65.x55 would be wonderful. it is ever-so-slightly rimmed, making it an extra good candidate. pressure's right for the handi, too. it's loaded by several manufacturers to boot.
.30-40 krag would be grand, with that rim and handi-happy pressure. it is still loaded, too.
.375 winchester would be a good one, too. hard to find loaded ammo, but components are easy as pie.