'Easing' the forcing cone does not affect the chambering and should sort out your problem. Lengthening the chamber will require re-proof. I also recommend trying different shells. Recently, I tested a Dickson round action for a client and forend that the case ends ragged when using Eley First, Hull High Pheasant and Eley VIP but not when using Eley Grand Prix. A lot of guns favour one cartridge over another - find what that is and stick to it.