Abstract: The basic relationship between fractal dimension D and the amount of shot bullets was found by collecting the data from actual measurements and analyzing the topography of bullet surfaces worn out by rifle of a gun. A dynamic equation of fractal dimension D was therefore proposed and, based on it, a fractal model for dynamic prediction of surface wear-out condition was given to establish the direct relationship between wear rate and amount of bullets shot. The fractal dimension or wear rate may be used as a characteristic parameter to examine the topography of worn-out by rifle. The dynamic prediction shows its conformity to experiment data and its available to judge the extent of wear-out of a gun through an analysis of the properties of its rifle worn out in turn by the surface of bullet. Therefore, the model is helpful to provide theoretically not only a basis to judge gun failures but a reference to infer the maintenance and applications of a gun for investigating the cases involving guns.