In this report results from numerical simulations of a steel projectile penetrating a concrete target are discussed. The simulations are performed with the Autodyn software in 2D and 3D, and the mechanical properties of the concrete are described with the RHT model. This advanced material model incorporates elastic limit, failure and also residual strength of the crushed concrete under pressure. As a consequence, the RHT model contains a large number of constants. In this report, a study of the relative importance in 2D- and 3D simulations of some of the parameters on the penetration is performed. An advantage with 3D simulations is that more realistic penetration cases with oblique impact can be done. However, 3D simulations lead to a large number of elements and thus also to long calculation times. Therefore a parallel processor system has been tested. The numerical results are, when possible, compared to experimental ballistic tests done in 1999. The study shows that the RHT constants describing damage evolution and residual strength of the concrete are very important for penetration calculations. Another important finding is that the impact geometry must be well defined, since even a very small deviation from normal impact results in a large effect on the calculated penetration path.