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Thompson Barrels

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#1 Doug Richardson

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Posted 25 September 2005 - 10:31 AM

A while ago a company trying to manufacture Thompson barrels approached me. They did not have the drawings or the Thompson expertise necessary, so they wanted me to provide assistance. When I found out that they did not intend to use the proper steel, which they felt was too expensive and difficult to work with, and that they were not willing to have custom chambering reamers made, again because of the cost, I withdrew. I did not want my name associated with an inferior product.

As original Thompson parts become less and less available, the price rises to the point where people will start manufacturing them. I have no problem with that as long as the parts are made in accordance with the original drawings.

Coping an existing part is not manufacturing to the original specifications and is, at best, only a guess as to what the original design dimensions were. The tolerances allowed cannot even be guessed at because the design dimensions of the mating parts determine them.

Unless expensive laboratory analysis is done, there is no way of determining the material used or the heat treating procedures. I have now added the following information in my catalog which some of you who would find it useful might not see. So here it is:


Thompson barrels have a unique chamber design. Standard .45 ACP chambering reamers cannot be used. The U.S.Army specified heat-treated 4150 ordnance steel. Quality commercial barrels are usually made of the lesser grade 4140 steel, which is not suitable for machine guns. Making a new Thompson barrel to the original specifications (which requires having the original drawings) is therefore too expensive based on the quantities that can be sold. So, the only way to produce new finned barrels meeting all the original specifications at a reasonable price is to machine fins on new surplus M1A1 military barrels, which is what I do. Beware! If anyone offers to sell you new made Thompson barrels, ask what grade of steel was used and where they got the chamber specifications and heat-treating requirements. Reverse engineering is never a substitute for the original drawings.
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#2 wildwilly2002



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Posted 25 September 2005 - 12:39 PM

Thanks, for the info Doug.
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#3 PK.


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Posted 26 September 2005 - 11:51 AM

It would be interesting to know who has contacted Doug; without a doubt, more new manufacture Thompson parts will become prevalent in the very near future.

Replacement barrels ranging from poor to mediocre quality have been available for a long time, for this very reason I have been researching and preparing to manufacture my own as I am not satisfied with any product currently available.

While I agree with Doug on many points of this topic, I have to take issue with a couple of particulars.

“Quality” of steels is not determined by the alloy, but rather by the manufacture of the product; 4140 in not “lesser quality” than 4150, just a variant of the same family. Both are Chromium Molybdenum alloy steels identical in every way except one. The last two digits of the number indicates the amount of carbon present (.40%/ .50%). The additional carbon in the 4150 allows that alloy to be hardened to a higher degree (approaching Rc60), but this is irrelevant as the hardness required for barrels is about Rc25, and at that hardness, the properties of the two are virtually identical. It’s kind of like putting racing fuel into your car and driving 55 mph.

4140 has been the standard for barrel steel since the war for good reason; at the hardness required it provides all the desired properties, and is certainly suitable for machine gun use.

Many of the aftermarket barrels have been made of truly inferior materials and have not held up well.

While these alloy steels were available prior to the war, they really came into their own during and since, and the quality of the product is unsurpassed today. It should not be surprising that specifications for steels used in the manufacture of these guns and other products may not be considered as appropriate today as when the forges of democracy were banging them out; much experience has been gained since. Many of the alloys in use then have since been replaced altogether.

One other area that has concerned me regarding GI barrels is the quality of manufacture. We often have an attitude of “GI is best” and this is very true of the majority of circumstances regarding aftermarket parts for military guns. On the other hand, it is clearly recognized that the best commercially manufactured parts are usually quite superior because they were not made under the duress of war time production and generally meet higher quality standards. Remember the old phrase “good enough for government work”?

It has been my experience that while GI Thompson barrels are better than any currently available replacements that I know of, they are still lacking. If you put them between centers, they often look like crank shafts when turned, being inconsistent in both straightness and concentricity. Diameters very considerably in some areas, and the resultant variance in the wall thickness of the “tube” is not desirable. This is particularly evident in the 28 barrels with milled fins. Keeping in mind that TSMG’s are not match rifles, the resulting deficiencies in accuracy are not overwhelming, but they certainly can be improved upon. With the supply of surplus barrels drying up, the time for a quality replacement is at hand.

The requirement to produce parts to the original dimensions is unquestioned; you can not guarantee the interchangeability or even usability of a well established part or system through reverse engineering. The chamber dimensions Doug mentioned are a classic example. I know a major player in the Cl3 mfg. community who attempted to make a 45acp subgun but had great difficulty until he discovered the difference between the TSMG chamber and the standard pistol chamber he was using. These are the ”secrets” that have been carefully worked out in a mature design, and you need to know them to field a successful part or system.

Respectfully submitted

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