The best ones have been Hillman's 4-6 x 7/8 but then they cheapened them down to where the rim that would protect the chamber was so thin that it wasn't protective enough. Awhile back they came back with a jar of 625 pcs from Amazon that were the thicker ones again and was a great bargain but I'm not sure if they are still available. I just looked and don't see them. They still list them in a 100 pack but not sure if it's the older good style or the newer thin ones.