The basic problem of an intense light pulse pistoning into a plasma ('hole-boring' (HB) radiation pressure acceleration) when there are multiple ion species is re-examined. An alternative analytical treatment of the problem is presented, and it is argued that the HB dynamics are determined by the light intensity and composite (i.e. total) mass density of the target alone. One-dimensional numerical simulations are used to validate this theory, and it is shown that it correctly predicts the dependence on composite mass density, charge state and numerical proportion of the species. Only in certain extreme cases is the analytical model found to be inadequate. These results challenge previous findings.