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M1928/M1A1 Bolt


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OK. The thing to do is make the tests. We all know how numbers
can be. Back in the day I would have used my own pocket calculator
to run the numbers but in this age of the internet I found a "scientific"
website where you filled in the values and it did it all for you. So who
knows. I will get on it but it will take a little while to make the tester.

Bob

Edited by reconbob
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Bob, i applaud your willingness to do the research tests of the various systems, i think this is long overdue and we are, today, in a better position to accurately accomplish it than ever before. I strongly encourage you to use accelerometers and strict scientific methodology. I look forward to seeing the results.

 

From a strictly empirical standpoint, firing a 28 next to a M1 would not lead one to believe that the 28 bolt is striking harder than the M1.

 

Standing by...

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If one is concerned about the rear bolt pocket being weak on the 28, why not reinforce the pocket, much like Diamond K does to the the locks at the front of the receiver, with a machined insert that is hard soldered in. Or maybe that would be to much change and deviate from the original design.



PS, I have been think about that bolt pocket insert. Kind of defeats the whole idea if just swapping in a m1a1 bolt, it would not be keeping it simple.

Edited by Paladin601
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Ninety per cent of this conversation is over my head! However, I do like the concept and discussion by many learned members of this Forum. Taking the post by Paladin601, above, a step further, would it be possible to design and manufacture a receiver insert with a "safer barrier" type approach that NASCAR now utilizes at all of its racetracks? Perhaps, something that could absorb the recoil while protecting the rear of the receiver and still allow the safe operation of the machine gun. And would be removeable (not a permanent modification), even if only by a skilled gunsmith. No doubt this would require a custom pilot, perhaps a new recoil spring and the redesign of an M1/M1A1 type bolt but still keeping with the concept of a drop-in type unit.

 

Simply stated, could reconbob make a few post sample 1928 receivers out of the same steel used to manufacture WH receivers to allow for actual testing of any designed drop-in type kit. An active machine gun rental range would be the perfect place to test such a design for long term durability. Or bring it to a couple of Thompson Show & Shoots and encourage members to do C drum dumps. I believe the line would be long for that adventure!

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The problem with a rear insert is that it takes up space so the bolt would have to be shortened by whatever distance the insert used up. That would mean a lighter bolt. Even with a stiffer spring the lighter bolt might be hitting at higher velocity. Since the force of impact is F = 1/2(M*V^2) the impact force could easily be higher than it was with the stock parts. Re-establishing the balance with that setup could be a real trip down the rabbit hole. The obvious solution would be a tungsten bolt and a really stiff spring but the economies of that might prove unworkable. It's an interesting project but it's not going to be a simple one

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if you want a heavier bolt but a tad shorter could you not hollow out the bolt and pour lead inside? better yet the length the bolt is shortened could be replaced with a custom molded urethane insert to absorb the impact, in all cases the 28 and M1A1, does the bolt always hit the back of the receiver?

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I seem to remember that the British in WWII North Africa had some luck in simply milling the lugs off of the Blish lock and then using what remained of the lock to secure the actuator to the bolt. Apparently, the arrangement worked fine. They bought into the argument that the Blish locking principle served no real purpose in the gun, and that the weight of the bolt and actuator were what mattered in what became a straight blow-back mode. I'm not advocating this, just saying that it's been tried.

 

Bob, I'd wonder about the size of a market for your idea. So few guns were made prior to 1986. Today, they're either working fine as they are or they've already been subjected to corrective action of some kind.

 

I also recall that PK has done what reminded me of hip replacement surgery, where he had completely replaced misaligned bolt slots by setting in new inserts. I can't find the photo, but the work was brilliant.

 

Excellent post, I really enjoyed reading through this, and with fear of upsetting the apple cart, I read this a while back:

"Scientifically however, the “Blish Principle” of metal adhesion does not exist. In reality, the effect Blish was seeing was that his lock merely added mass to the gun's bolt, which, in a blowback gun, simply slows the travel of the bolt"

https://www.popularmechanics.com/military/weapons/a25414/tommy-gun-thompson-submachine

 

I am lead to believe this was backed by observations made during the evaluation phase of the Thompson automatic rifle by the U.S. Ordnance Board. Major General Julian Hatcher, then a young officer on the board, who following extensive tests with the weapon, concluded that the “Blish principle”, simply did not exist. I have not read his book, Julian S. Hatcher, Hatcher's Notebook, although been looking for a 1947 edition to confirm it, but maybe someone out there has a copy and can confirm what he wrote, the quote I read, stated pages 44-46 as reference?

 

Finally a photo of a British modified Blish lock, which I found in a recovered former IRA 1928A1, with the serial number ground off:
No.14 Modified H Block.JPG
Stay safe
Richard
Edited by rpbcps
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What about a Bolt the weight of a 1911 slide and a recoil spring in the 14-18pound range? It is the same ammo.



Blish principle "Metal adhesion" does not exist? I don't know, I am not a physicist. I do know that dissimilar metals with machined surfaces have been pressed together and formed a non permanent bond. I believe what Cmdr Blish saw on the naval guns was the result of a "pressure lock" between the breech and barrel until the projectile left the muzzle. But it was found that the lock was not needed.

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What about a Bolt the weight of a 1911 slide and a recoil spring in the 14-18pound range? It is the same ammo.

 

 

 

Blish principle "Metal adhesion" does not exist? I don't know, I am not a physicist. I do know that dissimilar metals with machined surfaces have been pressed together and formed a non permanent bond. I believe what Cmdr Blish saw on the naval guns was the result of a "pressure lock" between the breech and barrel until the projectile left the muzzle. But it was found that the lock was not needed.

 

Well, I am no expert, just playing the devils advocate.

 

Incidentally have just managed to buy a copy of Hatcher's Notebook through a 2nd hand book dealer, so when that arrives, I'll have the opportunity to read up what General Hatcher had to say about the subject.

 

Stay safe

Richard

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Yep. Hatchers Notebook pages 44-46 comments on the Blish principle and also

the Thompson submachine gun. When describing the Blish principle he says that

there is some "dispute" among engineers as to whether the Blish principle actually

exists. He describes the principle regarding the Thompson but does not really

pass judgement on it.

 

Remember that the prime directive of Auto-Ordnance was to manufacture firearms

using the Blish principle and that is what they did.

 

I am looking forward to doing these tests because while I have seen a lot of numbers

thrown out there over the years I have never seen an actual test like the one I envision.

The farther the weight moves when struck by the bolt, the harder the bolt is hitting

the receiver. And especially to see M1921 vs. M1928 and Blish lock vs, Blish lock

with no wings.

 

Bob

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I've got nothing on N2 but 1141 is 61 KSI yield, 99 KSI as rolled but is usually heat treated to a much greater strength of 75 KSI/110 KSI so still quite a bit stronger and also a lot harder harder than 12L14

N2 Steel data sheet

 

https://www.steel-grades.com/index.php?m=pdfmetal&c=index&a=pdf&catid=18&id=7309?-ASTM---Grade-N2-Datasheet-Down.pdf

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Yep. Hatchers Notebook pages 44-46 comments on the Blish principle and also

the Thompson submachine gun. When describing the Blish principle he says that

there is some "dispute" among engineers as to whether the Blish principle actually

exists. He describes the principle regarding the Thompson but does not really

pass judgement on it.

 

Remember that the prime directive of Auto-Ordnance was to manufacture firearms

using the Blish principle and that is what they did.

 

I am looking forward to doing these tests because while I have seen a lot of numbers

thrown out there over the years I have never seen an actual test like the one I envision.

The farther the weight moves when struck by the bolt, the harder the bolt is hitting

the receiver. And especially to see M1921 vs. M1928 and Blish lock vs, Blish lock

with no wings.

 

Bob

 

Bob,

Thanks for that confirmation.

 

I am looking forward to receipt of Hatchers Notebook and also the results from the tests.

 

Stay safe

 

Richard

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Yep. Hatchers Notebook pages 44-46 comments on the Blish principle and also

the Thompson submachine gun. When describing the Blish principle he says that

there is some "dispute" among engineers as to whether the Blish principle actually

exists. He describes the principle regarding the Thompson but does not really

pass judgement on it.

 

Bob

 

Engineers, designers Draftmen and even the shop mechanics will always argue whether a part really has a purpose or not. And if it has a function. That is why thing take so long to develop now, especially since the DOD tries to let everyone have a piece of the contract.

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"Metal adhesion" may not exist but friction certainly does. Admittedly the coefficient of sliding friction for lubricated steel on bronze is pretty low but what is critical is the time it takes to slide the Blish lock out of the way so the bolt can move. The delay that the Blish causes occurs at the highest load point of the cycle, immediately after ignition, so it's going to have a disproportionately large effect on the rearward velocity of the bolt and therefore on the force with which it impacts the rear of the receiver. It may not do what Blish thought in the manner that he thought but it certainly does something to slow the cyclic rate

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Hi All,

 

Maybe in a perfect world the spring/mass/recoil system could be set to recoil a distance great enough to catch the sear and not so far as to hit the rear of the receiver. I've been able to do this with some simple guns like the Sten Mk II and Uzi. They run very smoothly with custom tuned handloads.

 

Just thinking out-loud.

 

Grasshopper

This sounds like a great idea. If you minimize/eliminate impact of the bolt with the rear of the receiver, wouldn’t that lessen the possibility of cracking a WH 1928 receiver and make the gun smoother to shoot? If the weight of the bolt is increased, would that not be a way to accomplish this? Tungsten inserts perhaps? This as been done to slow the ROF on mini Uzis. A person used to sell these on Uzitalk.com

Further, M11/9 bolts have been made entirely of tungsten, doubling the bolt weight to achieve Uzi-like ROF. This has been well documented on Uzitalk as well.

Would tungsten inserts in an M1A1 Bolt (modified in the way reconbob suggested to function in 1928 guns) be a way to keep the bolts from impacting the rear of the receiver on WH 1928s, thus minimizing the possibility of cracks? It also would slow the ROF. Or I could be completely incorrect.’just trying to get another idea out there.

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Hi All,

 

Yes. Wolfram (Tungsten) slug to increase bolt mass. Not inexpensive. Also easier on an M1 or M1A1 style bolt. I need to get to the range on a sunny day. I've 1 camera that will do 1000 frames/second. (You need a sunny day for shutter times

 

Just thinking out loud. Times are tough with the covid and ammo situation.

 

Stay safe,

 

Grasshopper

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Just a thought, but it seems a waste to ignore the two slots inside the receiver where the ears of the blish lock go back and forth. If someone with more design/manufacturing skills than I have were to design two sliding weights that run up and down those slots along with the bolt, it would add more mass to the whole assembly without resorting to horribly expensive tungsten. It would have to be a separate part and connect to the bolt with a tab in slot arrangement but it could be incorporated into the charging handle and left in the receiver during cleaning.

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The problem with the West Hurley Blish slots is in the angled part up in the front of the receiver. PK did the repair on my WH Thompson and that's where he did his surgery. I'm thinking of the empty space that runs along the length of the receiver from the angled slots in the front to the rear where the oiler sits. You could use those two empty spaces to hold simple weights (steel or even brass) that would be coupled to the new bolt under discussion and become part of it for the purpose of adding mass (if needed).

 

I know I'm doing a lousy job of explaining this, but from the rear, it would look kind of like two saddlebags on a horse, with the bolt body being the horse. You couldn't make them a permanent part of the bolt, as there would be no way to drop the bolt into the receiver, but as a separate assembly it could be placed in the receiver while it's upside down for routine disassembly and then the rest of the bolt dropped on top of it, where a tab on the bolt would engage a slot on the weight or vice versa.

 

In the M1/M1A, the empty space was addressed by making the top of the receiver narrower. For this new 1928 bolt, it could be utilized to add mass instead of just sitting there empty.

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I think what you are proposing still requires that the angle

cuts for the bronze lock be correct. The trouble is that so many

guns have incorrectly placed or machined slots. I have had a

couple of guns in here lately where the slots were un-evenly

machined with one slot being over 1/16" of an inch forward of

the other so the other slot did all the work and was never contacted

by the H-lock. By adding weight you do not fix any of this.

 

The original question was would an M1A1 bolt - as far as I know

the slowest bolt - be "easier" on the back of the receiver than a

M1928A1 bolt. Numbers, calculations, and opinions abound but

no one has ever done the test I drecribed.

 

I talked to Doug Richardson about this and he said it does not

matter what the rate of fire is, the bolt will strike the back of the receiver

with the same force at 640 rpm as it would at 770 rpm. Doesn't

make sense to me, but that's why I want to do the test.

 

Bob

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Yeah, I'm obviously not doing a very good job of describing my idea. The angled slots in the front of the receiver are totally irrelevant to what I was envisioning. The weights I am trying to describe would move back and forth alongside the body of the bolt as the gun cycled. The test bolt we are all (hopefully) discussing wouldn't have the cut-outs for the Blish lock and wouldn't have the separate actuator, so it will be a big solid chunk of steel that will presumably be heavier than the standard 1928 bolt assembly. I'm just saying it could be even heavier (if needed) by adding weights that would fill in the empty space in the receiver that is normally used by the ears on the Blish lock as it goes back and forth in the receiver.

 

They would have to be a separate assembly, as there's no way to get into the two side channels in the receiver by dropping a solid bolt straight down like we do now.

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  • 1 month later...

Bob said: “I talked to Doug Richardson about this and he said it does not

matter what the rate of fire is, the bolt will strike the back of the receiver
with the same force at 640 rpm as it would at 770 rpm.”

 

Assuming DR is correct, that impact (and corresponding impulse energy burst beating the s#!t out of the receiver) still happens 130 times less per minute. Maybe simplistic, but seems like a lower ROF would have some positive effect on longevity.

 

OTOH, I’m a electrical engineer, not mechanical, wtf do I know?

 

interesting discussion, thanks to all who have contributed.

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