I asked this recently regarding fitting an 11mm dovetail riser to my 13mm BSA scope rail. So the scope will actually be 1mm off to the side with respect to the dovetails ( and presumably the bore ).
It's all to do with verticals.
If the scope vertical ret is set up vertical ( plumb line ) and it remains vertical ( so put your spirit level on the scope and set the vertical of the scope with your plumb line ... and then set the spirit level, ON THE SCOPE, to level [ bubble in middle ]), then the ret will alway be vertical and just the offset away from the bore.
So in this case we are now talking crossover and not cant. So if you have a 2mm horizontal offset from crosshair to bore and you zero at 30 yards, then at 60 yards you miss 2mm one side ... and at 0 yards you miss 2mm the other side. So at FT ranges you don't need to worry about it.
The problem occurs re cant when the cross hairs drift off vertical ( they are basically rotating around the point of the centre of the bore ... or actually the bore is rotating around the point of the cross hairs because your eye is the point of reference and your eye is lined up with the cross hair centre ). So now the 'cant' trajectory is no longer true vertical, and in the same vertical line with the cross hairs ... it's rotated around the axis of the centre cross hair re bore. So the further the target is now away from the rifle, the greater the horizontal error of poi. The higher the cross hair from the bore the greater the problem.
You can try and understand it more by thinking triangles ...
At the rifle you have a triangle ... the cross hair centre is say 50mm above the pellet when it leaves the muzzle. It's also 2mm to the side ( if your offset is 2mm ... it would be 1mm on my BSA ). So we have a triangle that is 50mm vertical and 2mm horizontal. As long as the cross hair stays true vertical the vertical trajectory of the pellet ( providing there's no wind ) will remain in that same vertical plane. So that horizontal part of the triangle stays at 2mm.
If you cant the rifle ( and hence the vertical cross hair ) then the pellet trajectory now isn't in the same vertical plane. It has rotated around the axis of the cross hair/bore. So the further you get away, the wider becomes that horiontal part of the triangle and your poi's will get increasing errors horizontally ( and less so, vertically ).
Just how I understood it when thinking about the BSA problem.
Basically ... stick the scope on ... get the scope vertical ret true vertical and use a bubble on the scope if you want something to check. If you are getting more than a few mm's sideways error at various FT distances then you are either canting the set up, you don't have a consistent eye position ... or ... it's windy.
Last edited by skires; 3rd November 2015 at 08:42 AM.