Cerrosafe

Cerrosafe is a fusible alloy with a low melting point. It is a non-eutectic mixture consisting of 42.5% bismuth, 37.7% lead, 11.3% tin, and 8.5% cadmium that melts between 158 °F and 190 °F. It is useful for making castings whose dimensions can be correlated to those of the mold or other template due to its well-known thermal expansion properties during cooling. The alloy contracts during the first 30 minutes, allowing easy removal from a mold, then expands during the next 30 minutes to return to the exact original size. It then continues expanding at a known rate for 200 hours, allowing conversion of measurements of the casting back to those of the mold.

How to Make a Chamber Cast

1. Disassemble the firearm so that you have good access to the chamber area. Take it out of its stock if applicable.
2. Clean the chamber using a dry cotton patch and cleaning rod. Pistol cleaning rods with a looped end really work well for this.
3. Plug the bore using a tight-fitting patch and cleaning rod. Measure the approximate distance to the end of the chamber area or visually check that the plug stops 1/2″- 1″ shy of the chamber area. I insert this patch or plug from the muzzle end of the barrel.
4. Position the barrel vertically in a bench vise to allow for a clear entrance to the chamber. Heat the chamber area of the barrel until it is uncomfortable to touch.
5. Melt the Cerrosafe in a metal ladle that has a pour spout. Take care not to overheat.
6. Pour the Cerrosafe into the chamber until a slight mound builds up at the back of the chamber.
7. Let the casting cool for a minute or two.
8. Remove the chamber casting using a cleaning rod inserted from the muzzle end of the barrel. I insert the cleaning rod and simply drop the barrel an inch or two onto the end of the cleaning rod. One or two drops will normally do the trick. I don’t use my premium cleaning rods for this, but you should use one with a protective coating to avoid damaging the bore. Brownells recommends removing it within 30 minutes of pouring the casting.
9. Measure the casting exactly 1 hour after pouring for the most accurate results. Measuring as soon as you remove it will get you close enough to identify an unknown caliber most of the time.