There is an alloy casting material called cerrosafe. It can be bought at Midway or Bownells here in the US. It goes to liquid form at a very low temperature using a double boiler. You then stop the barrel up, in your case just passed the chamber and pour in the liquid cerrosafe in a few minutes it hardens and you can gently knock it free from your gun. You now have a perfect casting of your chamber to measure and trim your cases or send to someone to make brass casings for your gun.