Recently I picked up a very cool little tool that has helped get my scopes aligned much easier and more accurately then the methods previously mentioned.
It’s called EXD engineering vertical reticle instrument. You can use it to precisely align the rifle barrel and the scope objective. Once this is done you can either along the crosshairs to a plumb line or in a dark room you can shine a light into the objective of the scope and the reticle will cast a shadow against a wall and be leveled to a hanging string.
I just figured some of you might be interested in it also.
I use a good padded vise, 12" torpedo level and a 4' level.
Level the action with 4' level and lock action in the padded vise use 12" torpedo level to level the scope in the rings then once scope is secured in the rings check it with the 4' level and it should be near perfect. Works for me.
I shoot a tall target for most of my scopes . my regular point and shoot hunting rifles I mostly use the leupold Boone and Crockett reticle . if the scope is not on properly you will get windage when using the longer distance hash marks . on my long range rifles I dial my elevation . again if you don't have the scope aligned properly you will add windage when you dial elevation . I can remember sighting in my first scope , it was mounted by the local gun shop . as I changed my elevation I would also have to change my windage to get back to the center of the target . I didn't understand what was going on at the time . here is a link that explains the tall target .
Some of the scopes have a flat underneath that is level with the reticle ... I've used shims & more recently pin gauges to make sure that the base of the scope is dead parallel to the action.
I put my gun in a gun vise level it and across the room I use a string with a weight on it.
It is not fast method as It usually takes me 4 plus hours to mount a scope. Yes I want it perfect.