Conventional gun propellants exhibit the phenomenon of nitrocellulose (NC) decomposition. Besides an effect on thermal stability of propellants, decomposition of NC has an effect on the mechanical integrity of the propellant grains. Enhanced grain fracture may lead to unacceptable changes of the pressure build-up. The effect of ageing on NC molecular weight, mechanical properties and performance has been investigated for the main propellants of 155-mm cartridge-bags. Grain fracture appears to affect the peak pressure much more than the muzzle velocity. This effect is strongly related to the loading density and can for a great part be explained by internal ballistic simulations.