Previous papers by the author [1,?2] pointed to a discovery by Fisher [3] that an equation by Fano [4], when used to predict blast impulse from cased munitions, did not fit the available data. These previous papers showed that an alternative equation for casing-modified blast impulse could be derived directly from an equation by Gurney [5] for the kinetic energy balance between the mass of casing metal and the mass of explosive gases. However, this equation was derived for very ductile casings that are accelerated to their ideal Gurney velocity before they fracture. Many real casings, even under high dynamic strain rates, fracture before they can receive the full drive available from the explosive gases. This paper shows how the equation in reference [2] can be modified to allow for casing fracture at finite dynamic strain and provides validation for this modified equation from previously unpublished AWE archive experimental data.