Listen to uncle Buck. The black stuff inside the cases does nothing to the finished ammo. It does NOT "build up", it's simply carbon left over from combustion of the powder. Same goes for the crud in the primer pockets.
My method is simply to use 20/40 grit corn cob from drillspot.com. It's super fine grit cleans and shines more quickly than the coarser grits. Carsinc, that pet store stuff is okay as long as it's used on big pistol cases, but wait till you use it on rifle stuff under 35 caliber! You'll invent some new cuss words getting it out of those bottle neck cases, I know I did!