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Guns & Ammo Network


How to Determine an Unknown Chambering

by Reid Coffield   |  April 18th, 2012 10

This Austrian-made Voere combination rifle/shotgun was originally produced with a standard 12-gauge barrel over a .22 Magnum rimfire barrel, but the caliber marking below the rifle barrel is no longer valid. The rimfire chamber has been reamed, lengthened and enlarged, and the new chambering is not marked.

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.

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!

  • mrwedge

    There. I learned something today.

  • Robert Dunn

    Excellent article and it will hopefully save someone from getting injured. Gunsmithing is not a "close enough" or a guessing type of job.

  • Alan_T

    I'd like to know if there's any way to remark the chambering on the gun for safety , that doesn't look like hell ?

    • Guest

      Maybe TIG weld to fill the now incorrect lettering, grind smooth, and reletter? Maybe engrave or etch the new lettering rather than stamping it?

  • frenchy

    I'm surprised the .22 WMR barrel and chamber were able to to be used for the .219 Donaldson Wasp, or had the previores gunsmith already changed barrels etc.?


  • Ronny G. Jones

    I have encountered similar problems-Purchased a very old "custom made" rifle marked 300 mag and that was all. No way a 300 wim mag or a 300 weatherby mag or 7mm mag wouled go in but a 270 win and 30-06 dropped outta sight. So I used a wide variety of "tools' including grounddown/whitteled dowell rods to make a dummy cartridge and an investigation of reloading specs.-It was a 300 H & H mag and had been "built' from a 98 mauser in about 1936-1938 and at that time (I believe) it was the only mag around. and another "project" rifle that someone had buffed down (and took off all the markings) except the name and model. The guy at pawn shop told me he throught it was a 7X 57 and I got a buddy to "slug" or "plug" the barrel for me and it was .308 win so this problem is pretty common among those iof us who"deal" with a lot of guns. RGJones (dba-great guns)

  • Gary

    I traded for a ruger number 3 that had been a 22 Hornet, it to was rechambered to a wildcat on a 30-30 case. I casted the chamber with parifin, and measured the casting. It took a lot of research and I was able to find it is a 219 zipper ackley improved with a 28 degree shoulder. So many so called gun experts at the gun show where I traded for it were trying to stuff everthing you could imigine into it and telling me what it was and none of them were close. then when I went to the next show I was telling some of them about what I found it to be and they all told me that ackley never made a 219 zipper improved with a 28 degree shoulder, I told them thats funny, it is in his book that he wrote. case in point, not all guys selling guns are true gun experts.

  • Jim K

    I'm just finishing up a Mauser in 7X57. I plan to inlet a case head in the bottom of the stock's handgrip; but, I'm undecided as to where to mark the barrel and/or action. I had intended in using the bolt knob, but I did too good a job on hardening/annealing the knob and don't want to screw up the amber coloring. I'm always concerned about visibility and finish integrity.

    I do really like Cerrosafe and use it whenever I get a suspect firearm. It's also useful in evaluating erosion and to determine when a chamber has been incorrectly reamed (something that experienced in my personal Springfield M1A).

  • Neal M.

    I'm sure that you'll be tickled with you .219 Donaldson Wasp. I had Bullberry make a 21 inch barrel for a Contender frame….it's a real varmit killer. Cases cn be formed more easily from 25-35 brass.

  • creid

    Love this. Thanks for the post makes me wish i had become a gunsmith

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