The paper attempts to obtain an approximate distribution of oxidizer/fuel (O/F) flamelets on the burning surface of a composite solid propellant, mainly of the ammonium perchlorate (AP)-hydrocarbon (HC) binder variety commonly used in rocket propulsion applications today. A computer model is developed to simulate the random packing of oxidizer particles of different size distributions in the propellant. The focus of the present work is on the distinction between pockets of fine AP particles amidst the binder that may burn in a premixed O/F flame and the sufficiently large AP particles that burn with the surrounding vapor flow in an attached diffusion flame. A transition occurs between premixed burning and attached diffusion flamelet-burning over each oxidizer particle as the ambient pressure is varied. A simple criterion evolved earlier to delineate the two regimes of burning in the particle size-pressure domain has been employed to determine the flamelet type that prevails over each exposed particle of the model propellant at a given pressure. Delaunay triangulation is adopted to deal with regions of fine AP-binder corresponding to premixed burning. The results are compared with previously reported experimental observations of burning rate for specific propellant formulations that exhibited distinct jumps in their burning rate with variation in pressure. It was argued that the burning rate jumps corresponded to a concerted shift in the burning regime of a majority of fine particles. The present simulation supports this argument.