Try this: Get a rifle vise and lock your gun down. Mount the scope but do not tighten the ring screws down all the way. Mount a sight in target on some cardboard.( use a t-square to make sure it is squarely mounted.) Put the target across the room from the vise mounted rifle. Use the small grid lines on the target to square your crosshairs. Then tighten the screws down. This usually works for me.

GATA