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What Makes A '21 Barrel A '21 Barrel?

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The question of defining the characteristics of Colt Thompson barrel fins has been in the air for many years. The general consensus of the public seems to have been that 21’s have thin fins with rounded edges and military 28’s thick fins with square edges. In this discussion, ’21 barrels would include all Colt made guns as ‘28 Navy and 1927’s started life as model 1921’s .


I have been doing some research on the subject (preparatory to manufacture) and have consulted with several notable collectors in the Thompson field who have participated in a survey measuring and noting the characteristic of original ‘21 model barrels; the results may be somewhat surprising to many.


It would appear that the notion of fin edges having a full radius is not correct. Only one barrel has been so noted to date and it is believed this gun was re-barreled many years ago (perhaps with a 1919 part). It is generally held that the 1919 guns had this feature, but the vast majority of ‘21 models surveyed have barrels with the edges of the fins broken only. This would be consistent with other clues as to how they were actually made.


The original run of Colt made Thompsons were the “A” model and had a 5/8” diameter boss on the end of the barrel to which the front sight was press fit. When the Cutts was introduced, some of these guns had the sight removed and this boss threaded for attachment of the new device (5/8” thread), becoming the “AC” model. Later, new barrels were made and the boss was increased to ¾” diameter to accommodate a ¾” Cutts (2nd generation on). Guns subsequently returned to the factory for fitting of a Cutts compensator were re-barreled to make the change. It is interesting to note that the fins of these two separate production periods of barrels did not seem to measure any differently in the survey.


The 1919 dated A/O print has a specification for the width of the fin and pitch (spacing), but most barrels measured do not comply with these dimensions. The original specification was for a fin width of .040-.050, but this was changed in a revision dated 1-19-1935 to .070-.080 (which generally corresponds to the parts measured).


How could this be if the barrels were made in the early 20’s? One thought is that A/O had let their engineering staff go when they moved to Hartford. Problems likely occurred trying to produce the thin fins, and the change was penciled into the print on the shop floor. With no engineering staff to maintain the drawings, and the production completed with no follow-up contract looming, the changes were not finalized on the drawings. In ‘35-’37 A/O was preparing to sell the company and it is likely that cleaning up the drawings was part of that preparation. This is speculation of course, but it does make sense.


While the groove width seems to remain consistent within the given part the pitch varies, resulting in a variance of fin width. This would seem reasonable if the grooves were formed individually with the same tool, the operator moving that tool for each cut. As the tolerances stacked up, this would also result in a changing of the location of the “21st fin” that is supposed to rest on the pad of the grip mount. Many examples have been noted where the fin was barely on the grip mount pad and one has been reported where the pad actually fell between the fins. The operator likely broke the edges of the fins with a file, and subsequent polishing rounded them a bit, but no example of machine formed full radius has been noted outside the one questionable specimen mentioned above.


Between individual barrels, the groove widths will vary dramatically. This is also understandable as tools were worn, sharpened and replaced. The existence or absence of a radius in the corners of the groove could be accounted here as well. While the print clearly shows a radius, it was not specified and the tool grinder was on his own.


So, what are we talking about here? 14 guns have been recorded to date.


Groove widths in the survey ran from .115 to .145, averaging .1265.


Fin thickness often varied .010 or more in the individual barrel (sampling 3-10 fins per), running from .055 to .081 averaging .0733.


For reference, the GI (1942) ’28 barrel print shows a fin width of .065-.085. The GI barrels I have measured seem to be within this range, most about .070. It is also interesting to note that the pitch is more consistent in these war time barrels as it would appear the grooves were formed simultaneously with a gang of cutters, either on a lathe or mill. The latter milled fin barrels have inconsistent walls due to run out problems; ok for shooting the enemy I suppose. Because these barrels were not polished, the edges of the fins remain somewhat sharp.


Contributors to this survey were selected (in part) because their accepted standing as knowledgeable persons concerning original Colt produced guns and parts, their access to original guns and their ability to measure these guns. I wanted to be as sure as was reasonably possible that the data collected was credible. Some of the attributes were subjectively observed. Obviously, this is not a comprehensive study, but It’s a start. There are many more of you out there who could be qualified to expand this data.



The difference between the ’21 and ’28 barrels is fairly obvious to the knowing eye, but perhaps not by the width of the fin or the condition of it’s edge only. These other difference are fodder for another battle however.


Thanks to all who participated and gave freely of their time and energy.


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Excellent study, PK. I have noticed the differences in Colt barrel fins just from observing many guns over the years. I always believed that these differences were attributable to several different workers making barrels at Colt.


Thanks for taking the time and energy to study the issue.

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When will your 21 finned barrels be available and what will be the approx. turnaround to install one? Thanks.
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PK, that is great data!


I have always been wondering how dimensionally feasabile it would be to create a 21 replica barrel from a current production 27 (finned) barrel. Seems like it is do-able, and all I need to do is get that lathe here from my folks house... Of couse the comp threads are different, they would have to be like the early 5/8 barrels. Anyone got a spare early comp available? http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/wink.gif


Thanks again to all for the data! http://www.machinegunbooks.com/forums/invboard1_1_2/upload/html/emoticons/hail.gif


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Great info PK!

Thanks, C

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I have been using my own barrels in house for a while and hope to have them available for spares sales soon.


If you wanted me to install one on your gun you can have it now. Because I stock these as semi finished, any configuration can readily produced.


I would be happy to discuss your particular needs via e mail.



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