FS and RS four-velocities in the ideal MHD limit α_m → ∞, for unshocked ejecta with γ_ej = 500, h_ej = 1.001, . Higher ejecta magnetizations lead to greater four-velocities for the plasma inside the blastwave, as well as for the forward shock. For the shocked ejecta total pressure is stronger than the thermal pressure generated by the FS for all possible FS propagation velocities. The dashed lines denote RS quantities.