In vulnerability simulations, when simple penetrations models are used, it is essential to describe the retardation force on the projectile as accurately as possible. For that reason, a number of AUTODYN simulations have been made, and the retardation force for a "rigid" projectile has been calculated. In these simulations have primarily the retardation force depending on the inertia forces been studied. Accordingly, the part which depend on the penetration velocity and the target density. Another objective with the simulations was to compare the solutions when Lagrange- respectively Euler- formulation of the target was used. The simulations were made with the impact velocities 845 m/s respectively 2000 m/s. The comparisons show that the Euler formulation underestimates the influence from the inertia forces. The Lagrange formulation has on the other hand given a more reliably result. This results have also been reported to the developer of AUTODYN (Century Dynamics), because it is obvious that a programming error is causing the difference. For high velocities (2000 m/s) can the difference be quit large. Additionally, has it been established that it exist a critical velocity when the diameter of the penetration channel becomes larger than the projectile diameter.