The scope mounting base should have a little pin or some type of protrusion thingy (for lack of a better term) and a indent in the top of the receiver that keeps the scope assembly from moving back. Sometimes it is not "locked" in or has worn. It may be a screw under the scope base. Hard to say without seeing it. Take the scope off and base also and check.. It also could just be the scope itself, as mentioned there are specific "springer" scopes.

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Still Running Against the Wind