Gordie - a small quantity of Cerro-safe low melt point metal can be ordered from Brownell's. I will make a very accurate chamber cast quickly, easily, and safely. Alternatively, sulfur can be used, but be careful with melting it. Clean and oil the barrel and chamber. Place a paper plug about an inch past the chamber and into the bore. Using gloves, long sleeves, and a face mask, melt a small quantity of sulfur in a small sauce pan (the yellow powder will turn to a sorta golden brownish liquid). Carefully pour into the chamber just to fill. Allow to cool. Use a solid rod of near bore diameter from the muzzle end to punch the cast out. Measure the cast and compare to chamber drawings of candidate cartridges ("Cartridges of the World" is an excellent reference. Should you discover that you have an uncommon chambering, consult "Cartridge Conversions" for tips on how to put the 'ole smoke pole back in service.
