Under the best conditions, a shot size should represent 50% of the capacity of the injection cylinder (barrel). This will result in processing the material for one cycle while preparing the material for the next cycle. Thus, a mold requiring a 30g shot should be run in a machine that has a barrel with a 60g capacity. 

           A  50% shot-to-barrel ratio is ideal but can go as low as 20%,  and  as high as 80% depend on the heat sensitiveness.