The macroscopically steady combustion behavior of AP (ammonium perchlorate)-HTPB (hydroxyl-terminated polybutadiene) propellants containing boron and magnesium was examined using thermocouples and fiber optics and compared with previous results for similar aluminized propellants. Addition of magnesium at constant AP-HTPB ratio was found to increase propellant burning rate moderately while boron increased burning rate substantially. These increases were greater than those previously observed for similar aluminized propellants, A sensitivity analysis was performed to determine the relative importance of various mechanisms such as conductive heat feedback, radiative feedback, and condensed phase heat release. For boron and magnesium the propellant burning rate was found to be most sensitive to changes in condensed phase heat release while the aluminized propellant was found to be most sensitive to radiative feedback. These observations are attributed to the properties of the various metals and their respective oxides. Mg and B have a relatively high propensity to react while on the surface and in the condensed phase of the propellant, thus producing a significant increase in burning rate through condensed phase heat release. Aluminum, being less prone to react in the condensed phase and on the surface, imparts its (relatively weak) influence on the burning rate primarily through enhanced radiative heat feedback.