I use a rock tumbler I bought for the kids years ago. For media, I use walnut chips with a very small amount of car polish (a blob about a thumb print size). Also add three or four strips of a new (or used) dryer strip. The strips always come out black and can be tossed after each tumble.
De-prime brass before cleaning. The walnut chips are expensive when bought as a 'reloading' product, but you can get a 50 lb bag of it at Princess Auto as a 'sand blasting' product, really cheap. That will last a VERY long time. I never tried corn cob chips because the walnut worked so good.
A good rock tumbler is made to operate for weeks/months at a time, so it should last a while on brass. Normally a few hours should do it, but I've forgotten to check on a few tumbles and the brass really sparkles after an all nighter.