The revolving barrel gun is the principal component of the close-in weapons system (CIWS) that provides important terminal defense against anti-ship cruise missiles that have penetrated fleet defenses. The muzzle flow field of the revolving barrel firing is extraordinarily complex. The 3D computational model was formulated to illustrate the details of the flow field produced by the revolving barrel gun firing. The algorithm of a second order monotone upstream-centered schemes (MUSCL) approach with the advection upstream splitting method (AUSM) solver was used to simulate the high pressure muzzle flow field. The interior ballistic process was coupled with the simulation. The predicted muzzle velocity and maximum bore pressure were in good agreement with those measured in gun firing. Moreover, the muzzle flow field was obtained during the revolving barrel firing and was subsequently analyzed. The maximum lateral velocity of the first and second projectile fired was about 1.6 and 3.8 m/s.