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!

What is a wildcat cartridge?

A wildcat cartridge is a custom made cartridge, usually made by modifying an existing commercial cartridge, in order to optimize cartridge performance (e.g. more power, more velocity, better accuracy etc.) Since these are custom made, they are more expensive than normal cartridges. Also, wildcat cartridges are usually used mainly by very serious shooters and hand-loading fans, there is only a limited market for them and larger cartridge manufacturers generally don't make them.

In terms of variety, there are more wildcat cartridge varieties than commercial production cartridges. However, many of these varieties are produced in very small quantities indeed.

In some cases though, some cartridges started out as custom-made wildcat cartridges, but gained enough popularity that they began to be commercialized (i.e. rifles chambered for them are now available commercially) and SAAMI standards were specified for them. Examples of such cartridges are the 6.8 mm. SPC, which was originally developed in collaboration with some members of US SOCOM. The 6.8 mm SPC is based on a .30 Remington cartridge, modified to .270 caliber and then further modified in length to fit in an magazine that can be fitted into the magazine wells of the M16 rifle. Therefore, any M16 or AR15 type rifle only needs replacing of the barrel, bolt and magazine to use this new cartridge. This cartridge is more lethal than the standard NATO 5.56x45 mm. cartridge fired by the M16 and while it is not officially adopted by the military, it has found use by special forces troops in Afghanistan and Iraq and is gaining popularity as a commercial civilian round. Another example is the well-known .357 magnum cartridge developed by Smith & Wesson, which was originally developed from a .38 Special cartridge. The .357 and .38 Special cartridge are both the same diameter externally and only differ slightly in length, because of safety reasons. Early versions of .357 magnum were actually identical dimensions to .38 special cartridges and the length was only altered so that people could not accidentally load the more powerful .357 magnum cartridges into a firearm not designed for the additional pressure. Another example is the 6 mm. PPC (Palmisano & Pindel Cartridge, named after its inventors, Lou Palmisano and Ferris Pindell). This cartridge started out as an improvement of the .220 Russian cartridge, which was itself based on the venerable 7.62x39 mm. cartridge used on the AK-47 and AKM assault rifles. The 6 mm. PPC case is made by forming the .220 Russian brass case into a new shape and is specially geared for single-shot bench rest shooting. It is one of the most accurate cartridges available up to 300 yard ranges and has been produced since 1975 and used in several competitions.

As mentioned earlier, wildcat cartridges are generally used by very serious shooters mainly and quite a few require barrel modifications also in order to use the modified cartridge. The modified barrels are usually supplied by custom barrel makers, who typically work out of small shops. The custom barrel makers generally also supply the buyer with reloading tools and dies, so that buyers can make their own cartridges. Some barrel makers also supply data about how different powder brands, powder quantities and bullet weights perform with their barrels. Therefore, most wildcat cartridges are developed, either by the custom barrel makers themselves, or by someone who is working in conjunction with a custom barrel maker.

.243 Winchester Ackley Improved wildcat cartridge on the left, compared to a normal .243 Winchester cartridge on the right.

Note the reduced case taper and sharper shoulder angle in the Ackley Improved version, which leads to more case capacity and therefore, more propellant.

Image copyright Arthurh at wikipedia and licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License. 

There are many reasons why people develop wildcat cartridges, such as:

• Increasing the case capacity (as in the Ackley Improved cartridge pictured above) allows adding more propellant to the cartridge, which increases the velocity of the bullet and therefore, the energy transferred by the bullet.
• Reducing the bullet's caliber increases its velocity, thereby increasing its resistance to wind drift.
• Better consistency can be achieved by tuning a bullet's diameter, weight and velocity to a particular amount and type of propellant, which leads to greater accuracy.
• Feeding issues of certain types of ammunition can be fixed. For instance, it is not possible to reliably fire hollow-point bullets with .45 ACP pistol ammunition because of feeding issues from the magazine. Hence, the shape of the cylindrical cased .45 ACP cartridge was modified to a bottlenecked .45 cartridge to solve this issue.
• Some shooters like to use rifle ammunition with pistols, for greater accuracy. In this case, one starts out with a rifle cartridge and then reduces its case capacity so that it can be used with a pistol.

A wildcat manufacturer generally starts out by using a commercial cartridge case and changing its shape to new dimensions. Usually, this involves pushing the shoulder of the cartridge backwards or forwards as needed to modify the case capacity and also changing the diameter and length of the cartridge neck. This process can be done by either cold forming (i.e. the case is pushed into a die and pressure is applied to change the shape of the case) or fire forming (i.e. the case is placed in a chamber of a different dimension and loaded with a light gunpowder charge. Upon firing the charge, the case takes the shape of the new chamber). Sometimes a mixture of both methods is used to make the final case shape of a wildcat cartridge. Next, the manufacturer trims the case to the appropriate length, because cold forming or fire forming generally tends to increase the length of the case's mouth and the excess length needs to be trimmed. Then the diameter of the neck is changed as needed for the new bullet. The cartridge is then hand-loaded carefully and a bullet is crimped on.

In quite a few situations, firearms can be very easily modified to use the new wildcat cartridges. For example, the Ackley Improved cartridge shown above could easily be used by rechambering an existing firearm. Better still, a firearm that is chambered for the "Improved" cartridge can also fire standard factory loaded ammunition as well, which allows the owner to use less expensive and commonly available ammunition if there is a shortage of wildcat cartridges. Ackley Improved family of cartridges were developed by Patrick Otto Ackley, a prolific gunsmith and author, who produced many improved versions of commercial cartridges in several different calibers.