jwp may have a point. I use a bubble level on my ring base so that the rifle is "cant free" when firing and therefor my verticle crosshair is plumb because thats the way I set it up. The problem with setting a plumbed crosshair onto a canted rifle is repeatability. Without a mechanical reference of some sort, I would image that you would be holding the rifle slightly different each time. Over loongrange this would induce both verticle and horizontal error in shot placement.