In two earlier papers [1, 2] the effect of interstitial gas on milling was reported. It showed that these effects were well correlated with changes in powder porosity and consequently with the changes in powder mobility due to variation of gas viscosity and pressure. In this paper the cause of this correlation is further investigated. Starting from a close visual inspection of the processes in a ball mill, a model is developed in which the milling characteristics are correlated to the ball flow and the powder behavior during ball collisions. From experiments, it follows that both are influenced by powder mobility.