The main reason for tumbling brass is to get it clean,,--BEFORE it goes into your dies. The decapper pin will push any media out with the spent primer.
To avoid plugging up flash holes when tumbling
after sizing, get finer grit media. One source of the fine corncob is here;
http://www.drillspot.com/products/52...bs_blast_media
And it ships free! 40 pounds of that will last you and others a very long time. It's small grit size will not get hung up in flash holes.
As for walnut, look around on drillspot, they may have that in finer grit as well.