Gun barrel damage is attributed to factors such as erosion and wear growth, primarily resulting from the substantial mechanical stresses and fatigue that occur during firing. To ensure the safe and continuous use of the barrel, it is crucial to accurately assess the rate of barrel degradation and predict its lifespan. The authors, through this work, succeeded in detecting and localizing damage in the gun barrel. They explored the numerical simulation method using the Lamb wave, an ultrasonic-guided wave, to detect damage in the barrel, utilizing time-of-flight (ToF) information to gather data on the pristine and defective state of the barrel. The damage is then localized using the highest intensity location of the residual signal as per the calculated ToF. The proposed damage imaging algorithm accurately localizes the damage location, depicting its efficacy in the field of structural health monitoring (SHM).