It is not the little stainless rods that do the peening, it is the way the cases tumble against one another. I use a wet tumbler with NO stainless or other media other than the liquid and the cases themselves. I get the slight case mouth peening and I get very clean brass inside and outside. I even leave the spent primers in and the pocket floors are "almost" totally clean. I dry mine in a convection oven to be sure the pockets with spent primers still present are totally dry. It is over kill, but I use 250F with 30 minutes after the whole batch comes fully up to the "pre-heat".
prs