If you have it, use Cerrosafe, a very low melting point metal alloy. If you do not have it, paraffin wax works well but put a piece of string in the chamber to go with it. If the casting breaks, the string keeps it together..
Basically take the cylinder out and block the front end to keep the wax in then pour molten wax in the hole. Push the cast out with a piece of wood or similar
cheers mooncoon