The flow at the muzzle of a gun during launch of fin-stabilize d projectiles is investigated both experimentally and analytically. Optical and flash radio graph surveys are used to define the muzzle blast growth and projectile r dynamics. Based upon the experimental results, an analysis is developed of the loadings exerted by propellant gases upon the projectile. It is shown that both in-bore and muzzle blast flowfields must be considered. The inbore flow is modeled as a one-dimensional, unsteady expansion, whereas the muzzle blast flow is approximated as a two-dimensional, quasisteady, underexpanded jet. The effect of launch gasdynamic loadings upon projectile motion is calculated and is shown to compare favorably with experiment. Nomenclature = area of a single fin = lift and moment coefficients = speed of sound = gun tube diameter = distance between obturator and fins = projectile transverse moment of inertia