If white rice is all you can get, it may work to clean your dirty cases somewhat but not for long and may not end up being cheaper than walnut/corn. I did test white rice as a tumbling media but found it only worked OK just for a few batches and I needed to replace the rice in the bowl to keep cases from getting smeared with grayish residue on the case surfaces (especially if you shoot lead bullets) - yuck!.
Crushed walnut/ground corn cob work much faster/better and last longer (cheaper than rice in the long run). Use with NuFinish polish and walnut/corn will do an even better job of polishing the cases (No, you CANNOT use any liquid polish with white rice so no hope of "mirror" polish with rice).
You can get crushed walnut from pet stores (like Petsmart, Petco, even Walmart) and Harbor Freight carries them in two grit sizes. I prefer the
fine grit (24) over
coarse grit (12) as media won't get stuck in flash holes and seems to clean/polish faster.
Because walnut media lasts so much longer (walnut is so hard, it actually don't "wear" - just gets dirty), even $23/25 lbs HF walnut media will turn out cheaper than $15/25 lbs white rice because you will have to replace the rice more often.
If you want brilliant polish, corn cob and NuFinish will do an excellent job and costs less than walnut ($34/40 lbs shipped) -
http://www.drillspot.com/machining/finishing-supplies/pneumatic-blasting-media/
BTW,
Powder Valley sells corn cob for $18/40 lbs (Go to Powder Valley > Maintenance > PVI > Ground Corn Cob).