How to Make a Cerrosafe Chamber Casting

There are many reasons you might want to make a chamber cast. You may have acquired an older rifle and need to verify the chamber dimensions. Or, if you have a new reamer, you may want to check the exact “cut” dimensions against the blueprint specs. A chamber casting is also valuable if you run across a firearm that you believe has a custom barrel on it and you want to find out the dimensions of the chamber. Lastly, you may want to prepare a chamber casting to be used in the making of custom dies. (Most reloading die makers know how to work from Cerrosafe chamber casts.)

Cerrosafe is a metal alloy that has some unique properties which make it ideal for chamber casting. First, it has a relatively low melting point of 158 to 190° Fahrenheit. This makes it easy for the handloader to melt the Cerrosafe in his home shop. Second, it shrinks slightly during cooling which allows it to be extracted from the chamber easily. It then re-expands to the chamber’s original size after about one hour at room temperature. After cooling for about 200 hours, the chamber cast will expand to about .0025″ larger than the actual chamber size.

One of our Forum members has done many Cerrosafe castings and he offers this smart advice:

1. Remove the barrel from the action to make the pour much easier. If you don’t remove the barrel, it can be hard to pour through the action (even with a funnel) and can make a mess if you’re not careful.
2. Pre-heat the barrel for 5-10 minutes in the oven on the very lowest setting (170° F in my oven). (DON’T overdo it!). Allow to cool for a couple minutes so you can pick it up and it is under 120° F. Pre-heating the barrel helps the Cerrosafe stay liquid as you pour the casting. This helps ensure a good, complete fit to the chamber.

Brownells has an article about Cerrosafe chamber casting that explains how to use this unique material:

How to Use Cerrosafe for Chamber Casting
The basic ingredient of Cerrosafe is bismuth. Bismuth is a heavy, coarse, crystalline metal which expands when it solidifies, up to 3.3% of its volume. When bismuth is alloyed with other metals, such as lead, tin, cadmium and indium, this expansion is modified according to the relative percentages of bismuth and other components present. As a general rule, bismuth alloys of approximately 50% bismuth exhibit little change of volume during solidification. Alloys containing more than this tend to expand during solidification and those containing less tend to shrink during solidification.

What all this means for the gunsmith is that you can make chamber castings using only Cerrosafe and a few, simple hand tools. To make a chamber casting, first clean and degrease the chamber. Use a tight-fitting, cotton patch that’s wrapped around a bore mop or brush to plug the bore just ahead of the throat. I usually leave the cleaning rod attached to the plug until it’s time to remove the plug. Melt the entire bar of Cerrosafe in a heatproof container that you can easily pour the hot Cerrosafe out of. You can use a propane torch or heat over a hot plate or the burner of a stove. Cerrosafe melts easily at 158°-195° F. While the casting metal is still liquid, stir very well, skim off the dross, and pour your chamber. The real trick with Cerrosafe is not to overheat it. If you heat the solid slowly, and keep it within the required temperature range, you shouldn’t get any dross.

Note the time the casting was poured. The casting will take only a very short time to solidify, usually within a minute. Wait 30 minutes and then remove the plug from the bore. Turn the muzzle upward and the casting will fall from the chamber. At 30 minutes after initial solidification, Cerrosafe shrinks slightly, so removal is very easy. Allow the new casting to cool thoroughly then measure the casting exactly one hour from the time it was cast. The casting will give you an exact measurement of the chamber. Cerrosafe casting metal can be used over and over. Remelt the entire amount back together and pour the Cerrosafe into a small mold of the appropriate size. Always melt the entire Cerrosafe ingot to make a chamber casting. For best results, never cut off, or use, just a part of the ingot.