When I was dry tumbling (now I wet tumble) i once tumbled my .223 in corncob media and forever regretted it. The corncob media was just the wrong size. It got stuck inside the case and the primer pockets.

From that point forward I'd do .223 in crushed walnut media. And my pistol rounds went in corncob. (Since, in my experience, corncob did a better job of shining up the cases.) But from a pure cleanliness perspective, walnut media was just fine.