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.

Cerrosafe is infinitely reusable, it never wears out. Be careful to avoid overheating Cerrosafe as it will make it ineligible to use it for recurring purpose. Cast it into holes for missing screws, to learn thread size. Use it to grip small fragile parts while machining them. Cerrosafe is quite different from Woodsmetal, which expands after cooling and cannot be removed from a gun chamber. This alloy can be used for a precise bore measurement. One needs to measure dimension only approximately after an hour in order to get the exact measurement.