We use 1-D shock-tube experiments to investigate the dynamics of rapidly-decompressed gas-particle mixtures and associated shock waves, with application to the initial stages of Vulcanian and Plinian eruptions. For particle sizes 45--150 ¿m and pressure ratios 1--70, experimental particle Reynolds numbers reach 104 and impulsive accelerations reach 150 g. The experiments suggest that particles hinder gas motion via an interphase drag force, reducing shock strength and velocity. Gas-particle mixture velocities decrease with increasing particle diameter for a given initial pressure ratio and are less than those predicted by pseudogas approximations and existing interphase drag relationships due to imperfect phase coupling and unsteady flow during high-acceleration stages. We present a new analysis for predicting shock strength and velocity for gas-particle mixtures, and apply our improved interphase drag terms to the high-acceleration, eruption initiation stage of Vulcanian eruptions.