How about a bit more info.. We know that heating of the barrel is causing the horizontal spread... First things first though.. Is this a new rifle. If it is perhaps it is best to let Winchester have a shot at fixing their problem.. Because some of the things we may do may void the rifles warranty. We will assume that the bases/rings are tightened down securely and the scope is a known quantity. It sounds as if the barrel is being pushed to the side as it grows with heating. The taper of the barrel is forcing the tube as it elongates to move away from the contact point. (The barrels slight taper acts as a wedge) Be certain the barrel is free floated. You will need clearance for a dollar bill (2 at most) to be looped around the barrel and be moved to the rear to withing about 2 inches of the receiver. This will allow the barrel to grow as it heats without interference. I would bet there is contact along the left side of the barrel channel and high on the side of the barrel....
