Been pondering the same thing for the last little while. I finally have a rifle were the scope "must" be exactly square w/the bore.
Here's how I solved it after a few iterations.
1) Made a plumb bob (cartridge w/a knotted string through the primer hole and a bullet inserted into the neck.
2) Destroyed a cheap spirit level (bubble) to get the bubble device out of it. Layed the bubble thinger on the scope mount.
3) hung the plumb bob w/a thumb tack in the center of a door way.
4)Set the rifle as far away from the string as the house would allow (it was night time plus the wind messes things up outdoors).
5) Leveled the rifle then set the scope parallel to the string.
I suppose that I will find the weakness of this process as I extent my shooting out towards 1K but it sure seems to work pretty well.
The next thing I did was drill a hole in the bottom of a scope ring the proper size to glue the bubble thinger into and mount that on the scope. I should be able to look at the level and get any cant out of the rifle as I make my shot.
Twas kind of fun to do and is much better than lining up on the utility pole behind the house.
Hope this gives you some ideas.
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!
Having the same dilemma for a while (I could play with the scope and get it close, but never got perfect results I was confident of), I picked up a Wheeler Engineering Level-Level-Level gadget, and have been fond of it since it arrived. If you mount scopes often, it quickly pays for itself in time saved. A comparable device could be jury rigged with two bubble levels to work off of a scope base (this product levels the gun off of the bolt rails) but it isn't terribly overpriced to begin with.
I've used several scope leveling devices and never found them to be much good. Here is what I do:
Mount the rifle on a solid bench at the range. Level the rifle, removing any cant, by whatever means is available on your rifle. Leveling the bottom of the forearm on a varmint/target rig is easy but you may have to use the centerline of the buttstock or place a level on a makeshift bridge which spans over the chamber and rests on the stock.
Once the rifle is cant-free, I take a 3' level and place a solid black vertical (or horizontal) line on a rigidly mounted target backer @ 200yds. Rotate the scope until the crosshairs match your black line exactly. You will have to recheck to assure that the gun is still cant-free before you completely secure the scope.
This procedure is faster than it sounds and has worked well for me.
I use the same process that Varmint Hunter uses except at 100 yds. however, I do one more test, I shoot at the verticle line I've drawn and then begin to click up the elevation to see if the bullets will walk the line. If they do, I'm inline with the bore, if not, more adjustment is needed.
Hmmm interesting. I use that wheeler engineering kit and I dont think it works all that great. The bubble level that goes into the action or bolt doesn't usually stay very good for some reason. I usually just try to sit my rifle in a bench, then get the scope close, then put the other bubble leveler from wheeler on top of the scope and turn untill its suppose to be level. I like the way Varminthunter does it. I think I will try it that way the next couple times and see how it works. What is the disadvantage of a scope not being properly centered over a bore at long range??? What exactly does it do to the bullet??
I don`t think it matters that the scope is not alligned to the bore. A barrel will vibrate in all directions anyway, plus which the chances of a mount being exactly in line with the bore is remote.
I think the idea is to align the scope to the bullet path. Adjust the scope to the bullet path with the windage adjustment - horizontal plain.
Leveling a scope allows you to adjust the vertical component (elevation) without affecting the horizontal component. In other words its plumb. I sort of do it with three levels - one on the rifle, one on the mount and one on the elevation turret with the cap removed when setting a scope up and test on a target as others have said.
Well thats my understanding of it, most willing to learn where I am going wrong.