Abstract: In motion process of projectile in bore, its bearing band is subjected to the high strength impact loads and high-speed friction, and the microscopic mechanism of transient plastic deformation under high temperature is very complicated. According to the load characteristics in bore, the microscopic mechanism of plastic deformation was studied on the basis of metal material theory; the regulation of hardness distribution and microstructure evolution was analyzed; then, the microscopic nature of macroscopic deformation was explored. The research results show that the bearing band has obvious layered characteristics after extrusion; fibrous tissue on its surface is heated up quickly and recrystallized partly because of severe friction; the large scale deformation does not occur inside the bearing band; the melted metal membrane on the surface is an important factor to restrain the plastic deformation extend to the inner region.