What is the best procedure for cleaning spent hulls before reloading?
You tumble the hulls in a vibratory or a rotary tumbling machine filled with ground corn husks or ground walnut shells.
The shaking of the vibratory type of cleaning machine, tumbles the spent cases within the cleaning media. The ground material physically rubs the powder residue off the brass. Some reloaders add some brass polish to the mix and the result is a bright polished finish, one that is brighter and more reflective than never fired brass.
Though it may be customary in centerfire rifle and pistol reloading to clean brass prior to reloading, it isn’t common practice in shotshell reloading, but I see no reason why the same cannot be done to fired shotshells.