Chamber Identification With Cerrosafe

With a couple of cartridge books at your disposal, you can easily add chamber markings to any firearm.


There will always be a need to identify chamberings of firearms that have been inherited or bought at yard sales. The lure of buying for bottom dollar and discovering a real jewel is ingrained in most of us, so a little thing like no chambering marks on a rifle or handgun is no deterrent to a gun fancier.

This gives rise to a gunsmithing service for which little experience is necessary. Anyone can become proficient at identifying firearm chamberings with the help of one or two good cartridge books, along with a little knowledge from which to extend the cartridge drawings to chamber dimensions.

Chamber identification is done with a lead-like metal which has an extremely low melting point and is called Cerrosafe in the firearms industry. It is the safest known way to make molds of chambers, and can easily be taken from the chamber and measured. This metal fills the chamber when hot, but shrinks during the first 30 minutes of cooling to allow its removal from the chamber. Cerrosafe then expands back to the chamber’s size for about 200 hours, allowing the casting to be measured with great accuracy. After about 200 hours, the metal expands approximately 0.25 percent, a negligible amount for most uses. Reamer manufacturers and toolmakers are used to working with this slightly expanded metal and know how to take that factor into account.

The metal was originally developed to be used as fuse links in fire sprinkler heads and other heat-sensitive applications; it melts between 158 and 190 degrees Fahrenheit. This means that it will melt in boiling water, and can therefore be melted in a double-boiler system.

Another early use for this fascinating metal was the casting of draft-free spaces for machine shop usage. This practice was what led directly to Cerrosafe’s use with firearms, because it doesn’t solder itself to the base metal like lead and tin do. It can also be used to measure the bore of a firearm which has an uneven number of grooves in the rifling (although I usually just use a soft lead bullet for this).

Cerrosafe is lead-like in color and softness, and shines like smoothed lead. It casts into ingots very easily, but I don’t do that because I don’t want to get it mixed up in my bullet-casting supply. The density is almost that of lead as is its melted viscosity, further adding to the difficulty of keeping them separated. In spite of its similarity to lead, it doesn’t seem to wick its way through coffee can seams the way lead does. It could be that the lead is occupying all the space in the seam and will not allow the Cerrosafe through. In any case, we can make use of this factor, and utilize a small coffee or other can for our heating and pouring container.

The equipment required to identify chambers falls into six groups: heat source, container, cleaning, safety, measuring, and literature. Almost any heat source such as an electric hot plate, stove, propane or acetylene torch, or even a heat-shrink gun will suffice. I use an inexpensive electric hot plate that I also use for tempering springs.

The containing/pouring utensil choice is wide open. I use an “International Coffee” can, a rectangular can about 5 inches by 3 inches, by 3 inches tall. I always make sure that one corner is bent properly—to facilitate pouring the molten metal—before I start melting the Cerrosafe. This offers an advantage over a double boiler; wives view gunsmith activities differently than do the ’smiths themselves. A bullet-casting ladle or melting pot might also serve well, although this would be inviting a bullet-casting catastrophe.

The firearm to be identified must be clean before casting, and cleaned again after the casting for safety. Therefore, a well-equipped chamber-identifying bench will need a cleaning rod, bore brushes, mops or cleaning patches, and bore oil. Along with the cleaning things, you may need to have the tools necessary to disassemble and reassemble the firearms.

I recommend the use of eye protection, an apron, and gloves, as there is always the possibility of a spill or splash of the molten metal. Even though the metal melts well below the smoldering point of cloth, it is nearly impossible to get the metal out of clothing. I’ll admit to owning a couple of shirts with metallic spots.

Once the casting is made, you will need some method of measuring the casting in several different places. These measurements will need to be taken to three decimal places in inches or two decimal points in metric. Dial calipers reduce the incidence of errors and are much quicker to read, making them superior to vernier calipers for this use. I sometimes use my electronic calipers because they quickly convert to metric with the push of a button, but these are not necessary. (One millimeter is equal to .03937 inches or about 39 thousandths of an inch. Multiplying a metric number in millimeters by the number .03937 gives you the same reading in inches. Dividing inch measurements by the same number [.03937], gives you the reading in millimeters.)

You will also need a reference book or other literature with which to compare the casting’s measurements in order to make a good judgment. Most reloading manuals only give the dimensions needed to reload cartridges. The Sierra, Hornady, and Speer reloading manuals give some cartridge dimensions, but are not intended to be used for identification purposes. The most common authority on cartridges is the book written by Frank Barnes called Cartridges of the World, now in its 6th edition. It is very complete and you will definitely need it. It can be found in most gun shops and places where firearms manuals are sold. However, if you can afford it and want to get direct chamber drawings as well as cartridge dimensions, order the RCBS’s Cartridge and Chamber Drawings, sold in a ring binder. These are complete design engineering drawings of both the cartridges and chambers, showing the allowable tolerance deviations. I also started collecting sample cartridges, removing the primers where possible, to actually test-fit into the chambers after analysis.

One of the nice things about this endeavor is that there is no need for vast experience. In most cases, no experience will be required, and nothing more than some assembly/disassembly manuals will be needed in more complicated cases. I highly recommend The Gun Digest Firearms Assembly/Disassembly line of books, the NRA’s Firearms Assembly books, and The Gun Digest Book of Exploded Firearms Drawings for any level of gunsmithing. The last book gives more models for the buck but only exploded views, while the others are superior in the details involved in the separate steps of disassembly. I find an unfortunate lack of complete firearm reassembly instructions in the available books. Careful notice of the spatial relationships between adjacent parts should answer your reassembly questions for you.

In preparing the area for the casting work, choose a clear space on a table or bench with an electrical outlet nearby. It helps if there is good ventilation—most metals in their molten form give off vapors which can be harmful. Brownells, the source of Cerrosafe, doesn’t list any dangers from the fumes, but it is always a good practice when melting metals to have plenty of fresh air.

Disassemble the firearm so there aren’t any extra parts with nooks and crannies into which the molten metal might flow and harden. It is important to think the process through to the end before starting, eliminating rude surprises. This means determining how the casting will be removed from the chamber without having any undercuts or obstructions upon which to hang. Because the shell must go in and out of the chamber, it should be easy to duplicate this. However, it is easy to overlook a seemingly insignificant undercut in some firearms. Also, look to see if there is any danger of the molten metal running into the trigger mechanism. (Just in case it does, we will go through the method of recovering from this later.)

Plug the bore by pushing a small piece of felt, cotton, or rubber into it, stopping just into the rifling near the chamber. This will allow you to capture a small part of the rifling, and also inspect for erosion at the start of the rifling. If there is very much erosion, you may have to go to the section on recovery.

Place the barrel, cylinder, or receiver in a vertical attitude with the muzzle end down, and secure it. Then begin heating the Cerrosafe.

Precautions to take when heating any hot metal or liquid include making certain that nothing is near the edge of the workbench, that the heat source is steady and the cord isn’t in the way, and that the heating container is steady on the heat source. If you heat with a torch of any kind, see that the heating container is held even more securely. Heat the Cerrosafe in a reasonably slow fashion, as this is usually safer until you get used to how the metal behaves. Be ready for the metal to melt much sooner than you would expect—158 degrees Fahrenheit isn’t very hot. Don’t use any more heat than necessary to melt the metal; I have found no literature which tells me at what temperature Cerrosafe vaporizes. The metal will turn the prettiest chrome-like color when it is melted and can be poured as soon as it is all molten.

Pour the molten metal carefully to avoid getting it anywhere you don’t want it, but pour fast enough to be sure that it isn’t solidifying prematurely and layering in the casting. The only way I know to do it your first time is to proceed as rapidly as you can safely pour the hot metal. Watch carefully so that the level of the casting does not reach the location of the breechface of the bolt or receiver. There can be no Cerrosafe in an area with a diameter larger than that through which the casting must be removed. If the Cerrosafe is left in the casting for more than 20 minutes or so, it can be very difficult to remove, and the integrity of the casting’s dimensions may be lost from scuffing.

Remove the casting by pushing a cleaning rod through the bore or the front of the cylinder, tapping lightly if necessary to aid it along. When removed, inspect the casting for scuff marks or scrapes which may affect the dimensions. Set the casting aside to cool for 30 minutes to one hour. (The casting will have shrunk from the chamber dimensions, and must have time to return to the exact size of the chamber. This is a fortunate characteristic, as it allows easy removal from the chamber and then returns to the as-cast size.)

What if the casting refuses to release from the firearm? Believe me, this will happen sooner or later. As they say, fall back to “plan B.” This involves slowly heating the chamber area of the firearm with something like a heat gun, hair dryer, or other heat source that does not produce a flame that can affect the firearm’s blueing or mechanical parts. I strongly recommend the hair dryer, using it with the gun barrel up and the chamber down over some container. With a hair dryer, it is fairly easy to heat something up to about 170 degrees Fahrenheit, so don’t overdo it.

When the metal is recovered, determine what held the casting in place and repour, taking more care in that area. One stuck casting is usually enough to make the next attempt better.

After the casting has cooled and “regrown,” measure the bore diameter first. This can be difficult if the number of lands and grooves is odd, but we must start somewhere. The groove diameter will tell us generally where to look in the cartridge information at our disposal. The next measurement should be the body diameter or case length. If these don’t give you a definite clue, jot down all the measurements such as the neck diameter, neck length, head diameter, and shoulder location and angle. You might notice whether the firing pin hits the breechface in the center or at the edge, narrowing the search if the cartridge is deemed to be a rimfire.

The charts at the back of each chapter in Cartridges of the World give these dimensions for each shell in that chapter, and are by far the handiest of any of the sources which I have. When you have found the caliber which most closely matches your chamber measurements, settle on this as the most probable. There will almost always be some dimension in the charts which will eliminate the cartridge if it isn’t the correct one. Remembering that the chamber must be slightly larger than the cartridge in order to allow the insertion and removal of the case, your measurements should be larger than those in the charts by several thousandths of an inch. Chambers need some length to allow for shell length variation, and the diameter includes a tolerance for the shell to expand when it is heated and thousands of pounds of pressure hit it at the same time.

If the casting shows irregularities on the walls of the chamber, look into the chamber to see if there is an area matching the blemish on the casting. If there is, bring this to the attention of the owner and, if you are not experienced enough to determine the safety of the firearm, suggest that he take it to another gunsmith for a safety check before shooting it. I would hesitate to make a blanket safety assurance without more thoroughly checking by Magnafluxing or with an x-ray. The courts have proven themselves too quick to rule against gunsmiths in liability litigation.

The firearm must now be cleaned and reassembled before returning to the owner. Take care to look into recesses and other places where some of the Cerrosafe could be hiding. Nothing is more embarrassing than having an owner return after going out to shoot with a gun not working because of something you overlooked.

One of the most valuable things you will learn as a gunsmith, is how to learn from your mistakes and how to recover from difficult situations. The most common mistake in using Cerrosafe is failing to disassemble the firearm enough before pouring the chamber full of molten metal. It took me a solid trigger group before I learned to slow down when pouring Cerrosafe into a partially disassembled rifle.

Brownells charges less than $10 for a half-pound ingot of Cerrosafe and a little more than that for a one-pound ingot, which should last forever. The metal doesn’t wear out, and you should lose very little each time you work with it.

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