Well, think there has been one thing that you said was found defective. I would try a friends scope that is tried and trusted. Also, maybe the way the scope is mounted. If your using dovetails for the front on a two peice base, and adjusted the windage screws in the back alot, this puts alot of stress on the scope while it is tightened down in the front.

I really don' t see zero being off from the gun or barrel stress' s unless your taking the action out of the stock every time. That might do it.