Chamber Casting

The purpose of making a chamber casting is to get the internal dimensions and learn information about a firearm chamber. This information includes: 

1. Does the chamber have any damage or excessive wear that might cause function and reliability issues?
2. What are its chamber measurements for optimizing reloading?
3. Which details are needed to make a custom bullet mold?
4. Does the marking on the barrel match the actual chamber on a used, and possibly modified, firearm?

Since it's nearly impossible to measure the inside of a chamber with conventional tools, standard procedure is to make a chamber casting that results in a negative impression of the chamber and is much easier to measure. These measurements should be the same as the inside dimensions of the chamber.

The most common material available used for making a chamber cast is sold under the trade name Cerrosafe. It’s an alloy of bismuth, lead, tin, and cadmium and is available from Brownells as well as other vendors.

The most important attributes of chamber casting metal are:

• A low melting point, generally between 160 and 190 degrees Fahrenheit. 
• It  must also shrink slightly as it cools for ease of removal, yet an hour or so after casting it returns to its cast size for accurate measurement. 
• When employed properly, it’s nearly infinitely reusable.

An ingot of Cerrosafe, as sold by Brownells

Making a chamber casting is fairly straightforward, but it does require some preparation and care:

1. Make sure the barrel and chamber are clean and dry.
2. Remove any parts, such as the bolt, that would prevent easy access to the barrel. If possible, remove the barrel from the gun.
3. From the muzzle end of the barrel, insert a patch jag with a snug fitting patch to form a plug for the casting material. The plug should not be too snug, or it could complicate removal, nor too loose as that could allow casting metal to leak past. Ideally, the plug should be positioned no less than an inch or so past the start of the rifling.
4. Place the barrel in a secure fixture, muzzle pointed down.
5. Once this is set up, heat the casting metal in a small ladle. The cast iron style ladle that’s used for bullet casting is ideal. An electric hotplate or small gas torch are good heat sources.
6. Note the time, then carefully pour the molten casting metal into the chamber, using a metal tube or funnel if necessary. Fill the chamber to just above the edge, but not so much that it spills over since this can make it difficult to remove the casting.
   
   

   

   Pouring the chamber casting

   
   
7. If too much casting metal is added, just invert the barrel and heat it with a heat gun until it comes out. This won’t harm the metal of the firearm, but may damage a wood or plastic stock, so be careful.
8. It usually only takes a few minutes for the chamber casting to cool sufficiently for removal. Look for the metal to dull as a sign. 
9. Once the casting metal has firmed up, carefully push the chamber casting out from the muzzle using a cleaning rod or a brass or wooden dowel. If the casting seems to stick, strike the base of the rod or dowel with the palm of your hand. It should only take a few gentle blows to loosen.
10. Take care that the chamber casting doesn’t get damaged during removal. Have a soft pad, such as a folded towel, in place to catch it. 
11. Remember when I suggested noting the time? Wait at least one full hour after casting to take any measurements. This allows the chamber casting to return to as cast size.
12. If it’s necessary to keep chamber castings for later reference, make sure they’re clearly labeled regarding the source firearm and stored in a container that will protect them from damage. Brownells offers a selection of plastic tubes and caps that will work well for this purpose. If there’s no need to save the casting, it can be re-melted for the next chamber cast.

A completed chamber cast, showing the case neck and rifling

While making chamber castings isn’t an everyday skill, being able to determine specific details about a firearm chamber can be extremely useful. It's another tool skill to keep in your toolbox skillbox.