A simple one-dimensional model is used to examine geometrical and scale effects in the penetration of thick metal targets by projectiles. The model is used in two forms, one assuming the penetrator deforms by mushrooming and the other assuming it is eroded. The mushrooming version gives good predictions of depth of penetration at low velocities where the erosion version overestimates depth of penetration, but at typical ballistic impact velocities the models bound the experimental data from below (mushrooming) and above (erosion). Both versions of the model give good predictions of depth of penetration for low length to diameter (L/D) ratio penetrators at all velocities. The model solutions match experiment in simulating the effect of penetrator L/D ratio and scale and this is attributed to the inclusion of radial inertia and shear effects which are not considered in modified hydrodynamic models of penetration. Differences between penetrator materials based on penetrator strength are also evident in the model computations.