How to Measure the Chamber Dimensions

Assume that you've just purchased a firearm and brought it home and found that you don't know what cartridges the firearm takes. This could be because the firearm was manufactured a long time ago, when manufacturers did not stamp the cartridge dimensions on the outside of the weapon. Or, the previous owner could have a custom barrel made for the firearm. Perhaps a previous owner liked to experiment with wildcat cartridges. Or perhaps, you would like to measure the dimensions of the chamber to make sure it isn't too worn out.

As you can see, there are plenty of reasons why someone might need to measure the dimensions of a firearm chamber. So how is it done? We will study that in this post.

One of the most common and reliable ways of measuring chamber dimensions is to use a metal alloy called "cerrosafe chamber casting alloy". This is an alloy made of 42.5% bismuth, 37.7% lead, 11.3% tin and 8.5% cadmium. Cerrosafe alloy was originally used to produce castings of toy soldiers and because of its low melting point, it was also used for fuse links in fire sprinkler heads.

A bar of cerrosafe alloy. Click on the image to enlarge. Public domain image.

Cerrosafe has some properties that make it very useful for the job of measuring chamber dimensions:

1. It is relatively cheap to buy and easily available. A bar of cerrosafe weighing 1 lb. (0.45 kg.) can be purchased for prices ranging from $25 to $45 or so, from various sources.
2. It is generally reusable, unless the user overheats it too much. Therefore, the alloy can be reused multiple times for many years.
3. It has a low melting point and melts at temperatures between 158 to 190 degrees Fahrenheit (or 70 to 87.77 degrees centigrade). Note that this is below the boiling point of water (212 degrees Fahrenheit or 100 degrees centigrade). This means it can be melted by using devices commonly found in every kitchen or garage (e.g.) stoves, hot plates, blow torches, small lamps, a double boiler etc. This also means that hobbyists can use it without purchasing any special equipment.
4. An unusual property of cerrosafe is the way it shrinks and expands as it cools down. Initially, the cerrosafe shrinks slightly during the first few minutes of cooling, just like any other normal metal or alloy does. This makes it easier to remove from a firearm chamber. However, after about 30 minutes have passed, it starts to expand while cooling. After about an hour or so, it returns back to about the original dimensions of the chamber and after about 4 days (96 hours), it expands to slightly larger than the dimensions of the chamber it was cast in (it expands about 0.0025 inches per inch of size after 96 hours).
5. One more useful property of cerrosafe is that it does not bond itself to the barrel metal like plain lead or tin do. This also makes it easier to extract out of the barrel.

In case the user thinks that cerrosafe is too expensive, there is another product made by Rotometals Inc., called Rotometals chamber casting alloy. This is practically the same composition as cerrosafe, but doesn't come in a nice stamped bar with letters on it, but is sold as a conical ingot instead. However, it has identical properties as cerrosafe, but costs about 50% less (A 1 lb. ingot of  Rotometals chamber casting alloy only costs about $15 to $20 from various sources).

An ingot of Rotometals chamber casting alloy. Click on the image to enlarge.

The following table shows how cerrosafe (or rotometals) chamber alloy's dimensions change with time, as it gradually solidifies.

Time	Contraction/Expansion per inch
2 minutes	-0.0004 inches
6 minutes	-0.0007 inches
30 minutes	-0.0009 inches
1 hour	0.0000 inches
2 hours	+0.0016 inches
5 hours	+0.0018 inches
10 hours	+0.0019 inches
24 hours / 1 day	+0.0022 inches
96 hours / 4 days	+0.0025 inches
200 hours	+0.0025 inches
500 hours	+0.0025 inches

As you can see, the alloy initially shrinks for the first 30 minutes or so, then it starts to slowly expand. After approximately 1 hour, it returns back to the original dimensions of the chamber and the alloy continues to expand, until about 4 days later, when it reaches its maximum size.

So how does the user measure the dimensions of a chamber using this alloy then?

1. First the user disassembles the firearm as needed, to get access to the chamber of the firearm.
2. Next, the user cleans the barrel and puts a small amount of oil in the chamber and then pushes a cleaning patch into the barrel so that it is just ahead of the throat of the barrel. The cleaning patch serves to block the barrel after the chamber.
3. The cerrosafe (or rotometals) bar is heated until it melts. It is only necessary to ensure that the bar is not directly heated by open flame. This means it can be heated in a small iron ladle or coffee can, using a stove, electric hot plate, propane torch, oil lamp, candle etc. It can also be heated by placing a small container containing the cerrosafe into a larger container of water and then boiling the water. Remember that the melting point of cerrosafe is well below that of the boiling point of water.
4. The molten cerrosafe is then poured into the chamber of the firearm, until the chamber is full. Since the melting point of cerrosafe is so low, it doesn't affect the barrel or the cleaning patch that is blocking the barrel on the other side of the chamber. The cerrosafe alloy is then allowed to cool until it turns a shiny silver color. As soon as the alloy has cooled enough that it is no longer a liquid, it can be pushed out of the chamber using a cleaning rod or wooden dowel (remember that cerrosafe initially shrinks slightly as it cools for the first few minutes, which makes it easier to push out).
5. After about an hour, the casting expands back to the original dimensions of the chamber. It is then carefully measured at various points to determine the exact dimensions of the chamber, using a vernier caliper or a dial gauge. The user can then use these measurements to figure out what cartridge size the firearm was designed to use. Reference books that list the exact dimensions of various cartridge types are readily available, therefore the exact cartridge model may be easily determined. A good reference book that is commonly used is "Cartridges of the World".

After this, the casting can be stored somewhere until it is needed to measure the chamber of a different firearm, as the alloy can be remelted and reused multiple times.

One of the nice things about this process is that the user doesn't need to be very experienced to do this and the tools are also generally available and cheap.

Here's a movie from the one and only Mr. Larry Potterfield, showing how to use cerrosafe to measure a chamber.

Happy viewing!