Actually using the cerrosafe is pretty easy. And it gives you exact dimensions on your chamber. I sure understand your frustration, I've been in that position several times.
The sharp points on a digital caliper may get you pretty close to getting an idea of your case thickness.
Here's Brownell's instruction sheet for using cerrosafe. It can be used over and over.
CERROSAFE™ – The Gunsmith’s Secret Weapon
By: Andrew Swan
Those of you who’ve worked around guns a lot know it’s inevitable that someone will walk into your shop with a rifle, shotgun or handgun that they found in Grandpa’s attic and want to know what it is and if it is safe to shoot. You take a look at the gun and, for the life of you, you’re just not sure what it is.
There aren’t any markings on it, and it looks like somewhere along the line someone may have done some re-chambering. What do you do? Knowing what round the firearm shoots is only second in importance to its safety. It’s time to dig into your work bench and dig out the Cerrosafe™ - use it to get a positive copy of the chamber, neck, throat and bore and then you can answer both questions for your customer.
is an alloy containing bismuth, lead, tin, cadmium and indium. Bismuth alloys of approximately 50% bismuth, like Cerrosafe™, exhibit little change of volume during solidification. Cerrosafe™ shrinks slightly during initial cooling, and then expands to the original size of the casting about one hour after cooling. After 200 hours of cooling, the casting will expand approximately .0025” over
the actual chamber size. Cerrosafe™ is completely re-usable; the chamber cast can be re-melted and reused after all necessary measurements have been taken.
Chamber casting with Cerrosafe™ is relatively simple and requires only a few hand tools. Cerrosafe™ has a melting range from 158°F and 190°F. It should not be overheated, as this can cause a separation of the components of Cerrosafe™, resulting in dross, which must be skimmed off prior to casting. Melting Cerrosafe™ in a double boiler, in a Stainless Steel Dipper
using a torch, or any method of providing indirect heat is best. Slowly melting Cerrosafe™ will ensure that no separation takes place and will give you the best results. Make sure you melt the entire ingot each time for proper results. Cerrosafe™ can be reused over and over again, and can be poured into any size ingot for future use.
To determine the chamber size, disassemble the firearm as needed to gain access to the chamber. Remember; always check to see if the firearm is unloaded before beginning any work on a firearm. If you have a firearm such as a bolt or lever action rifle that does not have ready access to the chamber, you must make a pouring tube to pour the molten Cerrosafe™ into the chamber. Tubes can be made of steel, brass, or aluminum tubing and should be flared into the shape of a funnel at one end.
Keep the tube as short as possible to prevent the Cerrosafe™ from solidifying in the tube. Make sure that the barrel and chamber are clean, degreased and dry, using a product such as Brownells TCE Cleaner Degreaser
. Insert a tight fitting cotton patch on a cleaning jag attached to a cleaning rod into the bore from the muzzle end. This will serve as a “dam” to prevent the Cerrosafe™ from running too far down the bore. Position the patch about ½” to 1” ahead of the throat in the chamber. Heat the barrel at the chamber just to the point of being uncomfortable to hold in your hand.
Heat the Cerrosafe™ as directed above and carefully pour the Cerrosafe™ into the chamber until it shows a slight mound at the rear of the chamber. If there is too much material in the chamber, removal of the casting can be difficult. After the Cerrosafe™ has solidified (usually in about a minute) the chamber cast can be pushed out of the chamber.
The chamber cast must be removed within 30 minutes
after casting. If more than one hour elapses after casting before attempting to remove the chamber cast, the Cerrosafe™ will start to expand will have to be re-melted and allowed to cool in order for it to be removed.
To remove the casting, clamp the barrel in a padded vise and place a folded towel under the chamber to catch the casting when it drops. Tap the cleaning rod with your hand and the casting should slide free of the chamber. Although the casting is relatively hard, it can be damaged if it falls onto a hard surface.
Revolver chambers can also be cast with Cerrosafe™ if a plate of smooth, hard material such as aluminum or Masonite is clamped over the front of the chamber mouth. Disassemble the cylinder as completely as possible, removing the extractor star from double-action revolver cylinders. Be sure to cut a clearance hole for any gas ring or bearing surface at the crane or center pin location on the cylinder.
When Cerrosafe™ is used to determine chamber dimensions for identification of unknown chambers, make sure you allow for full expansion of the cast. Remember that a chamber will almost always be several thousandths of an inch larger in all dimensions than the cartridge to allow for proper feeding and functioning.
Once you have determined the size of the chamber, you can use the Books of Chamber Prints
to determine the correct cartridge to use. You will also be able to determine if the chamber can be safely fired. After determining the cartridge needed (and determining that it is safe to fire), stamp the caliber on the barrel to prevent the wrong cartridge from being used. You can use the 1/16” or 3/32” Figure Stamp Sets
for this process.
Your customer will appreciate knowing the correct ammunition to use in Grandpa’s old shooter, and odds are, they will come back to you for all their gunsmithing needs.