In order use material models in (numerical) calculations, the mechanical properties of all materials involved should be known. At TNO an indirect method to determine the dynamic flow stress of materials has been generated by a combination of ballistic penetration tests with an energy-based engineering model. For visco-elasto-plastic materials, like clay, not only the dynamic flow stress is strain rate dependent, but also its Young’s modulus. Experiments using 7.62x39 MSC bullets have been performed on Roma Plastilina nr. 1 at several target temperatures. The dynamic flow stress and young’s moduli have been determined using an adjusted ductile hole growth engineering model for weak materials.