Abstract: In order to incorporate minor perturbations in the simulated operation of a weapon (to simulate dispersion between batches of ammunition or individual rounds, and/or the effects of changing environmental phenomena), a variable is generated at a rate related to the type of ammunition to be simulated. For ammunition exhibiting a high degree of dispersion between rounds, the variable changes rapidly and an accurately-aimed shot is deemed a hit only if the value of the variable at that instant is less than the predetermined probability of a hit with that ammunition. In the case of ammunition exhibiting some consistency between rounds but not between batches, the variable changes slowly, and its value controls the extent of a displacement applied to a laser projector before the projector is energised and scanned for a hit/miss determination.