Several metallized solid rocket propellants, of the broad family AP/Metal/HTPB in the ratio 68/18/14, were experimentally analyzed at the Space Propulsion Laboratory of Politecnico di Milano. In general, they feature the same nominal composition, but different metals (micrometric and nanometric Al, B, Mg, and a variety of dual metals) are implemented as high-energy fuel powders and contrasted to a conventional micrometric aluminum (30 μm average grain size) taken as reference. The fundamental ballistic properties of the formulations under study were compared to that of a conventional aluminized propellant used in flight since longtime. It is shown that the propellant microstructure plays a fundamental role in controlling the critical aggregation/agglomeration phenomena occurring below and near the burning surface. Two specific effects of microstructure in terms of steady burning rate and average agglomerate size are illustrated. Understanding of these effects opens the path to improved ballistic performance in terms of delivered specific impulse for solid propulsion. Likewise, metallized solid fuels can assist in improving performance of hybrid propulsion in terms of higher regression rates, larger specific impulse, bigger density, and reduced throat erosion.