Cerrosafe metal is no doubt the "best" material for a chamber cast because of durability. But, I only use powdered sulphur ("Flowers of Sulphur", from a drug store) for my chamber casts because it works as well, it's easy to find and inexpensive too.
The sulphur is also a low heat material. I use a large candle flame and put maybe a tablespoon full in an old metal 35mm film can as a pot with some wire twisted around the can for a handle. It needs to be heated slowly or it will turn a very dark brown and congeal into a jelly like mass that can't be poured. Sulphur is flammable so don't get silly with it. (I keep a pot of water handy both for any burns I may suffer or fire I may start, never had to use it but thinking ahead seems good.)
Clean and dry your chamber and bore, plug your bore just ahead of the chamber; a cotton ball or bore patch will do nicely. Make a long thin funnel out of paper and pour through the action until the camber is full, NO more or the base of the cast will be too large to get out of the rifle without a ram rod and hammer to remove and then start over! (I ain't saying how I learned that!) Both the metal and sulphur will solidify in a few seconds, remove the cast as soon as it's cool and hard.
The most critical dimension, to me, is chamber neck diameter. With that I can KNOW how much room is in the neck so my reformed cases can then be turned to a proper (snug) fit in the rifle.
