Corncob IS the best. For media, I would tell you that I use Brasso and have never had problems, but then a gazillion people would jump all over my case about the ammonia being corrosive and such and causing weaker brass, etc. So I won't tell you to use Brasso.
Use something without ammonia.
I tumble WITHOUT primers, but then I clean the primer poket out with a little wire primer pocket brush. Gets all the gunk out of the very corners or the pockets, allowing for easier insertion of the primer.