How to Determine an Unknown Chambering

How to Determine an Unknown Chambering

It's a problem most gun owners never encounter, but it's one just about every gunsmith has seen. Someone brings in a gun with no caliber designation. The question becomes what caliber is it and how can that be determined.


Over the years I've encountered this situation a number of times and have seen it dealt with in many different ways — some good, some bad. Perhaps the worst and potentially most dangerous were the cases where folks just kept trying to chamber different cartridges until the bolt closed on one. As far as they were concerned, that was the correct caliber!



Needless to say, that procedure is dangerous. In fact, I heard recently of a young guy who did it and ended up touching off a 7.62x39mm round in a rifle that was actually chambered for .243 Winchester. There was considerable damage to the rifle, but fortunately the shooter's injuries were minimal. I also recall a guy who came into my shop years ago with an old, beat-up Japanese Arisaka rifle in 7.7mm. I happened to ask him what he was shooting in it since ammo in that caliber was kind of hard to find at the time. He proudly pulled out a box of Remington 8mm Mauser cartridges! I remember being absolutely speechless at the time. I couldn't believe it. It was truly a testament to the strength of the old Arisaka that neither he nor his rifle had been injured.


There are better, infinitely safer, and more accurate ways of determining an unknown caliber. Not long ago a friend who has a gunshop sold me an Austrian-made Voere combination gun. It has a 12-gauge barrel on top and a rifle barrel underneath. The calibers are plainly stamped below the breechblock: 12 gauge and .22 Magnum. The problem is someone in the past had done some rather interesting gunsmithing and had converted the rimfire to centerfire. The unknown gunsmith had also re-cut the chamber.


My buddy was not at all sure what the new chambering was and was understandably very reluctant to sell the gun because of liability issues. Consequently, I got a great deal on a very interesting and unique firearm. But before I could use it, I had to determine the identity of the unknown rifle caliber.

I should point out that my friend thought the rifle might be chambered for .219 Zipper, and that certainly made sense because the cut in the extractor for the case head appeared at first glance to match the diameter of a .30-30 case, which was the parent case of the Zipper. However, there were several variations of the Zipper, including the .219 Donaldson Wasp and the .219 Improved Zipper. It could be any or none of them.

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Casting the Chamber

There have been quite a few wildcat cartridges based on the .30-30 case, which is especially suitable for single-shot and break-open firearms like the Voere. The only way to accurately identify the correct cartridge is to determine the dimensions of the chamber. That necessitates making a chamber casting. A chamber casting is basically just a model of the chamber made by pouring a liquid into the chamber that then hardens. The hardened casting is then removed and measured to determine the specific caliber.

The first step is to take the barrels off the frame. Then carefully and thoroughly clean the rifle barrel and chamber. For this report I used Birchwood Casey Gun Scrubber. I placed special emphasis on removing all fouling and oil from the chamber. Since I planned on making a casting of the rifle chamber, any residual oil would create imperfections in the chamber cast, and these imperfections would make accurate measurements of the casting difficult or possibly impossible.

There are several different materials you can use to make an accurate chamber cast. Years ago, when I first started gunsmithing, I was taught to use sulfur. Sulfur makes a very nice casting, but heaven help you if you overheat it. It'll burn, and your shop will literally smell like hell! Take it from one who has experienced it, avoid the use of sulfur.

A much more appropriate, safer, and cleaner casting medium is Cerrosafe. Sold by gunsmith suppliers, this alloy, I believe, is made primarily of bismuth, lead, and tin, and it'll generally melt somewhere between 160 and 190 degrees Fahrenheit. In fact, it's so easy to melt, I used my hot air gun right at my bench. By the way, once you have finished using the hardened casting, you can melt it down and use the Cerrosafe over and over again. With a little care, the Cerrosafe will last for years.

After cleaning the chamber, use a cleaning rod and jag with a tight-fitting patch to plug the bore about a quarter-inch or so ahead of the chamber. Then position the barrel vertically (with the chamber up) in a padded vise. Warm the chamber area with your hot air gun and pour in the Cerrosafe. It takes just a few minutes for it to harden, and once it's solidified, use the cleaning rod to push the casting out of the chamber.

Measure the Casting

Then use a micrometer and a digital caliper to measure various points on the casting. I made a crude drawing to record the dimensions, and then I started doing some research to find the cartridge that matched the dimensions on my drawing. Two books were especially helpful: Wildcat Cartridges by Wolfe Publishing and The Handloader's Manual of Cartridge Conversions by John J. Donnelly.

Based on the dimensions from my casting and the information found in my resource books, it appears I am the proud owner of a .219 Donaldson Wasp. Many years ago I swore I would never own a firearm chambered for a wildcat. Well, so much for swearing!

Now I'm going to have to look at picking up reloading dies, getting into forming cases, and then working up loads. Actually, that doesn't sound so bad now that I think about it. In fact, I just might enjoy owning a wildcat after all! And oh yes, I will mark the chamber with the proper caliber designation.

Until next time, good luck and good gunsmithing!

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