Cerrosafe is fine for chamber cast I'm sure but as I don't have any, and like to keep things simple, I just use an old candle. First prepair the barrel - put in some newspater an inch or less past the chamber, then oil it lightly so it will not stick. Also use some paper or masking tape to avoid wax in unwanted places. Then melt the candle and pour it in. Wait some till hard and tap it out with a cleaning rod. Put in fridge and take measurments later (gently).

With kind regards,
Jani