PURPOSE: To prevent the burning characteristics of propellant grains from deviating from the reference even when off-loading is carried out on the propellant grains by forming an inner cavity after weight reduction into an expanded inner cavity which is gradually receded by an almost equal distance from each point of the inner surface in the normal direction, in relation to a reference inner cavity of the propellant grains under a full-load condition.

CONSTITUTION: The shape of an inner cavity 10 after weight reduction is formed into an expanded inner cavity which is gradually receded by almost equal distance from each point of the inner surface in the normal direction, in relation to the shape of a reference inner cavity 6 of propellant grains 5 under a full-load condition. When the inner surface of this new inner cavity 10 is to be ignited serving as an initial burning surface, the formation change thereafter progresses in the same manner as that in the reference inner cavity 6, and the relationship between them can thus be reproduced.