Abstract: In order to study how the damaged bore influences bullet's aerodynamic characteristics and the exterior ballistics process, finite element models of damaged barrels at 4 life were established based on systematic life tests of 12.7 mm machine gun barrels to obtain bullets' surface morphologies at the muzzle. Simulations of the aerodynamic parameters for the high-speed spinning bullets with different surface morphologies were carried out adopting the computational fluid dynamics (CFD) method with the shear stress transport (SST) k-ω turbulent model. Uniform design method was used to arrange the interior ballistics computing process influenced by random factors, and the random response states of bullets' disturbances at the muzzle were acquired. A model of 6-D exterior ballistics of rigid bullets was set up, and the numerical calculation software was programmed combined with bullets' aerodynamic parameters and muzzle disturbances. The simulated results of the dispersion circle radius and the ratios of elliptical bullet holes of bullets shot from the barrels at 4 life fit well with the experimental data. The computed results show that the increase of bullets disturbances at muzzle, and the antedisplacement of pressure center of aerodynamic forces as well as the increase of Magnus moment coefficient are the major reasons for the decrease of bullet's flight stability and the terminal of a barrel's later life.