How about just adding on a round flat bulkhead/disc? You can epoxy or screw it on, drill two holes for the shock cord attachment (or screw eye) and it negates the problem of ejection gases escaping through the shoulder and should make the nose cone act like a piston. Seems like a simple, easy, quick fix (if the angled shoulder is the problem) and you take up very little space and don't reduce the shoulder.