I tried the pet shop corncob. Its way to large of chunks. They get stuck inside my 308 and are no fun to get out. I liked the crushed walnut shells(lizard litter), because I add it to my small Lyman corncob for a brighter polish.
I pick out the primer hole pieces as I inspect the brass.
My tumbler takes 6lbs of media, but it lasts for 10000 case cleanings. I used Meguires polishing wax and every one in a while I throw in 6 little strips of cotton, to clean up the dust from the media and bowl.
Starting with new media, it takes 2 hours to polish 300 45acp cases, at the end of the media life cycle it takes 5 hours to get the same results.
Some people use wheat, barley or oats, I haven't tried them, because they are very dusty and require way more additives. I asked my Farmer shooting buddy if he tried flax seed or canola seed, not yet was the answer, they might even come out prelubed, ready for resizing.