Impact fracture properties of three nitrocellulose—nitroglycerine gun propellants have been measured in a three-point bend mode at moderate impact rates with an instrumented drop-weight impact tester. Dynamic moduli and loss tangents were measured over the temperature range −100 to +120° C, and three transitions were identified. A transition at about −30° C was found to increase the low-temperature fracture toughness of the higher nitroglycerine content propellants. The fracture data were analysed in terms of plane stress and plane strain fracture modes using a simplified model. It was found that the fracture toughness in zones undergoing plastic deformation under plane stress conditions was approximately twice that in zones under plane strain conditions. The plastic zone radii were greater than 0.3 mm at 20° C, falling to about 0.1 mm at −45° C. Strain energy release rates were calculated from fracture load and modulus, and from fracture energy. Good agreement was obtained between the two methods.