This ^^^. Its free and effective. Just take the upper off and set it on a rest. Aim it at something across the street, yard, or whatever. Look down the bore then through the scope. Adjust the scope accordingly.Call me old fashioned, but I just look down the bore and compare it to the scope.
Best method I found. Works great on bolt guns, or uppers, and it's free and fast. You just need a solid, secure rest.This ^^^. Its free and effective. Just take the upper off and set it on a rest. Aim it at something across the street, yard, or whatever. Look down the bore then through the scope. Adjust the scope accordingly.