Grip mount Design

[Sgt]

I've always wondered why the Thompson grip mount was designed to be held in place by the barrel. Given the slotted fit for strength, why wouldn't one or two set screws hold it in place, and not use the part that goes under the barrel? That would make both barrel change or grip mount change less difficult. 

[StrangeRanger]

When they designed the gun, they probably didn't realize that the grip mount was going to be one of the highest rate of failure parts on the gun.  They expected it to live forever

[Sgt]

That sounds reasonable. I got an ignorant reply from Google ai saying they couldn't trust the set screws to hold the barrel in place. 

[StrangeRanger]

There's a reason that it's called artificial intelligence

[DINK]

It was a totally different design philosophy back then.  Set screws would be looked upon as the mark of a clumsy and untalented designer.  Look at the Luger pistol- the only screws in the whole thing are the two that hold the grips on.  The rest goes together and each part locks in place.  The Mauser Broomhandle goes together like a Chinese puzzle- once again, the only screw in the pistol is the one that holds the grip panels in place.  The same goes for Browning's designs of the period.  Look at how the 1911 or the Woodsman come apart.  Not a screw, bolt or set screw to be found.

On the Thompson, the only screws on the whole gun are those involving the attachment of a piece of wood.

 

[jirahice]

I'm sure they viewed using the barrel to secure the grip mount as more elegant. Doubt they considered the difficulty it would cause the amateur Thompson enthusiast a hundred years later. And to my knowledge, they only began to fail once they went from a solid milled design to a two-piece riveted solution late in the war. 

[StrangeRanger]

2 hours ago, jirahice said:

And to my knowledge, they only began to fail once they went from a solid milled design to a two-piece riveted solution late in the war. 

Not true.  My Savage M1 (not M1A1) had an original 1 piece grip mount.  The tiny lug that abut the barrel just in front of the threads and holds the mount into the receiver sheared off.  The result was a "pump action" TSMG.  Replaced it with a repro from Waffenmeisters which I expect to last another 75 years.

It has been alleged elsewhere on this site that the grip mount has the highest failure rate of any TSMG part

[TSMGguy]

Had always thought that the grip mount does not hold the TSMG barrel in place. Rather, it's the properly torqued barrel that holds the grip mount in place.  

It's some serious abuse indeed that breaks a solid steel grip mount. 

[inertord]

I have never seen a grip mount break on a Colt TSMG. Have seen plenty of sheared actuator knobs, broken knob ears and bulged barrels. 

[TD.]

I think it would be best to distinguish between the Colt, Model of 1928 and the M1 series of Thompson guns when talking about grip mounts and breakage.

There is a section in Great Britain- The Tommy Gun Story beginning on page 60 that deals specifically spare parts for the Thompson gun used by the British military during World War II. I believe this is very important because the British military had a lot of experience with using the Thompson submachine gun in battle prior to the USA entrance into the war in December 1941. What I found was the British military only wanted a small number of spare parts per 100 guns ordered and received. The US wanted to greatly expand the number of parts and a bureaucratic battle of spares began that was fun to read (and write) about. I also included 5 charts developed during these discussions of the parts argued about starting on page 125. The grip mount or "Mounts, grip" is not even listed until the 4th chart. The number of spare parts listed for the "Mounts, grip" per 100 guns is "Nil" or none. If any "Mounts, grip" are needed, the source for this part is "Recover." I assume from battlefield used guns retrieved on the ground and refurbished for use after an engagement. 

The 5th chart is the U.S.A. Ordnance Scale (page 133), and it lists the requirement for (Mount, grip (Plates fore grip), as only 2 grip mounts per 100 guns during a 12-month maintenance schedule. The number of spares needed for many other parts are quite a bit more, i.e., barrels - 120 per 100 guns, actuators - 23 per 100 guns, etc.

The resource material for the battle of spares is dated from 1940 through 1942 (see footnotes) and only encompasses the Model of 1928 and US Model of 1928 A1 Thompson submachine guns. There is a short reference in January 1943 on page 66 about a "new model" SMG being introduced and how spares for this new SMG would be under discussion in the future. I encourage all of you with, Great Britian - The Tommy Gun Story, that is interested in grip mounts and Thompson spare parts to review the above material. Perhaps I have missed something. 

All good stuff!!!   

[reconbob]

       We have to remember that - except for a couple of oddball guns that had only a brief production life - the Thompson was the first submachine gun. And the gun designers were craftsmanship/commercial quality types who did not have simpler, later guns to copy or influence them. In those circumstances we get the one-piece, hardened steel grip mount. I also had never heard that the original pattern grip mount had a high breakage rate. Over all the many years I have been at this I have never seen a broken original pattern grip mount. But TD has put that idea to rest. 

Bob

[Frank I.]

While pursuing tables at SAR West 2025 I saw two Savage milled grip mounts with broken retaining tabs, the seller had no details.

I always thought that perhaps a carry sling attached to the front swivel put stress on the grip mount? And thus the introduction of the front grip barrel band, primarily  for the later riveted grip mount.

[StrangeRanger]

Steel parts can fail under very low loads after an extremely high number of  load cycles, meaning the impact of the gun on the tab during recoil.  How many cycles did a Thompson see in 80 years including at least one war possibly three?

[Doug Quaid]

There are few "quality" guns that use screws in their basic construction, especially internally, because when a gun is fired it violently vibrates like a tuning fork.  That provides plenty of energy to make screws vibrate loose.  Screws and nuts coming loose is a huge problem, so they only get used in guns where they can be easily tightened and where they are needed for easy disassembly by someone with few tools.

For example, the odds of any soldier needing to adjust or remove the rear sight of his 1911 are zero, so the sight got mounted in a very simple, robust and vibration-proof dovetail slot.  If the sight was adjustable and mounted with screws, it would not be better for combat use.  It would actually be worse.  Which is evidenced by the fact that this method of construction is still in widespread use for military handguns.

Some of you said that the lack of screws might be due to an desire to be elegant.  Normally, I would tend to disagree with that idea, since all gunmakers back then knew that screws are a bad feature.

But the Thompson was so feature laden and so beautifully made that it's obvious that they definitely would not have done anything that was inferior or cheap.  Based on the superb finish and build quality, I think that they came from a custom sporting arm background.

The evidence of the above statement is the fact that when WWII production started, they started stripping the features and build quality off the gun with no loss of function. 

If you examine a piece of machinery in detail, sometimes you can see the underlying idea behind it.  

It seems to me that the designers were trying to build a gun that did literally everything, be all things to all people.

For example, when you look at the cantilever mount on the 1921, you can see that it mechanically isolates the barrel from the foregrip and it also pushes upward on the barrel in exactly one spot.  That is how super accurate bench-rest guns are built.  That mode of construction allows the barrel to vibrate exactly the same every time it's fired, with no forces or damping from the user gripping the stock.

Then consider the fancy ladder sight.  Back in the old days, they had this thing called "firing for effect", where you would shoot at people who are far out of range just to keep their head down.  You can't pepper the Germans 500 yards away without a 500 yard gunsight. 

These two things suggest that they were trying to accurize the gun so that it could be used as a main battle rifle in addition to being a supporting arm.  The cantilever mount and the ladder sight cost a lot of money, you would not put them on a gun that was going to be issued to tank crews and for other CQB uses.

When they built the original Thompson, they weren't just trying to hit a home run, they were trying to smack the baseball clean out of earth orbit.   That's how it seems to me, anyway.

Please don't think the above is me playing the Great Swami of All Wisdom.  Anyone can look at a product that is 100 years old and have crystal clear 20-20 hindsight.  
 

Edited January 11 by Doug Quaid

[Doug Quaid]

10 hours ago, StrangeRanger said:

Steel parts can fail under very low loads after an extremely high number of  load cycles, meaning the impact of the gun on the tab during recoil.  How many cycles did a Thompson see in 80 years including at least one war possibly three?

It's really hard to know how a part will hold up to fatigue cycles without doing a lot of testing.

The primary load capacity of a steel shape can be very accurately calculated, but fatigue life comes from all kinds of unknowns, like work hardening, 2nd order stresses from strain compatibility, residual stress from hot rolling, discontinuities that cause stress risers, inclusions and flaws in the metal, etc.

So design code for fatigue is typically 100% empirical.  A certain detail will be assigned a fatigue grade and there is a corresponding allowable stress range for a given number of cycles. But it's really approximate.

Most people don't know this, but when you load something, there is usually more than one load cycle, because the part is elastic and it bounces.  Stand on a long bridge span sometime and you will notice it jumping up and down constantly.

Edited January 11 by Doug Quaid

[Got Uzi]

The other part of this is to realize we are running guns 50+ years past their “sell by date” these guns we’re never meant to be used this long, so maybe we need to stop and see what the service life of these parts were.