Interesting, I need help with a gun....no make or model, might as well be, I need help with the air. Obviously the front and rear action rings are not level under that streight edge, so they are not the same height, if square. A scope base or bases should be level and centered to the axis of the bore as mounted.
First thing you need are bases or a base for that make and model action. Forget shims or Fouxinh up a scope mount job wih epoxy.

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