© 2020 Acta Materialia Inc. This paper concerns experimental study and modeling of the ballistic impact (indentation) of spherical projectiles into thick metallic (Mg) samples, and resultant crack propagation characteristics. A previously-described procedure is first used to evaluate the Johnson–Cook parameter for the strain rate sensitivity of plastic deformation for this material (C~0.026). The main emphasis, however, is on study of its fracture characteristics (under high imposed strain rates), with tomographic imaging being used to obtain crack patterns for different projectile velocities. It is recognized that there are many situations, including the one being studied here, for which use of a critical plastic strain criterion for prediction of fracture is inappropriate. An approach based on fracture mechanics, and on use of FEM modeling to estimate the strain energy release rate required for crack propagation (i.e. the fracture energy of the material) is proposed and applied to these experimental results, leading to a value of the order of 2 kJ m−2. While such a procedure is unlikely to produce accurate values, partly because the crack propagation takes place under local conditions that change rapidly and are not well-defined, this figure is plausible for the case concerned. While there are several sources of complexity, it may be possible to develop this methodology, both as a technique for fracture toughness measurement (requiring only small samples of simple shape) and as a novel approach to prediction of ballistic impact outcomes.