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Originally Posted by fps plus
Wow there is definitely some confusion other this and it’s not a difficult comcept
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It is not as simple as many make it out to be. If you mount a round scope parallel to and in a dead line above a round bore, then the center of the scope and the center of the bore must by definition be in direct alignment. The issue comes in when you introduce a crosshair and an erector system for adjusting the vertical and horizontal impact of the bullet. At this point, the center of the cross hair may or many not be in the dead middle of the center of the scope. The erector system is designed to move the horizontal crosshair up and down and the vertical crosshair left and right. Any angle introduced into the alignment of the cross hair to perfectly vertical and horizontal to the world means a vertical adjustment will also create left or right adjustment to the POI and a windage adjustment will introduce vertical movement. The center of the scope and the bore has not moved, just the crosshair is moving up down or left and right.
With respect to cant affecting POI in the absence of any adjustment, this occurs due to the line of sight rotating away from direct alignment of the bore at the alignment that the rifle was sighted in at. If the crosshair was dead center in the middle of the scope, and no adjustments were being made, the cant in the crosshair would not affect the POI. Take a scope, mount it in a v-groove and center the crosshair on a dot or +. Now slowly rotate it 180 degrees, you will easily see that the crosshair moves left, right and/or up and down. This is caused by the reticle not being dead center in the middle of the scope. The further out towards the extremes of vertical or windage adjustment the more pronounced this effect is and is why scope makers recommend you use the windage screws on the mount to try and keep the reticle as close to dead centre of adjustment as possible.