A bump stock and interface system for a semi-automatic firearm. In use, the user grasps a handle with one hand and places an index finger of the same hand firmly on a finger rest. When intending to fire a round of ammunition, the user pushes the firearm receiver forward with another hand so that the trigger collides with the immobilized finger on the finger rest. Recoil force separates the trigger from the user's finger on the finger rest. The user again pushes forward the receiver to repeat the firing cycle. A lock is selectively activated to arrest longitudinal movement of the firearm relative to the handle, thereby returning the firearm to normal semi-automatic functionality. The lock includes a retractable pin having a tip that rides in a track having a rear stop to limit over-travel of the handle.