April 6th, 2014

Tech Tip: How to Cast your Chamber using Cerrosafe

by Bill Gravatt
(This article was written when Bill was President of Sinclair International, Inc.
Chamber casting is an easy task for the handloader to perform. A chamber casting is 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. Some gunsmiths will chamber a barrel and not mark it properly with the neck dimension or the exact cartridge name or specifications. We also get calls from some customers that have military firearms without cartridge stampings on the barrel; this will help these shooters identify their chambering.


Another reason to make a chamber casting would be for a die manufacturer to manufacture custom dies for you. A chamber casting is often required when fired cases are not available. Some reloaders will make a chamber casting that shows them the exact configuration of the throat and leade so they can determine what bullets to try. Shooters using cast bullets will make a cast so they can choose a mould that better fits their throat taper and grove/lands diameter.

A product called Cerrosafe is the most common, reliable, and the safest material to use for making chamber castings. 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. Most good reloading die makers are used to working from Cerrosafe chamber casts.

As we said, using Cerrosafe is fairly easy and comes with complete instructions.

Making a Cerrosafe Chamber Cast — Step by Step:

1. First, clean and dry the chamber and barrel thoroughly.

2. Disassemble the firearm as necessary to gain access to the chamber.

3. Insert a tight fitting cleaning patch with a jag into the bore from the muzzle end to form a plug for the Cerrosafe. The patch should be positioned in the bore, just forward of the throat by approximately ½” to 1”.

4. Heat a Cerrosafe ingot in a small ladle. A heavy cast iron bullet caster’s ladle works fine or a plumber’s ladle. Any source of heat will do (a small propane torch will work fine).

5. Pour the Cerrosafe into the chamber until a little mound forms at the rear of the chamber. Too much and it can become more difficult to remove the cast from the chamber. If this happens, simply heat the barrel a little and re-melt the Cerrosafe. Don’t worry, your barrel gets a bit hotter than 190 degrees during firing.

6. The chamber can be difficult to access, so some people find it easier if they make a pouring tube out of steel, brass, or aluminum tubing to funnel the Cerrosafe into the chamber.

7. After the Cerrosafe has hardened, the chamber casting can be pushed out of the chamber coming from the muzzle end using a cleaning rod or a wooden dowel. It is recommended that you push it out within a half-hour of casting the chamber. We usually push our cast out within a few minutes. If the cast does not push out easily, insert a cleaning rod from the muzzle and tap the rod handle with the palm of your hand to start the cast out of the chamber. You can put a paper towel in the action to catch the cast or lay the rifle on the bench with a towel or bench mat underneath it to catch the cast as it falls from the action. This will prevent damage to the cast.

8. Take your measurements shortly after one hour of cast, and then put the casting away until you need it again. A medicine container or something similar makes a great container. If you are going to keep the cast be sure to mark the cast or the storage container so you know which rifle it came from. If you have no need to keep the cast you can re-melt the Cerrosafe and use it again when you need to make another cast.

This article originally appeared in Sinclair International’s The Reloading Press Blog.

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