Ground walnut shells or corn cobs. You will find it in the section for lining cages or aquariums.

 

B-Kareful with the Corn Cob Stuff.

Once A-pon A-Time, I bought a bag of some-what coarse looking stuff that cleaned the outside of a case very nicely, However in the 223 cases I was cleaning it was just the Wrong size to exit the case. spent much time ,correcting that error the hard way. My 2¢, Russ. [Live & Learn]

 

Most of that pet litter stuff is too coarse, it can work OK on straight wall cases like pistol cases, but the larger stuff can get stuck inside a bottle neck rifle case, and it's so much trouble to get out it's just not worth it.

I buy bulk corn cob or walnut shells from Midway, if you are ordering some other stuff it doesn't really add much to the shipping cost, or if you want large quantities you can get it where they sell sandblasting supplies, but they usually like to sell at least 50 lbs.