Quoting shooting illustrated:
"In almost every riflescope manufactured, the reticle is not perfectly plumb with the adjustment turrets. Most manufactures hold that tolerance to within 5 degrees. Out to around 300 yards—the distance within which most people shoot—that error will be hardly noticeable. Leupold has an off-plumb reticle tolerance of 3 degrees for all of its scopes except the high-end Mark 6 and Mark 8s, which are allowed a maximum of 1 degree. Nightforce has a reticle-cant tolerance of .5 degree for every scope it builds; that’s right, the company mandates all its optics to be at most .5 degree off."
By leveling on your turrets you are introducing 3-5 degrees of cant. A bubble level is 1 degree of cant in accuracy.
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