Powder left in the reloader's powder measure hoppers for extended periods, overnight or several days, should be avoided.  Powder needs to be stored in original containers ONLY, when not in use.  Numerous modern smokeless powders are double base in composition, containing both Nitroglycerine and Nitrocellulose.  Many powder measures currently available use an inexpensive plastic containing polystyrene, which Nitroglycerine adversely effects when contact is made for extended periods of time, resulting in etching or misshaping the plastic.  Normal usage during the reloading process does not provide adequate time for this to occur, so simply draining hoppers into the original containers when the reloading is complete for the day prevents ruining the hoppers.