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M1928/M1A1 Bolt


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We periodically discuss the plight of those who have West Hurley M1928's

with defective angle cuts for the bronze lock. It occurred to me that if you made an

M1A1 bolt with a fixed ball knob cocking handle on top, and the recoil spring hole

0.100" below the centerline of the bore that you would be able to shoot the gun

and completely bypass the defective angle cuts without harming the gun.

This is how we did the bolt for the M1928A1 blank guns, although for them

we use the standard centerline bore positioning of the recoil spring.

Since we are already making M1A1 bolts it would not be a big deal on

the next run to make some bolts in this pattern.

Is there a need/market for such a bolt? All opinions welcome.

 

Bob/Philly O

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It sounds like it could be a fix-all 'drop-in' solution for this common problem. It would be a simpler mechanism while minimizing potential issues. I'd be willing to purchase and test one in my WH (even though I'm not experiencing that problem and have all Savage internals).

Edited by heavy artillery
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reconbob,

That is an interesting idea. I would suggest you contact PK and see if he would opine on your possible solution to the 1928 West Hurley's with out of specification Blish lock slots. I will state that one of the problems with the WH 1928 guns is the buffer pilot hole at the rear of the receiver. This hole is out of spec on many/most/all WH guns. The pilot wobble is definitely a problem and impacts function. Depending on the size of the hole, PK will manufacture a custom buffer pilot to fit the hole for each gun he remanufactures. I am not certain how PK repairs pilot holes that are too oversize for a custom buffer pilot - but I know he has a solution.

 

If the barrel chamber issues could be repaired, a custom buffer pilot made for an out of specification pilot hole in the rear of the receiver, and a M1A1 type bolt utilized to completely avoid the Blish slots, would this solve many of the problems facing WH Thompson owners? I think the concept is worth exploring. I would also suggest one of PK's polyurethane buffer discs.

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Bob,

I think the idea of modifying an M1A1 bolt to be used in a 28 receiver had been tried in the distant past.

The problem was the very thin receiver rear (28) compared to the really heavy duty rear end of the M1A1.

I seem to remember cracks developing in the 28 receiver.

Jim C

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I've heard of cracks when a 28 was run without the blish lock and that would be about the same idea.....maybe a bigger urethane buffer might help? Edited by john
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This would seem about equal to the Gun Machines de-eared lock that was blamed for cracking WH 28 receivers.

It used a pair of setscrews to lock the actuator and bolt together, essentially the same function as a modified M1 bolt.

Also, the WH receivers are made of 12L14 leaded steel, not as strong as the steel used in real Thompsons.

Edited by mnshooter
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Bob, you talk with DR a lot. This sounds like his "M2" design, right?

He may already have something out there that is just what you describe.

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Really, really bad idea. The TSMG and M1 are actually very different guns. The 21/28 receiver is designed to use the Blish lock, the M1 not, they don't mix. WH receivers with bad slots will crack from normal use because the lock can't do its job, eliminating the lock doesn't solve the problem, it exacerbates it.

 

IF i recall correctly, the Richardson set up was for blank firing adaptations, not for use with ball.

 

As others have said, we've been down this road before and it doesn't end well.

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Bob, you talk with DR a lot. This sounds like his "M2" design, right?

He may already have something out there that is just what you describe.

 

Is Doug still around? I know he was hit hard by the fire, has he got back into his parts business. he shut down his web sight and all.

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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.

Edited by TSMGguy
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TSMG guy,

You need to remember that the Brits weren't using West Hurleys and the fact that the guns fired after removing the Blish lugs doesn't mean the receivers weren't stressed.

The Blish system might not be necessary to make the gun go bang, but it might be needed so as not to stress the receiver.

Jim c

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On page 36 is Doug's take on the M1-bolt-in-'28-receiver question....Phil

 

 

http://www.sturmgewehr.com/dalbert/MGBoards/RichardsonCatalog72A.pdf

Thanks Phil, yes that is what I was trying to remember! page 33 says as mentioned - blanks only!

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Fear not, I am not going to bet the ranch so to speak) on making M1928A1/M1A1 bolts.

It just that I am in a unique position to make them.

 

But this has - to me - raised an interesting topic, and that is: with what force does a Thompson

bolt strike the rear of the receiver? This comes up in the current context of H-locks on West Hurleys,

but also when the British-in-the-desert and Gunmachines topics come up.

 

I am NOT advocating throwing caution to the wind and counseling anyone to

file off H-locks or dust off their Gunmachines bolt!

 

Consider the following:

 

The M1928A1 and the M1A1 Thompson use the same recoil spring.

 

The weight of the recoiling parts on a M1928A1 is about 1 lb. 11 oz = 1.687/lb.

The distance that the bolt assembly moves until it hits the buffer pilot is about 3.9"

The rate of fire of a M1928A1 using military ball ammo is about 770 RPM (Thanks to Jim c.)

 

So the bolt makes 770 "round trips" over a distance of 3.9". When you run it all out the

peak force of the bolt striking the buffer is about 138 ft. lbs.

 

The weight of the M1A1 recoiling parts is about 1 lb. 14 oz. = 1.875 lb.

The distance the bolt moves until it hits the buffer is about 3.75"

The rate of fire of an M1A1 Thompson with military ball ammo is about 640 RPM. (Thanks to Jim C.)

 

So the M1A1 bolt makes 640 "round trips" over a distance of 3.75". It is moving much slower

than a M1928A1 bolt and the peak force when it strikes the buffer is about 98 ft. lbs.

 

So an M1A1 bolt strikes the buffer with about 30% less force than a M1928A1 bolt

strikes the buffer pilot.

 

Now, is all this exact? Of course not. The duration of impact and deflection of the buffer or

buffer pilot and been estimated. But a M1928A1 bolt assy. is only 10% lighter than a M1A1

bolt, yet it is traveling 20% faster.

 

So of the two bolts, the M1A1 applies much less force to the back of the receiver. Remember,

once the bolt has unlocked and recoiled, the bronze lock is out of the picture and its just the

bolt recoiling and striking the buffer pilot.

 

Interesting....

 

Bob

Always willing to be corrected if I got it wrong....

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reconbob, I still think it is a great idea. I think the heel cracking could be addressed with a new buffer or a buffer of different material. Always thought the cracking was work hardening of the thinner metal on the heel. After all even with the locks, I believe there was mention of Cracking.

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You have to consider the differences in material between GI and WH receivers. That's the reason I asked about the steel spec for original 28s

Almost everything critical on firearms in that era was made from 4140HT or if impact resistance was a consideration 4340HT. The former is 105 KSI yield strength and125 KSI tensile; the latter is 110 KSI yield and 130 KSI tensile. The 12L14 that WH used in thier receivers is only 71 KSI yield and 79 KSI tensile, not garbage by any means but still only about 2/3 the strength of a GI TSMG (assuming that 4140HT is correct.) A GI 28 might survive with the Blish locks trimmed or with an M1A1 style bolt but a WH is must less likely to do so

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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

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Yes, of course the X factor is the 12L14 steel. Maybe this is one

of those counter intuitive things but the less force hitting the back of the

receiver, the better off you are going to be. And it seems that in general

an M1A1 bolt won't hit as hard as a M1928A1 bolt.

This may be unknown territory. Yes, people have modified M1928's

to shoot faster (Gunmachines) and filed off H-locks to make the gun

more "reliable" ( British in the desert) but has anyone thought to slow

the gun down?

The only case I know of is years back when I made Jim C. a heavy

actuator for a M1928A1 by machining plates that fit in the lightening

cuts in the side of the actuator. This extra weight did slow the gun down.

 

Maybe the thing to do is to set up a tester where when the bolt strikes

the back of the receiver it hits a weight which slides along rails ( like a Grease

Gun bolt) and the distance the weight moves would tell us the force. I have

some extra receivers so it would be possible to do this. I would think the bolts

to try would be M1921, M1928A1, M1928A1 with ears removed from H-lock,

M1, and M1A1. Did I forget anything?

 

Bob

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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

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Bob,

I have to admit you make a very good case for the M1A1 bolt in a 28 WH.

Enough for me to reconsider my opposion.

If the M1A1 bolt would , on the average, strike the rear of receiver with less force than the 28 bolt with Blish locks working properly, then you should proceed, full steam ahead.

This would mean that a 28, with defective Blish slots would strike the receiver rear much harder than the M1A1 bolt.

Since a shooter who spends thousands of dollars on a West Hurley is going to want to shoot it.

And since Paul K is 4-5 years behind in repairing Blish slots, you would be doing shooters a big favor by providing them with a better option.

Can't wait to hear Paul Ks response.

Best of luck to you.

Jim C

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