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Buffer: Circle vs. Square


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#1 Scrambles

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Posted 17 January 2018 - 03:07 PM

The 1921 Thompson was designed with a large diameter spring that, when bottoms out, compresses a small stack of fiber discs, in a ventilated buffer tube.

On the 1928 Thompson, we see the round buffer, and pilot, with the bolt impacting on the metal pilot, roughly 15% of the surface area. It is as though the pilot was designed with only the 1921 spring in mind, not the bolt, not the receiver...? The pilot also is shorter, leaving a gap about 1/2 where the spring can naturally bind. This generates rather high/uneven loading and stress.

"There's no engineering justification for the 1928 design!!!!"

Yet on the M1, the transition to a buffer that matches the bolt face occurs, as well as a method to insert the buffer/spring base. Eliminating the fiddly 28 system.

The "PK" hybrid buffer eliminates the drawbacks of the 28 spring, and "complexity" of the buffer discs, however leaving the metal to metal impact between the rear bolt face/actuator and the pilot, and requires the fiddly installation not seen with the M1.

With nearly an inch of extra/wasted bolt stroke in the receiver, there is plenty of room to experiment with new designs that may increase the longevity of an inevitability fatigued receiver.

Reverting to a 2 piece system that eliminated metal to metal contact would be wise. There are new materials we could use in place of fiber disc stacks, such as a urethane insert of varying durometer, or even a small piece of 28 recoil spring to speed things up, a la speed bolt.

Reading the patents, it's clear the inventor was limited by present day technologies and often spoke of using composites or sythetics, although out of reach.

I propose simply a 21 pilot, with recess for an M1 style buffer that matches the rear face of the bolt. A bit like the "2M2" 2B etc. as well as an variety of inserts for the 21 buffer tubes (which possibly could be lengthened to reduce spring bind)

Simply in an interest to keep all the old Thompson running well into the future, possibly better. Blown out rear ends will become a thing with enough rounds downrange.

Discussion is welcome,

Attached Files


Edited by Scrambles, 17 January 2018 - 03:16 PM.

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#2 benedw60

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Posted 17 January 2018 - 04:34 PM

Scrambles: a thompson is a thompson because its a thompson. Start changing a things then you just ruin perfectly good guns
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#3 buzz

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Posted 17 January 2018 - 04:37 PM

Well, the original 28 buffer is a phenolic resin washer, which was the commercially available plastic back them.

 

If you replace the phenolic washer with a urethane washer, you're going to soften the recoil impulse a whole lot, because urethane is 100 times as "springy".

 

So that alone would be enough to lower the impact load on the rear plate of the receiver by a large amount.

 

It doesn't matter if the bolt hits metal like in the M1 buffer as long as there is a layer of buffer material in there somewhere between the bolt and the rear plate of the receiver.

 

I don't think that the rear plate of the receiver is at much risk of failure anyway.  The WH guns are made of leaded steel and occasionally you hear about one cracking, but it's not known how the gun was treated.

 

Do the rental ranges ever report the rear plate cracking on their newly made guns that are made with ordnance steel?  How about WWII guns?  Any reports of cracking?

 

I would strongly urge people to not use any kind of speed bolt in their gun.  The faster the bolt goes, the more energy in the bolt, and therefore the more energy in the rear plate of the receiver.    Know what causes fatigue cracks?  When metal absorbs energy.

 

FWIW, this sort of thing (kinematics and fatigue) is something I studied in grad school for my profession.  


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#4 buzz

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Posted 17 January 2018 - 04:37 PM

Scrambles: a thompson is a thompson because its a thompson. Start changing a things then you just ruin perfectly good guns

 

 

If someone came up with a M1 type buffer assembly for my 28, I would not cry my eyes out.


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#5 Motorcar

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Posted 17 January 2018 - 05:47 PM

Blown out rear ends will become a thing with enough rounds downrange. 

 

Based on what evidence? WH guns as Buzz mentions above, WW2 production...has there EVER been one recorded?

 

Mine has been running fine since 1941, not going to "fix" it.


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#6 Piep

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Posted 17 January 2018 - 10:37 PM

My father always told me "If it's not broke don't fix it." and he was right. I never told him that, but should have...
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#7 Black River Militaria CII

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Posted 17 January 2018 - 10:45 PM

When Gunmachines was making parts to increase the rpm of Thompsons many years ago a number of Savage 28s eventually were damaged with cracks developing in the sidewall at the rear of the receiver. WH examples also were damaged and probably a larger number given their greater vulnerability. Allegedly a Colt or two was damaged as well but I did  not see any of them or talk to anyone who had one damaged by the Gunmachines parts. FWIW


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#8 laurencen

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Posted 17 January 2018 - 11:13 PM

the buffer was likely round because phenolic came in sheet or round bar, to make from sheet any shape it needed to be cut out to shape and drilled, with round bar they would drill a length and part off in a lathe, way quicker to manufacture from round stock


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#9 reconbob

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Posted 17 January 2018 - 11:42 PM

I think you know almost NOTHING about Thompsons. Prove me wrong:

- how old are you?
- do you live in a state where ownership of machine guns is legal?
- how many Thompsons have you owned and what models?
- how many rounds have you fired?
- how many years of experience do you have shooting and fixing Thompsons?

If you can minutely insert your expert comments in one of my previous posts
you can certainly answer these simple questions....

Oh - and the reason there is a 1/2" gap between the front of the buffer pilot
and the rear of the actuator that it fits into is because if the gap is shorter you can't
fit the buffer pilot into the receiver. I guess if you'd ever actually field stripped a GUN
you'd know this.

But I will say again. If you think the Brandon/Scrambles new high tech buffer is
such a good idea, make them. If people buy them, and over time the part gets a good
reputation like PK's buffers then good for you.

The "the receiver is going to crack" urban myth (with the exception of the Gunmachines
parts in WH receivers)is a favorite of armchair armorers but I have never seen an original
cracked receiver of any model and heard of only one or two that allegedly cracked but with
no actual documentation or photos.

I have made over 100 M1928A1s with a handful of M1921s thrown in for rental
ranges and I get the feed back. The only gun that stopped working was because the barrel
got shot out and the gun started to short recoil. These guns get many tens of thousands of rounds
thru them and they just keep going.The thing that is going to fail on a Thompson
is the barrel, not the receiver.

Contrary to your expert opinion that the design is not "justified" the excellent design has
been proven over the years.

The Thompson is unique. It was designed and made before a lot of modern technology
and was manufactured using techniques that would never be used - or even thought of today.
If they were going to make a Thompson today will the new developments and technology
their Thompson would be, well, an MP-5. And that is not a Thompson.

You think that different is better. OK. But different is not a Thompson.

Bob

Edited by reconbob, 17 January 2018 - 11:44 PM.

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#10 Paladin601

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Posted 18 January 2018 - 12:55 AM

the buffer was likely round because phenolic came in sheet or round bar, to make from sheet any shape it needed to be cut out to shape and drilled, with round bar they would drill a length and part off in a lathe, way quicker to manufacture from round stock

I was thinking that the buffers may have been cut out of a sheet, using a Hole saw.

 

 

(Btw, I don't know when a hole saw was put on the market)


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#11 RChapman

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Posted 18 January 2018 - 06:30 AM

I think you know almost NOTHING about Thompsons. Prove me wrong:

- how old are you?
- do you live in a state where ownership of machine guns is legal?
- how many Thompsons have you owned and what models?
- how many rounds have you fired?
- how many years of experience do you have shooting and fixing Thompsons?

If you can minutely insert your expert comments in one of my previous posts
you can certainly answer these simple questions....

Oh - and the reason there is a 1/2" gap between the front of the buffer pilot
and the rear of the actuator that it fits into is because if the gap is shorter you can't
fit the buffer pilot into the receiver. I guess if you'd ever actually field stripped a GUN
you'd know this.

But I will say again. If you think the Brandon/Scrambles new high tech buffer is
such a good idea, make them. If people buy them, and over time the part gets a good
reputation like PK's buffers then good for you.

The "the receiver is going to crack" urban myth (with the exception of the Gunmachines
parts in WH receivers)is a favorite of armchair armorers but I have never seen an original
cracked receiver of any model and heard of only one or two that allegedly cracked but with
no actual documentation or photos.

I have made over 100 M1928A1s with a handful of M1921s thrown in for rental
ranges and I get the feed back. The only gun that stopped working was because the barrel
got shot out and the gun started to short recoil. These guns get many tens of thousands of rounds
thru them and they just keep going.The thing that is going to fail on a Thompson
is the barrel, not the receiver.

Contrary to your expert opinion that the design is not "justified" the excellent design has
been proven over the years.

The Thompson is unique. It was designed and made before a lot of modern technology
and was manufactured using techniques that would never be used - or even thought of today.
If they were going to make a Thompson today will the new developments and technology
their Thompson would be, well, an MP-5. And that is not a Thompson.

You think that different is better. OK. But different is not a Thompson.

Bob

 

same, i have never seen a Colt /WWII receiver cracked, only one fading image of a WH, as far as i know they are made of a "softer" metal.

 

I attached one interesting piece of paper regarding the Thompson. I know thats regarding the H lock, but it should be enough to show that the buffer assembly / receivers, even with an altered H piece, are clearly strong

 

enough to withstand the beating.

 

now my question is, why use buffer discs instead of a stiff spring inside the 1921 buffer pilot assembly?

 

Attached File  h lok.jpg   131.76K   23 downloads


Edited by RChapman, 18 January 2018 - 06:43 AM.

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#12 Adg105200

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Posted 18 January 2018 - 07:17 AM

You pretty much never hear about a ww2 or colt era receiver cracking.  Other than on a WH, I don't think even a lighter bolt would really crack the rear of a receiver.  The 21 was designed with a higher rate of fire and since the bolt/actuator is lighter it isn't like it's a significant difference in kinetic energy even though it is a faster rate.  I know that the weight vs speed for kinetic energy probably isn't linear given the fixed dimensions of the receiver - maybe Buzz can help out with that one- I don't think you would have much issues with cracking an even slightly lighter bolt, unless you had spring or buffer issues.  But other than the M1, most 21/28 owners are using polyurethane buffers anyway.  It would be nice if someone would engineer a poly buffer for M1s.   Like Reconbob said, there is a gap for a reason.  I'm pretty sure they used the same math back then when designing lol.  Everything in the Thompson design is what it is for a reason and in ww2 or colt tsmg's you never hear of them being any less reliable than other FAs.  Unless you field strip a 28 wrong and lightly kink the spring you won't have any reliability issues (other than the consumables - extractor, shot out barrel etc.).  It is a given though that a M1 is a little easier to field strip given the design.   My 2 cents   Andrew


Edited by Adg105200, 18 January 2018 - 07:20 AM.

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#13 buzz

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Posted 18 January 2018 - 12:18 PM

I've kind of given up trying to explain physics on the internet. If I had a chalk board and could draw pictures and jabber for an hour, and answer questions, I could make anything perfectly clear. But typing a paragraph on the internet is hopeless, it's just simply not enough information.

 

I'm not playing the expert to impress anyone, I just happen to be a guy who studied these topics in great detail, for professional reasons.

 

Basically, a Thompson bolt is just a damped spring-mass oscillator, it's pretty simple.

 

You know how a screen door has a spring and damper on it?  If you violently kicked the door open with your foot every time it was just about to close, and it opened all the way and it banged against a wall when it was nearing the full extension of the damper assembly, that would be a rough duplicate of a Thompson bolt operation.

 

Anyway, if I was tasked with designing this gun, I would just have someone make up some actual receivers and test them to destruction.  

 

A lot of these privately owned MGs have not seen a lot of rounds down the barrel, so they're useless as data points.  BUT, some rental guns have seen enough rounds that they are wearing out multiple barrels. 

 

If those rental guns are not cracking, then you don't need to worry about it, it means that in general the little nasty stress risers in the receiver are below the threshold to form fatigue cracks in that tough, fatigue resistant ordnance steel.

 

On the flip side, the speed bolts seem to cause stress high enough to crack that not-so-wonderful leaded steel in WH receivers, so I would AVOID using a speed bolt in any Thompson.

 

Don't forget, the gun designers back in 1921 doubtlessly tested a lot of these receivers to destruction, this question was probably fully settled when they developed the gun.  


Edited by buzz, 18 January 2018 - 12:42 PM.

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#14 buzz

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Posted 18 January 2018 - 12:26 PM

Someone asked why they don't use a spring in the ass end of the Thompson instead of the hard washer.
 
Well, the washer is a spring.  A really, really stiff spring.  You put load on it, it deflects and pushes back, that's what a spring does.
 
Why didn't they use a much softer spring, like a neoprene washer?  They didn't have that back then, and anyway the durability of the neoprene is way lower.
 
Or maybe they didn't use a softer material because the softer material would damp too much, suck energy out of the bolt.
 
We don't really know how hard the Thompson bolt thumps the washer, if at all.  It might be there just as a safety for a hot round, or to keep from peening the bolt when firing proof rounds.
 
The only way to know any of this stuff would be to test it, and the testing has been done already in actual use.
 
I would say that using the neoprene buffer is a good idea, neoprene is stiff but WAY less stiff than phenolic, it will definitely reduce impact load on the rear plate.
 
But I don't think it's a big deal either way and i don't see any reason to use some fancy spring gizmo, based on the satisfactory performance of the gun.
 
Not broke, don't fix.


Edited by buzz, 18 January 2018 - 12:29 PM.

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#15 buzz

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Posted 18 January 2018 - 12:39 PM

You pretty much never hear about a ww2 or colt era receiver cracking.  Other than on a WH, I don't think even a lighter bolt would really crack the rear of a receiver.  The 21 was designed with a higher rate of fire and since the bolt/actuator is lighter it isn't like it's a significant difference in kinetic energy even though it is a faster rate.  I know that the weight vs speed for kinetic energy probably isn't linear given the fixed dimensions of the receiver - maybe Buzz can help out with that one- I don't think you would have much issues with cracking an even slightly lighter bolt, unless you had spring or buffer issues.  But other than the M1, most 21/28 owners are using polyurethane buffers anyway.  It would be nice if someone would engineer a poly buffer for M1s.   Like Reconbob said, there is a gap for a reason.  I'm pretty sure they used the same math back then when designing lol.  Everything in the Thompson design is what it is for a reason and in ww2 or colt tsmg's you never hear of them being any less reliable than other FAs.  Unless you field strip a 28 wrong and lightly kink the spring you won't have any reliability issues (other than the consumables - extractor, shot out barrel etc.).  It is a given though that a M1 is a little easier to field strip given the design.   My 2 cents   Andrew

 

 

Back when this gun was designed, there was no math to use in designing it.

 

At the corners and cut-outs and locations of "discontinuity" in the receiver, there will be little spikes of stress that load up the microscopic flaws and inclusions in the metal crystal lattice, which can eventually cause cracking.

 

To figure that out with math is a NASA computer modelling type thing, it's possible to do today but nobody would bother, it would be too expensive and pointless anyway.

 

The money you would spend on that approach, you could test 100 Thompson receivers with 200,000 rounds each and come up with a better answer.

 

The Thompson was an early gun, it has a very conservative and elegant design.  If they had continued to develop it, eventually it would have evolved to have a much simpler mechanism and had a lot of needlessly expensive features deleted from it. (cough M1 cough)


Edited by buzz, 18 January 2018 - 03:41 PM.

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#16 Scrambles

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Posted 18 January 2018 - 02:13 PM

..wow

That went great

Bunch of fantastic people here
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#17 buzz

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Posted 18 January 2018 - 03:49 PM

I think the problem was that the tone of your post was a flat declaration of truth about a topic that you are merely speculating about.

 

People find that a tad annoying.


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#18 benedw60

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Posted 18 January 2018 - 04:15 PM

If you enjoy ruining perfectly good guns, go sportorize a 1903.
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#19 Bridgeport28A1

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Posted 18 January 2018 - 05:05 PM

Buzz, I would sit through your classroom Physics lecture regarding a Thompson bolt cycling. Maybe a You Tube video of the lecture.
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#20 reconbob

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Posted 18 January 2018 - 05:55 PM

Yea Buzz - this is right up your alley. What is the surface area of the base of
the cartridge. The chamber pressure. The weight of the recoiling parts and the
bullet, etc. Cannot all this be used to determine the velocity of the bolt, the surface
area upon which it acts when striking the buffer, and finally the force exerted on the
back of the receiver?
I remember when I took machine design in engineering school there was
a graph that had a trapezoidal shaped box in the middle. The legs of the graph
we're forces and cycles acting on the machine component. If a plotting of those
forces resulted in a point inside the trapezoid your mechanism had the potential
for unlimited cycles.

Bob

Edited by reconbob, 18 January 2018 - 05:57 PM.

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