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Discussion Starter · #1 · (Edited)
Re-visit- Tommy gun issue.........pictures added

I have 2 Numrich guns, our shooters. One has the old Gun-machines telescoping recoil spring guide set up. I failed to get a second one before they folded. I loaned the gun to a guy at a shoot. He managed to break the telescoping part (???) either that or it was time.... Got it fixed, havnt shot that gun since, (fall '14). The other one has the original Numrich set up. Recent 200 rnds factory ammo battered the replacement buffer to the point of making it unusable. Last buffer was two 1/8" sheets of corded synthetic "rubber" (looks like a tire sidewall). Not the first of several different material buffers........... I saw a Numrich gun that was split, then welded, vertically up both side of the receiver rear............had to been from being battered by the bolt, what else is there.????

We had been shooting '21s until the Numrich guns, their bolts are considerably lighter, one effect of that is increased rate of fire. I lightened the bolts in the Numrich guns by drilling five half inch holes through the bolt bodies, crossways. Did that 35 years ago. Raised rate of fire noticeably, the desired effect. The original recoil spring ought to slow or stop this lightened bolt better than the heavy bolt it was designed for.....???? or am I missing something??? I have replaced the recoil spring w/new several times.

What am I missing here?????????

PJH

Picture of assembled recoil spring guide & new buffer......Sundays shooting event with this set up showed no ill effects.

Picture of front of recoil spring guide in its proximity to the bolt shows a slight mis-alignment. This is caused by the buffer material having a slight curve, not sure what the material is. Installing new US WW 2 1928 springs (may be M-1???) will cause the spring to pig-tail out of the space between the spring guide & the back of the bolt when the bolt is retracted by hand.......Usless.........

Picture of old telescoping recoil spring guide, blued one, slightly bent, can probably fix that & some replacements Barry made. Never finished the replacements.... Seemed like I didnt need them then...... I am sure the telescoping guide front section can be much shorter than the original 2" length, it only has to go into the back of the bolt a short distance to ensure the stiffer spring stays in place.




 

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Physics. Funny how that bites us. A lighter bolt will be moving much faster, and the original 28 recoil spring will not stop it's motion before it batters the rear of the receiver. The 21 had a different pilot/ recoil spring for a reason. West hurley guns had issues with receiver cracks in the rear anyway, and it sounds like you may be accelerating ( pun intended) toward the same. Instead of lightening the bolt, try reducing the travel distance / time by utilizing a thicker buffer. This works well for the UZI.
 

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decrease in weigh =higher ROF however lighter +ROF = higher inertial impact forces against buffer
Example M2 Vs M3 50 cal backplate design , M2 uses the fiber discs while the M3 they had to go to stacked wave spring backplate buffer with a significantly larger diameter "every von Unterstands dis !!" :confused: (my old Swiss diesel teacher :rofl: ).
 

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Discussion Starter · #4 · (Edited)
Stiffer spring????

More than a bit of a goof here..........I drilled the 1/2" holes in the inner piece of the bolt, not the main body itself. Still a lightening of the recoiling components, just not so much weight as initial post implied. Failure to proof read objectively.

Do these analyses indicate a stiffer spring is in order??? Receiver design doesnt seem to leave room for a regular coil spring buffer replacement (think-1" long piece of MG 81 spring) at the rear of the bolt encircling the factory spring??

Now, last buffer replacement repair is an increased thickness segment of a bit softer material with cord in it. Dont know what it is, almost like a conveyor machine belt, but only 1/4" thick.

I just examined another bolt assembly. When in recoil, the inner segment of the bolt is flush with the back end of the bolt. When the bolt is in full recoil mode, the only area of it that contacts the buffer is the U shaped bottom segment of the bolt, greatly reducing the area that contacts the buffer. How about cutting the back end of the bolt off until it & the other part are flush with each other during full recoil?? I cant see any problem with that in a quicky think through, unless it would cause the recoil spring to stack itself, putting all recoil stress on the little circular part of the guide, effectively bypassing/neutralizing the buffer. Surely a spring up grade would by pass that problem.

PJH
 

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As stated the laws of physics are involved here. Trying to slow down a lightened bolt with a heavy spring is just a crutch. The real issue is the bolt is too light. When the Thompson was designed I'm sure a lot effort went into making sure the bolt was heavy enough so the gun didn't beat itself to death.
 

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As stated the laws of physics are involved here. Trying to slow down a lightened bolt with a heavy spring is just a crutch. The real issue is the bolt is too light. When the Thompson was designed I'm sure a lot effort went into making sure the bolt was heavy enough so the gun didn't beat itself to death.
Or Pop a case due to early "release" .
This probably does hold true in almost all developments, but even the 1919, IIRC, was toyed with and the bolt lightened by an armorer in WWII to increase ROF. Don't remember the specifics or names, but I remember reading about it.......it's all a balance of components I guess - and time to research variations.....
 

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When the Thompson was designed I'm sure a lot effort went into making sure the bolt was heavy enough so the gun didn't beat itself to death.
Pretty much this. The modifications are breaking parts. It's his gun, but I kept my sub guns stock for fearing of killing a transferable. It also made troubleshooting problems a lot easier.
 

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Discussion Starter · #9 ·
Throw in the towel???

Doubt it.

I will put a "stock" bolt section in & try that to see if that beats the buffer. If so, I will have to try to get a stiffer main spring...........SARCO has none, Numrich lists them at $6.95. Price doesnt raise much hope of a real good quality. Anyplace else???

Anyone other than Wolfe make springs???

PJH
 

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PJ,

everybody has already gave you teh root cause of whats going wrong on your tommy...

we all know you SHOOT your stuff... so by making those bolts lighter, it was only a matter of time & cycling before you crapped out the buffer & probably ruined the spring.

a stiffer spring in these tommy's is only a temporary fix. & your gonna make it real hard to cock if you get a stiff enough spring to make up for the weight you lost on the bolt.

the buffer is no big deal... you can make those real easy. & the buffer pad/block can be made out of a piece of cutting board you get over at walmart or the dollar store near your house.

ive made these for lots of people when we do the semi conversions.

the buffer wont fix the problem of you beating it to death, only getting that bolt weight back up some will.


did you weigh the adjusted bolt? & then weigh the factory bolt? im guessing you took about 4oz out of that bolt from the post you made above..

thats about 20% of the bolt weight. thats gonna take a lot of spring to offset that & then you would lose the cycle speed you gained.

would be just as easy to put a regular weighted bolt back in it & a new buffer.

call me at work one day if ya need a hand.

sean
 

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PJ,

everybody has already gave you teh root cause of whats going wrong on your tommy...

we all know you SHOOT your stuff... so by making those bolts lighter, it was only a matter of time & cycling before you crapped out the buffer & probably ruined the spring.

a stiffer spring in these tommy's is only a temporary fix. & your gonna make it real hard to cock if you get a stiff enough spring to make up for the weight you lost on the bolt.

the buffer is no big deal... you can make those real easy. & the buffer pad/block can be made out of a piece of cutting board you get over at walmart or the dollar store near your house.

ive made these for lots of people when we do the semi conversion.




the buffer wont fix the problem of you beating it to death, only getting that bolt weight back up some will.


did you weigh the adjusted bolt? & then weigh the factory bolt? im guessing you took about 4oz out of that bolt from the post you made above..

thats about 20% of the bolt weight. thats gonna take a lot of spring to offset that & then you would lose the cycle speed you gained.

would be just as easy to put a regular weighted bolt back in it & a new buffer.

call me at work one day if ya need a hand.

sean


You pretty much got to the root cause here "engineering in the vernacular" generally produces poor/bad/worse results.
 

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I just examined another bolt assembly. When in battery, the inner segment of the bolt is flush with the back end of the bolt. When the bolt is in full recoil mode, the only area of it that contacts the buffer is the U shaped bottom segment of the bolt, greatly reducing the area that contacts the buffer. How about cutting the back end of the bolt off until it & the other part are flush with each other during full recoil?? I cant see any problem with that in a quicky think through, unless it would cause the recoil spring to stack itself, putting all recoil stress on the little circular part of the guide, effectively bypassing/neutralizing the buffer. Surely a spring up grade would by pass that problem.

PJH
Did you assemble it right? The bolt should not touch the buffer when in full recoil because it is behind the guide rod disc. See the TM's
I've had my WH 1928 for about 30 years still with the stock fiber buffer. It used to get fired a lot. The front of the guide rod disc has the imprint of the bolt but it has not damaged the buffer or the receiver. Also if you use the GI felt oiler it will help soften the impact.
 

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i like this picture a lot:

Auto part Metal Screw Steel

there really shouldnt be this much space between your guiderod & that spring. not sure if you got a mismatched rod or if maybe someone before you cut it off .... but none of my kits or builds i did had that much space .. thats just asking for that thing to catch off center & either hit the rod or jam the spring.


you did post another rod in a picture that was bent. did that bend from being caught by the bolt? or did it bend because it bottomed out ON the bolt in the well?

its one of those things where there wasnt a lot of QC back in the wartime guns & they had LOTS of variance in the rods/bolts & bolt tunnels.


in a perfect tommy world the spring cant bind because the bolt stops on the plate of the rod & buffer before it does... & this is also before it bottoms out the rod & bends it.

soooo many things that can get out just a hair. yet these guns are so amazing even now.
 
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