In this study, the effect of the initial temperature of a 120-mm mortar system on its interior ballistics was investigated using four different experiments: temperature-conditioned closed bomb firings for determining the temperature sensitivity of the ignition cartridge's M48 double-base propellant and instrumented firings of temperature-conditioned flash tubes, ignition cartridges, and an instrumented mortar simulator (IMS). The results of these experiments reveal that, for initial temperatures of –12 °C and greater, the mortar system and its subcomponents exhibited regular initial-temperature-dependent behavior, with increasing initial temperature causing monotonically increasing propellant burning rates and monotonically decreasing ignition delays, which produce monotonically increasing system pressures and pressure differentials in the flashtube, ignition cartridge, and IMS. However, some anomalous behavior was discovered for temperatures around –46 °C. At this initial temperature, the closed bomb firings indicated that brittle fracture of the M48 propellant granules used in the ignition cartridge occurs. This phenomenon explains the occurrence in the IMS firings of dramatically increased variation in pressure-time behavior and projectile muzzle velocity for charge 4 firings as compared to higher temperatures, as well as the occurrence of maximum tube pressures for –46 °C firings, being greater than those for 21 °C firings for charge 0. However, one of the –47 °C closed bomb firings does not exhibit evidence of grain fracture and yet produces a higher propellant burning rate than the –12 °C firings, suggesting that a fundamental change in reaction kinetics or flame structure is occurring at extremely low temperatures. This supposition is bolstered by evidence of a liquid layer existing on the surface of M48 propellant granules ejected from the ignition cartridge during the –46 °C firings—a phenomenon that does not occur at the higher initial temperatures and is not theorized to occur in double-base solid propellant combustion. Based on the flash tube experiments alone, the flash tube was determined to have a weak effect on the initial-temperature-dependent behavior of the mortar system; however, IMS testing with two different flash tube configurations revealed significant differences in longitudinal pressure wave amplitude and projectile muzzle velocity in charge 4 firings between the two configurations at –46 °C, suggesting that the uniformity of combustion product discharge from the flash tube could significantly affect the performance of the mortar at low temperatures.