Another thought would be the .204 Ruger or one of the various .20 caliber wildcats that preceeded it. Cooper builds rifles for several of them. The ballistics of a .20 caliber bullet, particularly one of the heavier ones, is surprisingly good. Check out the information available on the web site of the Woodchuck Den (I think it's http://www.woodchuckden.com. Todd Kindler will be happy to talk with you at length about .20s, as he was instrumental in reviving the caliber.