I just found that the small diameter of a Winchester 3006 casing will fit nicely over a 5/16 shaft with just a little sanding.  Also, if you cut off the casing's tapper they fit over a 3/8 shaft with minimal wiggle room.  A whole casing weighs about 190 grains.  So far I've been sanding down 3/8 shafts to fit the standard .38 casing, but I'm thinking of switching.