I've had the same problem with a couple of 700's. It appears that the receiver is not level on the top from the front to the rear of the top of the receiver. It was clearly evident by putting a straight edge on the mounted bases from one to the other. Making sure the straight edge (steel rule) is flat across the higher of the bases and measure the gap with feeler guages. That's the thickness of shim that you need under your base.
I would be hesitant to use credit card shims as plastic expands about 10 times more than steel due to temperature changes. I've had hunters miss elk due to this type of "shim" but that story goes under the "Sad stories of hunting big game" forum.
