1928 Thompson Barrel Questions....

[Blanksguy]

Questions about original Government contract Thompson SMG replacement barrels:

 

When the US Government contractors made up replacement barrels for the 1928 series (threaded-muzzles, with or w/o "fins")......when the outside of the muzzles were threaded......was a notch cut for the Comp./Front-Sight Pin (?).....or was this area only threaded (to later be reamed for the notch when the Comp./Front-Sight was mounted for proper location (?).

 

Was this the same for the M1 and M1A1 Thompson SMG Government Contract replacement barrels with the exception of not having the outside of the muzzle threaded (?).

 

Last question......Will a 1921 Model Front-Sight-Assembly (no-comp.) screw onto a 1928 barrel's muzzle......or does/will an M1/M1A1 type front-sight-assembly be required (?).

 

TIA, RichardS in MI.

Blanksguy2001@chartermi.net

[1921A]

The front sight pin groove was milled in after the sight was installed. Most M1928 replacement barrels were threaded for the compensator. Some were not.

 

1921A barrels are not threaded on the muzzle end - the 21A front sight was pressed on and then pinned.

 

The M1/M1A1 front sight is similar to the 21A sight and can be fitted to an unthreaded service replacement barrel if you can find one. It should be noted that the pin hole was not cut into the sight until it was installed on the gun. This was true with both types of front sights. It was milled or possibly drilled using a fixture of some type. I have never seen a picture of that particular operation in any of the reference books. Maybe PK could answer that one.

 

The easiest way to do this is to buy a non threaded aftermarket barrel from someone like Doug Richardson.

 

Greg Fox

[Uncle Dudley]

I just bought this 28A1 that has what looks like an M1 ring sight on the 28 finned barrel. I like the looks and shooting in this configuration. I guess I could change it back to a comp, if I wanted to. You can see the threads just behind the sight. I don't know when or where it was done. The barrel has a "block" S on the right side by the reciever (fuzzy picture).

 

UD

 

http://img.photobucket.com/albums/v104/overspray/tsmg/100_0236.jpg

 

http://img.photobucket.com/albums/v104/overspray/tsmg/100_0238.jpg

 

http://img.photobucket.com/albums/v104/overspray/tsmg/100_0237.jpg

 

[reconbob]

Yes, when Thompsons were manufactured the barrels were fitted

to the receivers, then the sights or comps were put on in the correct

position, drilled and pinned. So you won't see a new barrel with the

relief cutout for the sight pin. Likewise never-used sights are not yet

drilled for the cross pin.

You can fit the ring sight right over the end of a threaded barrel

as on the gun you had. I would assume that as the war progressed

and the transition was made from finned/comped barrels to smooth

barrels with no comp that there could be "factory" guns like yours.

And of course its also entirely possible that this was done either

during a legit ordnance repair or rebuild, or by some one else after

the gun was put on the registry.

 

Bob

[giantpanda4]

One addition to the comment about the 21As... they were not threaded from the factory, but all of them that were modified to 21ACs were threaded - with a different thread than you see on the 28s. It is a smaller diameter thread because there was not as much metal there as in the 28 barrels.

 

[artie in miami]

At Knob Creek last Spring, SARCO had a brand new, old stock bbl for the 1928 w/ compensator threads..still wrapped in some type of wax/gauze...They wanted around $300 for it then. It did not have the groove for the comp pin milled in...and I think that makes sense...what if when they screwed it into a receiver, the groove was not at 12 o'clock, dead center. I am not positive, but I doubt that all receiver threads were exactly the same !! I did not buy it, and I am sorry that I didn't. Shoudda/Woudda/Coudda!!

artie in miami

[richard w.]

The receivers on M1928/M1928A1 Thompsons had "qualified" threads, that start at the same place on all receivers.

 

According to Doug Richardson's Thompson Technical Volume 1 (p. 25):

 

QUOTE

... Its purpose is to enable any barrel to screw on to any receiver such that when properly tightened, the front sight is is positioned at the top of the barrel.  This is important for guns which have the sight pre-machined on the barrel.  It may have been that the Thompson front sights were fitted on the barrels before the barrels were fitted to the receiver.  If so, the witness marks [scribed on the side of the receivers] make some sense.  When the M1/M1A1 models were introduced, the system of qualified threads was abandoned. ...

[1921A]

Qualified threads.....

 

This is sure to start a war of words but....

 

These days there is a wealth of written information on the Thompson but, a great deal of it is "theory." And, because it is offered by persons we consider "reasonably knowledgeable individuals," we often "assume" it is fact. Example - magazines! For years it was accepted belief that Colt's made the early drums and stick magazines for the Thompson. They did not! Other examples; the so called "mixed markings" guns are super rare. They are not, the canvas "spare parts pouch" for the 1921 is the rarest Thompson accessory! Not true - they're popin up everywhere. In fact, they may not even be for Thompson parts! The list goes on....

 

I can't help but respectfully disagree with the "qualified threads" theory absent some concrete evidence to support it. I don't dispute the use of qualified threading, only the theory suggested. The notion that you could or would install front sights on 21/28 barrels before they were screwed into the receiver during mass production seems unlikely to me. Maybe reconbob could weigh in on that one.

 

Somewhere along the line, in my years collecting and researching anything Thompson, I seem to remember reading about sight installation and alignment fixtures used during production. The notion that the machines and technology available during the production era could regulate the alignment of fully configured, mass produced, "sighted" barrels seems unlikely. Remember, much of the machinery used was left over odds and ends from as far back as the Civil war. Just my 02

 

Any qualified opinons on this?

 

Greg Fox

[reconbob]

I think I've processed, studied, and measured more actual Thompson receivers than anyone. From a

1000 gun reweld project that never happened back in the 1970's, to processing many tons of torched

guns for parts retreival. I made I don't know how many thousands of 80% receivers to the specifications

of U.S. Ordnance drawings dated 1942, and manufactured shooting M1's in the early 80's. Through this

all, I have never seen any evidence that Thompson threads or barrels were "qualified" or "timed". Consider

the following:

 

The original drawings do not call for the thread in the receiver to be timed.

 

The original drawings do not call for the thread on the barrel to be timed.

 

Original in-the-grease barrels are not yet fitted with sights, do not have the cutout for the

sight pin yet, and most do not have a draw mark.

 

Over the years I have repeatedly attempted to qualify the thread in the receiver - easily done

in this day of computers controls - and while you may get it right for a series of barrels, not all

barrels are the same. If the barrels were made with the same qualified thread, then one receiver

thread would give the same result for all barrels - i.e. they'd all get tight with the sight in the

in the same position. This does not happen.

 

So whether the source of this statement is an old manual, or gun magazine article, or gun

expert, in this case the source is wrong. This happens because articles and manuals tend

to be repeatedly used as sources for future research and often statements become cliches

and are never questioned.

 

Maybe, and I've never seen anything to support it, the 1921 Colt production run had qualified

barrel threads, and maybe Colts production process was set up that way. There are no original

spare receivers and barrels to check. But without a doubt the M1928 and M1 series did not have

qualified receiver or barrel threads.

 

My $0.02

 

Bob

[1921A]

Hi Bob:

 

I thought you would have the answer on this one. The "timed or regulated thread theory" just did not seem feasible given the time period involved. I have never seen a NOS barrel that was already grooved, Colt or otherwise. I think the theory is just that - a theory...

 

By the way Bob - you need to go to Knob Creek this fall to talk MGs and drink beer!!!

 

Greg Fox

[richard w.]

Great thread, guys. Thank you for your input.

[reconbob]

They had the "technology" for timed or qualified threads. Remember the Krag, the Springfield,

the M1917, and the BAR had all been made before Colt made the Thompson. All those rifles

had "qualified" threads on the receivers and barrels because the barrels had sight bases,

gas takeoff holes, or extractor cutouts at the chamber. (The BAR has all three.) But even in

the production environment when barreling receivers you selected a barrel with the correct

"draw" from a pile, and might have to try at least several before you matched a barrel with

a receiver that could go in the tightening apparatus. But timing or qualifying threads is

difficult and time consuming and certainly if you don't have to do it you wouldn't. Annnd,

since a Thompson barrel has no sight bases, extractor cutout, or gas takeoff hole, no

reason to time it. Much easier, sensible, and economical to put the sight or comp on in

the right spot after its fitted to the receiver.

 

Bob

[PK.]

I just doesn’t make sense that wartime production guns would be made with timed threads, especially if there was no good reason to do. It is well documented that TSMG sights (28 and M1) as well as Cutts compensators found in the parts inventory are not drilled for the retaining pin. Neither are parts barrels. This evidence alone will support the premise that the sights were not installed until the barrel and receiver were joined. There is no other reason to time the barrel/ receiver threads on a TSMG other than sight alignment- so why do it?

 

I don’t have an answer to the above question, but I am convinced it was, in fact, done.

 

Several years ago when parts sets started to become readily available, I set aside some ’28 receiver noses from both AOC and Savage production. I noticed that the draw marks on all of the barrels that came off these pieces aligned (generally) with the draw marks on all the receiver noses. Understand, the 10 pitch thread used on TSMG barrel threads is quite coarse, and getting an exact match of the draw marks isn’t easy while applying adequate torque. Most do not match exactly from the factory, but are within a degree or two.

 

Since that time, I have run every GI barrel that came through the shop over those reference receiver noses and only one was out by any degree at all. I stopped taking notes long ago as there was no variance significant enough to record. You might say ‘well, it’s only a couple of receiver noses’, but keep in mind, each of those barrels had a draw mark that aligned with a receiver at some point, so if they now align with the reference pieces, they had to be made generally the same. I have not seen an original gun where the draw marks were not close.

 

It is easy to see a mark that has been made after the finishe was applied. All draw marks on barrels I have seen were blued after the mark was applied. I have never seen any evidence that this mark was applied after the barrel was installed. Many photos have been published on this board of NOS parts barrels with the draw mark inscribed. In fact, the only parts barrels that I know of that lack the draw mark are later M1 barrels.

 

I would love to find a GI print that gave the timing for the barrel thread in the receiver, but I have not been able to view such a reference. I do however have a GI print for the barrel (D53509 Dec 31, 1941) that shows the draw line and reads as follows: “Draw line- must match thread qualifying gage. See mating draw line on side of receiver- D35512”.

 

The significant amount of empirical evidence I have seen, combined with the lack of cross pin cuts in the parts inventory and the notes on the barrel print have convinced me that the receiver/ barrel threads on all WWII ’28 TSMG’s were in fact timed, even if they didn’t need to be and it made no sense to do so.

 

The M1’s would be another study but I haven’t amassed enough evidence to make a claim. I would venture that the later war guns dispensed with the requirement to time the threads.

 

I feel confident in saying: TSMG 28 wartime production guns made by both AOC and Savage had barrel and receiver threads timed.

 

FWIW

 

 

 

 

 

 

[reconbob]

Hey PK - thats some interesting info. I can agree with you to

the extent that various production runs had similar threads. But

I have also found that as I have had to make receivers for the

various generations of parts sets that have come into the country

that the threads are not all exactly the same. of course we're

talking assorted iterations of M1928A1, M1, and M1A1 parts

sets...over 20+ years. I recall at least a couple of these that

had Turkish markings on the trigger frames.

One thing I wonder about - if you look closely at some GI

barrels the minor of the thread appears to be generated from

small steps, unlike what you'd get if it was turned, and I wonder

if some of the barrels had the threads formed on some type of

milling machine where the thread would be the result of a series

of indexes - this would enable timing to be duplicated barrel

after barrel. What do you think?

 

Bob

[PK.]

I have seen “chatter” in the minor diameter of the threads, which would result if the barrels were threaded on loose tooling, either milling or turning. I think the barrels were threaded and then the draw line marked in a fixture traceable to the master gage mentioned in the drawing. Simple enough, I’ve done similar in recent projects.

 

Timing the threads on the receivers would not have been a big deal if all threading were done on machines set up for that operation and gauging went back to a master gage. That is why the AO and Savage parts are the same in this respect. It was common practice to have master gages back then, with working gages supplied to the contractors and checked against the master from time to time.

 

The M1 is another matter, and as I note, I think that at some point they abandoned thread timing all together, as they should have from the outset.

 

Did you actually try to interchange the parts you were working with to check the alignment? The threads would never be exactly the same; chip gets in the fixture, operator gets a bit off, etc. Doesn’t take much with 10 pitch threads. How much did you find the marks to be off? 5 degrees +/- (~1/4”)? That would certainly be within the “fudge factor” for this kind of thing.

 

Are you sure you haven’t mixed up the M1 stuff with the ‘28 in your memory? That would be easy to do if you weren’t looking specifically for this and would certainly affect your feeling about the matter.

 

I would like to challenge (if that’s the right word) you to pay close attention from this point on and interchange everything you can with a mind to determine this exact question. Although I am comfortable with what I have determined I would welcome some other investigation into the matter.

 

Nothing I have said should be interpreted to infer that any of Bob’s comments or recollections were negligent in any way.