A counting mechanism for detecting and registering the number of rounds fired from a recoil gun includes a housing that can be mounted to a component of the gun that exhibits recoil motion upon firing of a round. A counting device is contained within the housing and includes an actuator that is activated by the recoil motion of the gun component upon a round being discharged. A weighted mass is movable within the housing with the recoil motion of the gun component, and the actuator is operably configured with the mass such that movement of the mass resulting from the recoil motion actuates the actuator and increments the counting device.