Very few rifles are square and true. It's common for rails or mounting holes to be slightly off center. Barrels aren't always straight and concentric to the receiver and even stocks can be crooked. To compound matters, a lot of scopes (even relatively expensive ones) have crosshairs misaligned with the turrets. You can drive yourself crazy trying to square things that aren't plumb!

One of my pet peeves is crooked scopes. I see a lot of them that are way off, because the previous owner naturally cants the gun and just mounts the scope so the crosshairs look level to him. It doesn't really matter if you always shoot at the same distance. The misalignment is easy to see if you look into the scope from about a foot back. You'll only get a partial field of view through the scope, but you will see the crosshairs and can compare them to the rifle. Just mount the scope so it looks good and call it close enough. This method is fine, unless you do a lot of precision shooting at various distances.

Long range varmint/target shooters need the scope directly over the bore and square. The best way to verify this is at the range. Use a grid type target and make sure it's mounted square (take a level). Shoot off a sold rest and zero the rifle. Then make about a 10" elevation adjustment (either with the top turret or with the mil-dot/graduation marks) and shoot a group. If the group is directly over the center of the bullseye, you're good to go. If it's off to one side, slightly rotate the scope the direction of the error and start over.

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