In the case of the 76mm naval gun on PKG(Patrol Killer Guided-missile) and PKMR(Patrol Killer Medium Rocket), the spent case ejection hatch is always open from the preparation stage of firing to the completion therefore, there is a possibility of failure during the operation of the naval gun due to corrosion and fixation of internal components by seawater penetration into the gun. In this study, the actuating mechanism is improved to open the spent case ejection hatch only during firing, thereby improving the durability and reliability of the naval gun. It aims to improve the operational capabilities of new vessels.