In this paper the dynamic equations of the gun-projectile system and the corresponding two kinds of solving methods which are perturbation method and numerical method are established, respectively. After that the effects of the ultimate velocity of the projectile, the elevation of the gun, the acceleration of the barbette and the rotation velocity of the gun to the deflection at the end of the gun are studied. The numerical results of the two methods tally extremely well with each other ,which indicate that the theory established in this paper is reliable. Further research indicates that the main factors which influence the deflection at the end of the gun are the ultimate velocity of the projectile, the elevation of the gun, the transverse acceleration of the barbette and the rotation velocity of the gun and then a formulation of the deflection at the end of the gun about the forenamed factors under the barbette motion is put forward.