected for, but the bullet path would always be 1.5 left. this is where the issue lies. no one wants there bullet to always be 1.5 left.
so with the same scope rifle set up and sighting in at 100m we would dia the scope across 1.5 inchs so that at 100 the line of sight on bullet path would cross.. so this would be making a triangle with the shortest side beind 1.5 inchs(distance between sight and bore) the other side of the triangle 100m and the last side slowly slopping into the point. then at 200 the impact point would be 1.5 right and x low. at 300 would be 3 right and x low, at 400 would be 4.5 right and x low at 500 would be 6 right and x low and so on. this is because the bullet was sent on its way crossing the sight path on a left to right course. sighting in at 300 for example would greatly reduce the effect.
i think that is the confusing part explained. well unless i confused myself while writing this.
so the key is to just have the scope level and the bore axis verticly under the x hairs and ensure that the scope is level when shooting. cheers