The mechanisms of formation of gunshot fractures in flat bones inflicted by a semispherical bullet were investigated using expert and experimental materials. The process of crack formation was considered in terms of the Hertzian contact problem and the Hill-Johnson model. It was shown that the fracture develops as a result of combination of stresses and strains in the bone tissue leading to the formation of a hydrostatic nucleus prior to tissue fragmentation. Dynamic fluctuations (waves) generated in the zone of hydrostatic compression resulting from the gunshot injury propagate with the velocity of sound from the nucleus in the direction of the bullet movement. According to the Hill-Johnson model, the waves propagate in the direction of the impact within a parabolically expanding space; this accounts for the mechanism of formation of parabolic cracks and the specific shape of the defect that the bullet produces in the flat bones. The dynamic load applied by means of an indentor forms at a higher rate than the velocity of sound in the affected material. It gives reason to consider the effect of a bullet moving with the speed of 250 m/s as quasi-static loading. The results obtained in this study make a contribution to the theory of impact effect of a bullet and provide a deeper insight into the physical nature of the direct and sideway action of a gunshot projectile. Moreover, they explain the cause behind the widening of the outlet part of the perforating fracture in the flat bones.