Abstract: Recent experimental studies on muzzle blast overpressure histories of an 81mm mortar system have revealed a number of reproducible features following the main blast front. Some of these features are shock fronted, with pressure rises of similar magnitude to that in the main blast front. The time lapse indicates that these may be associated with the precise geometry of the projectile. A study was therefore undertaken, using our previously validated computational fluid dynamics (CFD) code, to examine the evolution of the flowfield behind the blast front. The primary concern is to achieve an improved understanding of the changing flowfield, and how this determines the features of the experimentally measured pressure history in an 81 mm mortar.