Mr Polecat,
Before you do any thing to the gun, try trimming a case neck short enough that it doesn't cause a problem with the "ring", then try .311-.312 bullets( first cast, then if a jacketed bullet "falls" into a fired case, try them. You can load the bullets to the normal OAL. A 30-30 case neck should be long enough to allow this. If it works, great; if not, you are no worse off than you were yesterday.
Mike