I did use some white rice a few months ago. I let them tumble for about 30-45 min. Not worth it. While the rice does change color, The brass barely gets cleaner really. It does get stuck in the flash hole and is kinda annoying but nothing serious.
Now, I just use corn cob and you can tell they shine pretty good for about 30-45 min in the tumbler.
Bottom Line - Don't waste good rice. Just get the corn cob or the lizard pet bedding, You will be much happier.