The invention provides a test rig for detecting spring life cycle of a firearm comprising a plunger having an elongated shaft body extended from a reciprocating engine, passing through a channel guide of a mount and to be in-contact with a spring specimen, characterized in that the reciprocating engine includes a motor for providing rotational motion, a cam mechanism linked to the motor, having one cam surface in-contact with the plunger for converting the rotational motion of the motor into a reciprocating motion to drive the plunger to impact on the spring specimen repeatedly, and a pulley system disposed between the motor and the cam mechanism to adjust the speed of the rotational motion so as to stimulate an impact force that are similar to firing of ammunition.