The channel is filled with oil from storage tank. The rotation of the bar creates turbulence which keeps it the centre of the channel. The higher the rotation speed the better centralisation effect therefore the magazine will help the lathe to achieve optimum cutting conditions.
If the diameter of the bar material is close to that of the channel little turbulence can be created by rotation but then the hydrodynamic bearing effect supports the centre of the channel.
FMB bar feed channel