Do Index Lines Always Line Up On Original Barrel/ Receivers?

[LDog2012]

Do the index lines always line up on M1 and 1928A1 Thompsons that have original barrels and receivers? I think i read here that the index lines on replacement barrels often do not. Thanks for the replies.

 

[reconbob]

     This is tricky. If the index lines line up, OK. But if they do not, I don’t know that this would certify the barrel has been replaced. If the line on the barrel is 1/2 turn away from the line on the receiver that would (in my opinion) be a rebarreled gun. But if the line is within 1/10” either way I would say normal. Plus, not all barrels have draw lines and ring sights and compensators are fitted after the barrel is on the receiver.

   Thompson barrels are different than M1903, Garand, M1 Carbine, etc. because they have no sight bases, extractor slots, or gas ports that must be precisely located.

   On a Thompson you just torque the barrel on tight then fit the sight or comp in the correct position.

Bob

[Arthur Fliegenheimer]

Bob,

This topic came up in 2011. We agreed on the index lines acceptable degree of misalignment for Colt/AOC factory installed barrels with 2nd pattern Cutts.

                                                                                                 ****************

Colt guns ordered, or returned to the factory for the (circa 1932) 2nd pattern Cutts Compensator installation, had new Remington barrels installed. This would account for the slightest misalignment .010(?)-.013(?) of the barrel and receiver marks. Doug Richardson stated he has seen this on factory Colt/AOC installed TSMG barrels on Colt TSMG's where the barrel line is either above or below the receiver line. Naturally, the original Colt 1921A would have perfect alignment marks, or it would indicate a barrel change sometime after the gun left Hartford for good.

May 2011

 

 

 

[Stampcollector]

Great question! I have wondered about this myself.

On 7/5/2025 at 8:37 PM, LDog2012 said:

Do the index lines always line up on M1 and 1928A1 Thompsons that have original barrels and receivers? I think i read here that the index lines on replacement barrels often do not. Thanks for the replies.

 

 

[Rekraps]

So, what if a gun owner removed the original barrel, then put it back on, and the index lines did not match up. I do think this is a real possibility if the gun was worked on or if the owner just wanted to do so. Maybe the original was removed, a shooter barrel installed, then later the original barrel put back. The scenarios' are endless.

[anjong-ni]

Why on Earth chisel an "alignment mark" AFTER the barrel is tightened "for good"?      ...Phil

[68coupe]

I know that the witness mark on my replacement barrel lined up, but it took a LOT of effort. I'm talking about a 4 foot cheater and a Hurculean "Ooomfff" to make that last .010. Perhaps I was just lucky.

V/R, Karl, 68coupe

[Rekraps]

9 hours ago, 68coupe said:

I know that the witness mark on my replacement barrel lined up, but it took a LOT of effort. I'm talking about a 4 foot cheater and a Hurculean "Ooomfff" to make that last .010. Perhaps I was just lucky.

V/R, Karl, 68coupe

You got the "Ooomfff" man!

[TD.]

I agree with Bob, a very tricky question.

I believe if the Model of 1928 or Model of 1928A1 Thompson submachine gun is in original condition with the original finish, then I think the index or alignment marks should match. I would guess that this alignment was an inspection point by government inspectors during World War II. With guns that have been arsenal or depot or civilian refinished and/or updated, I would lean heavily on a barrel change sometime in the past, especially if the index marks do not align. The use of corrosive ammunition and poor maintenance in the field during WWII was most likely the cause for many barrel changes.    

 NOTE: It is very common to find Savage guns under the 20,000 serial number range without index marks on the barrel collar.

Arthur,

We missed you! 

I enjoyed reading that 2011 thread, especially the part regarding taking off and reinstalling Colt Thompson barrels. I used to think (because I read it many times on this forum) that if one removed an original Colt era barrel from a Colt receiver, usually to create a shooter Colt without the worry of damaging the barrel, aligning the index marks on a future reinstall would be problematic. Now, 14 years later, I don't believe this is the problem everyone was talking about back then. I have a several friends with the Richardson tools that have removed and reinstalled a number of Colt barrels in the last 14 years and alignment was never a problem. More importantly, Gordon Herigstad used to travel around the country (and probably overseas) with his Richardson tools removing barrels on Colt's serial numbered under 1000 and gangster guns with obliterated numbers and never had a problem with realigning the index marks that I heard of. Yes, I do believe if you remove and reinstalled a barrel numerous times (over and over again) on the same Colt receiver, there could be a problem. But who does that?

I do agree that the index lines on Colt Thompson's are for the most part in perfect alignment. That said, I have seen a few Colt's over the years that I believe are in original condition where the index line on the barrel collar is a 1/4th or maybe a 1/3th of an index line high or low to the index line on the receiver. The index lines match, it is just not 100% perfect - maybe 98% perfect, maybe just a scooch off as some would say. It is almost like your eyes are deceiving you. But the lines were definitely good enough to pass Major Barrett's inspection.

The Colt's with barrels that have the Type 2 compensators are a mixed bag. Yes, I do believe some/many are replacement barrels installed by Auto-Ordnance or Auto-Ordnance subcontractors (for example, Fenn Manufacturing). What I have noticed over the years is a number of these guns appear to have the index line on the barrel collar marked after the barrel was installed on the receiver. It is easy to tell because the index line on the barrel collar does not go all the way across the collar. There are other peculiarities to many of these barrels too - but no issue with originality.  It would be an interesting area of study; one I have never undertaken.     

All this said, a complete examination of the gun in question, Colt's, Model of 1928s, Model of 1928A1s or M1s, will probably provide enough information to judge whether or not the barrel has been changed. Refinished guns, a 100% blue barrel on an 80% receiver, the position of the compensator, flattened compensator pins and grossly misaligned index marks are key indicators something is not original. When examining a Colt, if anything does not appear perfect, it is a clue to do the most through inspection you can before spending your money. If there is any doubt in your mind, walk away. The doubt will never get better. 

All good stuff!!! 

[Arthur Fliegenheimer]

2 hours ago, TD. said:

I agree with Bob, a very tricky question. I have a several friends with the Richardson tools that have removed and reinstalled a number of Colt barrels in the last 14 years and alignment was never a problem. 

I do agree that the index lines on Colt Thompson's are for the most part in perfect alignment. That said, I have seen a few Colt's over the years that I believe are in original condition where the index line on the barrel collar is a 1/4th or maybe a 1/3th of an index line high or low to the index line on the receiver. The index lines match, it is just not 100% perfect - maybe 98% perfect, maybe just a scooch off as some would say. It is almost like your eyes are deceiving you. But the lines were definitely good enough to pass Major Barrett's inspection.

The Colt's with barrels that have the Type 2 compensators are a mixed bag. Yes, I do believe some/many are replacement barrels installed by Auto-Ordnance or Auto-Ordnance subcontractors (for example, Fenn Manufacturing). What I have noticed over the years is a number of these guns appear to have the index line on the barrel collar marked after the barrel was installed on the receiver. It is easy to tell because the index line on the barrel collar does not go all the way across the collar.     

 

Tom,

Taking off an original Colt/Remington barrel with Cutts Compensator that was in sight alignment would no doubt   be reinstalled matching  up to the previous setting using the tools shown in the video from another thread.

Many Colt TSMGs did miss out on the JHB stamp. Whomever was doing the final inspection on those Colts no doubt adhered to the same standards  as Barrett.  

The variable associated with the barrels witness mark and the receiver  witness mark lining up is when the Cutts type II was installed.  The thread below concerned the FENN MANUFACTURING COMPANY  method of Cutts installation. Was the new Cutts fitted with the new(?)  barrel on the receiver or off the receiver?  Cutts pins that are pristine would indicate any adjustments to align the sights were done during the time the barrel and Cutts type II were joined together.

If the witness marks is a skosh off but the Cutts is perfectly aligned with the Lyman front sight would the artisans recrank the barrel forward  or backward  to get perfect alignment and then fine tune the Cutts before drilling the hole and grove in barrel to accommodate the pin?  

 

[Motorcar]

My Bridgeport '28AC marks are not in perfect alignment, and yet looking at the proof marks and finish on both barrel and receiver I have no reason to think it has been re-barreled at some point. May never know for sure.

[TD.]

Arthur,

Excellent points. 

Show below is an example of an Index line that is damn near perfect but not perfect. It is on a 1927AC. I would guess AOC (or some subcontractor) removed the barrel and installed the Type 1 compensator and reinstalled the barrel. There was no doubt in my mind the barrel is original on this Colt. There is no one answer on index marks with Colt compensated guns (other than a mark needs to be present). Off hand, I don't recall ever seeing a reproduction barrel with a Type 1 compensator. 

The JHB inspection mark seems to disappear on many Colt's over the 10,000 serial number range. Many exceptions exist to this observation. My guess is AOC was storing many more guns than selling when the last 5,000 or so came off the assembly line at Colt's and this stamp was omitted. Again, just a guess.

I am not up to date on Fenn Manufacturing Company and the manufacturing and installation of compensators. I know Chuck (gijive) and Tracie (Taliaferro) have researched and studied this area extensively. I believe the answers to the remarks and questions you have can be found in Volume 1 of The Thompson Encyclopedia series. I am in my Skeet season and busy doing another Thompson project for the All Thompson Show & Shoot next month (everyone should be there). I don't have time right now to get up to date on this area. I suggest those with Volume 1 comment to this thread. And those that do not have Volume 1, get a copy. It will provide hours of great reading! 

All good stuff!!!   

[Arthur Fliegenheimer]

6 hours ago, TD. said:

 

I am not up to date on Fenn Manufacturing Company and the manufacturing and installation of compensators. I know Chuck (gijive) and Tracie (Taliaferro) have researched and studied this area extensively. I believe the answers to the remarks and questions you have can be found in Volume 1 of The Thompson Encyclopedia series. 

Tom,

The 2016 thread  below discussed  the Fenn Company  and their involvement with Cutts Compensators.  Doesn't appear to be any new information in Hill's 2022 Volume 1  on how Fenn Manufacturing Company used their "device" for attaching Cutts Compensators to barrels. Don't know as of this date whether Chuck's inquiry to Fenn resulted in any response.

 

Here is the letter as shown in Hill's Volume I that was previously  uploaded to the 2016 thread. Still no resolution on whether the Cutts was installed with barrel on or off Colt receiver.

Edited July 10, 2025 by Arthur Fliegenheimer

[TD.]

Arthur,

I enjoyed looking over the 2016 thread. I don't have the answers nor the time to get deep in the weeds on this. Hopefully Tracie and Chuck will jump in. I was thinking Fenn Manufacturing invented some device that allowed them to mount the compensator on the Thompson barrel without removing the barrel from the receiver. This would have speeded up the process (time) and reduced the cost. It would be perfect for the new guns converted to the AC or NAVY models - of which there were many. Of course, if the barrel needed to be changed first, i.e., for a bulge or ring, then it really did not matter.  

[anjong-ni]

I believe that I mentioned, at the time, that a "device" to allow perfect threading of the barrel while it was "still on the receiver" would be simple to make.

A round bar that indexed into the buffer hole from the lathe chuck. and from there, an extension that went around, into the receiver and indexed into the chamber.

Use the live center at the muzzle, spin it up and cut perfect threads.

Something like this is undoubtedly what Fenn used.   ...Phil

[Arthur Fliegenheimer]

16 hours ago, anjong-ni said:

I believe that I mentioned, at the time, that a "device" to allow perfect threading of the barrel while it was "still on the receiver" would be simple to make.

A round bar that indexed into the buffer hole from the lathe chuck. and from there, an extension that went around, into the receiver and indexed into the chamber.

Use the live center at the muzzle, spin it up and cut perfect threads.

Something like this is undoubtedly what Fenn used.   ...Phil

According to the above letter from AOC to Colonel Cutts the Fenn device  solved the problem of avoiding damage to barrels during the process of  proper attachment of the 1st pattern Compensator. If your labor-saving solution  was how Fenn cut the threads to John T. Thompson's strict standards then AOC wouldn't be complaining to Cutts that the Fenn method was still time consuming and costly. 

Maybe the "clever device" was strictly  about  removing the barrel  and resembled the Doug Richardson tools pictured.

 

[Arthur Fliegenheimer]

On 7/11/2025 at 12:17 PM, TD. said:

Arthur,

 Of course, if the barrel needed to be changed first, i.e., for a bulge or ring, then it really did not matter.  

According to Tracie Hill's Volume I (as shown in the above post) the Marines attempted to install 1st pattern Cutts on their Colt TSMGs with poor results.  U.S.M.C. Richard Cutts blueprint didn't foresee the problems associated with fitting the comp to the barrel boss.

AOC replaced the bulged barrels and compensators when returned to them free of charge. Doesn't specify if the this meant the Marines sent their receivers along with the barrels. Opens up the prospect of  AOC attaching the Cutts with the barrel off the receivers.  

Richard Maclean (Small Arms Review contributing writer) wrote in American Gunsmith that the witness lines on the 15,000 Colt TSMG receivers and barrels were used for initially installing the barrels and then removing them for "bluing and sight installation. "

 

 

 

 

 

Edited July 13, 2025 by Arthur Fliegenheimer

[TD.]

Arthur - my comments in red font.

According to Tracie Hill's Volume I (as shown in the above post) the Marines attempted to install 1st pattern Cutts on their Colt TSMGs with poor results.  U.S.M.C. Richard Cutts blueprint didn't foresee the problems associated with fitting the comp to the barrel boss.  I have no reason to doubt this reason but do not know where this reason is documented. Of note, Doug Richardson and Tracie Hill both say the same thing. Whatever the reason, the compensator design was changed several years after introduction. Something obviously warranted a reason for the change. I will say that Colt Thompsons with Type 1 compensators properly installed seem to be working fine nearly 100 years later.  Regarding the Cutt's letters, there is no known complete collection of Cutt's writings. It appears all his letters and telegrams were scattered around after his death and/or when the Thompson assets of Maguire Industries, Inc. was sold. What readers do not know is what Cutts discussed in the previous or later letters on this particular subject. It is definitely a data point to recognize but may not tell the entire story. Cutts was a prolific writer. Many times, he wrote letters to the same person day after day. 

AOC replaced the bulged barrels and compensators when returned to them free of charge. Doesn't specify if the this meant the Marines sent their receivers along with the barrels. Opens up the prospect of AOC attaching the Cutts with the barrel off the receivers.  I believe I remember reading in Gordon's 6th Edition that AOC sometimes replaced receivers for the Marines when guns were sent in for repair. Some of these replaced USMC receivers were refurbished and later sold in the commercial marketplace. The Marines, later the U.S. NAVY, were good customers. I would assume customer service was excellent to these customers. 

Richard Maclean (Small Arms Review contributing writer) wrote in American Gunsmith that the witness lines on the 15,000 Colt TSMG receivers and barrels were used for initially installing the barrels and then removing them for "bluing and sight installation. " I would really like to know where Richard Maclean obtained this information. I always try to authenticate the source of "new" information I publish. Perhaps Tracie has the answer in his many archives or in his excellent Gage book. I understand from Turnbull Restoration Company the bluing process was different on the Colt Thompson receiver and frame versus the barrel in 1921/22. Making an index line on the barrel collar would make sense early on when the barrels were numbered as I doubt the bluing process involved only one barrel at a time. As for fitting the front sight, I would guess the barreled receiver was placed in a jig and the front sight was pressed on and pinned. There had to be a good reason for the index lines as it cost money and time to make these marks. Perhaps there is a notation on the blueprints or Colt/AOC contract.  It is interesting to note the early Savage Thompson guns did not have an index line on the barrel collar. This line or mark was added after production started, probably 4 to 5 thousand guns into production. One has to wonder why this mark was added if the earlier guns worked fine without the index mark. Oh, so many questions! 

[anjong-ni]

Hi Tom, love your books.

I do remember being pilloried years ago for suggesting that "the barrel reference line was staked" because the barrel was going to be "initially installed...then removed".

In reviewing pictures of the mark, it's obvious that SOME of the stakes  were chiseled with the barrel..."off".

The chisel deformation extends past the inside flange. Not possible to get a chisel in that close.

My guess?

The barrel line was "penciled-on" while the barrel was first attached.

Upon removal (for sight installation?) the pencil-mark was chiseled to insure correct re-installation down the line.  ...Phil