The validity of a deformation model proposed by M. L. Wilkins and M. W. Guinan is tested at a microscopic scale for copper projectiles. In particular metallographic investigations confirm the curved plastic front built up during the first microseconds of impact. At higher velocities the material present in the peripheral region of the impact end is recrystallized by thermal effects caused by strong plastic deformation work. The influence of thermal softening on the position of the plastic front is discussed.