Idea For Reducing Recoil Spring Pull Or Open Bolt?

[T Hound]

I had an idea for recoil spring pull reduction that involved taking the recoil springs out of the semi-auto and

replacing the firing pin spring with the recoil spring of a TSMG (after modifying the pilot of course).

 

But then it occurred to me that this would most likely cause the gun to become an open bolt system if I did the modifications.

 

Of course, there is also the issue of whether the gun will cycle properly after that.

 

What do you guys think? Anyone tried it?

Edited March 6, 2012 by T Hound

[reconbob]

This sounds like a BIG problem waiting to happen. I have mentioned this here

before. There are believers and non-believers.

On the full auto Thompson, and for just about any "blowback" submachine gun

the cartridge is fired a faction of a second BEFORE the bolt slams fully shut. The

result is the forward moving mass of the bolt (as opposed to the motionless shut bolt

of the semi) absorbs most of the recoil force generated by the firing of the cartridge

and the propelling of the bullet out of the bore.

This seems like a minor thing but its a BIG difference. How big? The semi-auto

Thompson fires from a closed, motionless bolt. The 25-30 lb. recoil springs are

needed to act as shock absorbers for the recoil impulse, yet even with those heavy

springs the bolt moves at high velocity and the cartridge is ejected violently.

We test fire Thompsons here all the time. We fire from a kneeling positon

and where we have the trap set up the cartridges ejected from a full auto fly a few

feet and hit against the front doors of one of the machining centers about 4 or 5

feet off the ground. The doors are 6 feet high.

Fire a semi-auto and the cartidges are violently ejected up over the 6 ft. doors

and fly completely over the machine which is about 8 or 9 feet deep and hit against

shelving another 3 feet beyond.

Not scientific, but you get the idea.

As I said, a lot of people don't buy this but there is not a better example than

full auto Thompson with its mild action and light recoil spring vs. semi Thompson with

stationary bolt and very heavy springs to show how much the forward moving bolt

of the full auto asborbs so much of the recoil energy.

 

Bob

Edited March 6, 2012 by reconbob

[T Hound]

No, actually Bob I think you are quite convincing. I was afraid that it would be something like that which

is why I wanted to pose the question to you guys before I did something stupid. I much prefer to

learn from other peoples experience and expertise than from hard knocks. Thanks!

 

John

Edited March 6, 2012 by T Hound

[winbar]

John,

I took the recoil springs off my WH1927A5(owned since 9/94)and reassembled (the upper) it with only those pieces missing. If you do that you will see that there is no tension to hold the bolt body against the front of the receiver.

 

I have never examined a Kahr to see the changes they did to the firing pin and/or bolt so I don't recommend you do the next step.

On my gun, the firing pin protrudes beyond the bolt face until the bolt contacts the front of the receiver.

 

(In the spring of 1995 I attempted to change a used WH1927 carbine to 10mm having bought 10mm barrels and bolts from Gun Parts Corp (unfortunately no magazines were available), so I have done some tinkering with them.)

 

Assembling my A5 receiver to the frame and cocking it,(then inserting a fired case in the chamber)I moved the bolt as far forward as it would go. The firing pin would not release because the bolt was not in battery (the disconnector works!) I then smacked the cocking handle so the extractor would jump the rim of the cartridge case and the bolt then contacted the front of the receiver. Now when I pull the trigger the firing pin releases.

As designed, the WH1927's will not fire from an open bolt by removing the recoil springs. Even if you pointed the gun straight down(using gravity to get the bolt in battery) you still have the protruding firing pin producing a misfeed.

Doug

[T Hound]

So basically the firing pin spring doesn't have enough force to properly move the bolt forward to seat a cartridge either? Sounds like an out of battery misfire waiting to happen.

 

I was thinking about what Reconbob said and came to some clarity in my mind. I have a background in physics, so I agree totally with him that if the recoil springs are not there then the blow back could really cause a problem in the form of the bolt going back and eventually causing damage to the back of the receiver with enough poundings. But I was sure that taking the recoil springs out would cause the bolt to stop with the firing pin and basically become an open bolt (not that that is what I wanted-my actual motive was to avoid any illegal configurations). But given the weakness of the FP spring, I can certainly see how it would fail to cycle properly from the "open bolt" configuration. Knowing how and when the Thompson cycle will fail is just as interesting and useful to know as when it will work.

 

But this also gives clarity to my idea. Perhaps, with the two SA recoil springs removed, instead a strong recoil spring surrounding the FP spring can be used

together with the SA firing pin spring in order to push the bolt forward with sufficient force to cycle properly AND protect the back end of the receiver from a severe pounding. But I do forsee the need for some sort of extended scaffolding with such a configuration-if possible.

 

John

Edited March 7, 2012 by T Hound

[reconbob]

For what its worth the fabled Polston semi-auto fired from a closed bolt

and had an easy and smooth cocking force very much like a full auto. His

gun used a fucntioning blish lock (so obviously the blish lock actually does

work!)

Joe H. here on the board, I believe, made a semi that used an original

bolt and blish lock, but had the spring loaded firing pin of the semi. Perhaps

he will weigh in on what type of cocking force his design has...

 

Bob

[winbar]

I took the ejector out of my WH1927A5. Guess what? The firing pin ejects the fired case. Farther case ejection perhaps depending on sooner ejection in the recoil cycle and higher angular acceleration(case ejecting from the center instead of the side.) I guess I'm going to have to go shoot my mighty WH!

Doug

 

[T Hound]

. His gun used a fucntioning blish lock (so obviously the blish lock actually does

work!)

Bob

 

Yeah, I've never believed it didn't work despite all the protests I've heard. Friction is friction brother! If it weren't for that

we'd all be crashing our jeeps!

[T Hound]

I took the ejector out of my WH1927A5. Guess what? The firing pin ejects the fired case. Farther case ejection perhaps depending on sooner ejection in the recoil cycle and higher angular acceleration(case ejecting from the center instead of the side.) I guess I'm going to have to go shoot my mighty WH!

Doug

 

Very, very interesting. It's the info bits like this that need to be documented! Course, maybe, just maybe, the ejector was there to spare wear to the firing pin. And then again, maybe not. Worth knowing tho.

[Lancer]

Yeah, I've never believed it didn't work despite all the protests I've heard. Friction is friction brother! If it weren't for that

we'd all be crashing our jeeps!

 

 

I doubt anyone would suggest that the Blish lock didn't add some friction, it's the concept of dissimilar metals adhering to each other under pressure in the Thompson that was in question. If the goal is only to add some friction, it would seem to me that there should be a less complicated way to do it.

[darrylta]

I've always been enamored at the genius of the early American gun makers,

especially Colt, Browning and General Thompson's designers. They were truly

out of the box thinkers.

-Darryl

[T Hound]

I doubt anyone would suggest that the Blish lock didn't add some friction, it's the concept of dissimilar metals adhering to each other under pressure in the Thompson that was in question. If the goal is only to add some friction, it would seem to me that there should be a less complicated way to do it.

 

But it wasn't just to add friction. The whole point was recoil reduction as I am sure you are well aware of. It is my belief that the Blish lock works due to friction AND to the geometry of its path which tends to redistribute the recoil force along the rearward path at angles other than straight backwards. The Kriss SMG does this same thing-but even better. The question then becomes whether it is a necessary recoil reduction. The US Gov didn't think so and all of us know they already uncomplicated it long ago in the form of the M1/M1A1 which I admit is hands down my favorite Thompson. But sometimes complication is necessary for innovation as in the Kriss sub gun.

[mnshooter]

There must be some appeal to the Kriss that I'm missing. Maybe I'm just jealous 'cause I can't own a FA version.

The idea of the Blish redirecting some of the recoil force seems sound. But, look at how elegantly the 90 year old does this compared to the Kriss. I keep thinking it looks like someone decided to hang their school lunch box underneath.

Edited March 7, 2012 by mnshooter

[Lancer]

But it wasn't just to add friction. The whole point was recoil reduction as I am sure you are well aware of. It is my belief that the Blish lock works due to friction AND to the geometry of its path which tends to redistribute the recoil force along the rearward path at angles other than straight backwards.

 

I'm not aware of this at all, must have missed that part in TGTMTTR.

 

The whole point of the Blish Lock was to momentarily lock the bolt using the Blish principle of dissimilar metals adhering to each other under high ppressure. Whether or not this actually worked is what is questioned & was found to be not needed in the M-1 even if it did. The fact that in actual use it added friction is, of course quite obvious. The angles involved just add to the friction. I serious doubt the Blish reduced recoil to any considerable degree. In the end, recoil in the Thompson isn't an issue because of it considerable mass.

[T Hound]

Well, in my case it is most definitely is jealousy.

[T Hound]

TGTMTTR? I don't know what that is Lancer.

 

The whole point of the Blish Lock was to momentarily lock the bolt using the Blish principle of dissimilar metals adhering to each other under high ppressure. Whether or not this actually worked is what is questioned & was found to be not needed in the M-1 even if it did. The fact that in actual use it added friction is, of course quite obvious. The angles involved just add to the friction. I serious doubt the Blish reduced recoil to any considerable degree. In the end, recoil in the Thompson isn't an issue because of it considerable mass.

 

Yeah, the "Blish" principle was certainly why it was designed that way, but Friction is always due to applied force and it is NOT obvious that what is going on is a redirection of force along the Blish pathway unless you are well versed in the force vectors of friction. The angles involved redirect the force at right angles along the path and that is absolutely a consequence of physical law. Whether this force redirection is actually significant is a

matter for experimentation and measurement and not subjective experience. But the US Gov didn't care

about that either. It just wanted a SMG that was easier to machine.

 

The mass of the Thompson is definitely more significant when it comes to FELT recoil. But if you want your

bolt and receiver to last then you have to pay attention to the internal recoil too.

Edited March 7, 2012 by T Hound

[Lancer]

TGTMTTR? I don't know what that is Lancer.

 

Yeah, the "Blish" principle was certainly why it was designed that way, but Friction is always due to applied force and it is NOT obvious that what is going on is a redirection of force along the Blish pathway unless you are well versed in the force vectors of friction. The angles involved redirect the force at right angles along the path and that is absolutely a consequence of physical law. Whether this force redirection is actually significant is a

matter for experimentation and measurement and not subjective experience. But the US Gov didn't care

about that either. It just wanted a SMG that was easier to machine.

 

The mass of the Thompson is definitely more significant when it comes to FELT recoil. But if you want your

bolt and receiver to last then you have to pay attention to the internal recoil too.

 

 

TGTMTTR = Helmer's book "The Gun That Made the Twenties Roar"

 

Your point is becoming muddled, I'm not real sure what it is at this point. If it is that the blish is needed for some reason other than it's intended purpose, I think the existence of the M - 1 is proof that it is not. If it is that the blish slows down the bolt through friction, as I said, this is obvious. If it is that the angles of the blish movement slow the bolt, I would say that if it does, it's because they add to the friction. I don't believe there is enough mass in the lock itself for it's redirection to slow the much heavier bolt to any considerable degree.

[T Hound]

TGTMTTR? I don't know what that is Lancer.

 

Yeah, the "Blish" principle was certainly why it was designed that way, but Friction is always due to applied force and it is NOT obvious that what is going on is a redirection of force along the Blish pathway unless you are well versed in the force vectors of friction. The angles involved redirect the force at right angles along the path and that is absolutely a consequence of physical law. Whether this force redirection is actually significant is a

matter for experimentation and measurement and not subjective experience. But the US Gov didn't care

about that either. It just wanted a SMG that was easier to machine.

 

The mass of the Thompson is definitely more significant when it comes to FELT recoil. But if you want your

bolt and receiver to last then you have to pay attention to the internal recoil too.

 

 

TGTMTTR = Helmer's book "The Gun That Made the Twenties Roar"

 

Your point is becoming muddled, I'm not real sure what it is at this point. If it is that the blish is needed for some reason other than it's intended purpose, I think the existence of the M - 1 is proof that it is not. If it is that the blish slows down the bolt through friction, as I said, this is obvious. If it is that the angles of the blish movement slow the bolt, I would say that if it does, it's because they add to the friction. I don't believe there is enough mass in the lock itself for it's redirection to slow the much heavier bolt to any considerable degree.

Thanks for the book info. Good to know.

 

No, the point is not muddled, the trouble is that too many different points are being made.

We are not fundamentally in disagreement about the Blish, the existence of the M1as proof against the Blish and the Blish angles. And as you say, some things, like the low mass of the lock and friction are obvious. What I said is NOT

obvious was that the redirection of force is NOT just friction, but the geometry of the Blish pathway too. It creates

contact forces in a different direction than the recoil for very brief periods of time (to be sure friction has a role in doing

this of course). If it was just friction alone then it would merely heat the frame surface. Even so, I already believe,

obviously like you, that it is not a huge factor in reducing bolt speed/recoil. But since older model Thompsons still use it, then it is worth considering.

 

To be absolutely sure however, you would have to place pressure transducers between the buffer assembly and the back of the receiver on both the M1 and the 1927/1928 in order to measure exactly what is happening. If the

difference is slight then the Blish lock is totally worthless. But if the difference is measurable, well that starts a

whole new discussion.

Edited March 7, 2012 by T Hound

[Joe H]

 

Joe H. here on the board, I believe, made a semi that used an original

bolt and blish lock, but had the spring loaded firing pin of the semi. Perhaps

he will weigh in on what type of cocking force his design has...

 

Bob

 

I built a 1928A1 semi using Bob's receiver & a parts kit. It's striker fired very similar to NK/Kahr. I also utilized the blish lock & oiler. The striker spring is the original TSMG spring cut to 6 - 1/4" . The return springs are IMO very similar to the EZpull springs and include the additional buffer. The return springs bear on the actuator, not the bolt. The spring load on the closed bolt is about 7# which is about the same as the TSMG. The pull is easy & smooth. Nothing like the Kahr. I also built an M1 but I used slightly heavier return springs 8+# on the closed bolt.

 

IMO Lancer is right, the blowback is controlled by the weight of the bolt. Althought the advanced primer ignition may come into play in the FA Thompson, IMO the closed bolt semi relies pretty much on the bolt weight. IMO the Blish lock does delay the blowback but it is not totally necessary.

 

The Orelikon 20 mm cannon is a classic example of the advanced primer ignition used in design. It does not have a locked bolt but does have massive springs. I don't think I'd want to pull the trigger on a closed bolt semi copy of that monster.

 

Joe

Edited March 7, 2012 by Joe H

[winbar]

Joe,

So, are you running 2 springs for the bolt (ala greasegun) and a separate spring for the striker?

Doug

[T Hound]

. IMO Lancer is right, the blowback is controlled by the weight of the bolt. Althought the advanced primer ignition may come into play in the FA Thompson, IMO the closed bolt semi relies pretty much on the bolt weight. IMO the Blish lock does delay the blowback but it is not totally necessary.

 

Yes, there is no question that the bolt plays the primary role. The point was whether the Blish lock plays any significant

role whatsoever. Since there are those Thompson fans out there that contend that it does, I am saying that

the only true way to be sure is to perform the two recoil pressure measurements. That will tell you if there

is a serious difference between the internal recoil of the M1 and the 1927/1928. FELT recoil however is

without a doubt going to be overwhelmingly controlled by the entire mass of the gun.

Edited March 7, 2012 by T Hound

[winbar]

I can say from personal experience that the blish lock does work. After firing my WH1928 once at 650 rpm, I removed the ears from the blish lock which raised the cyclic rate 200 to 250rpm. I also put a 3/16" thick aluminum plate in the back of the receiver as an extra buffer, which also shortened the recoil stroke.

 

Joe ,

I found your build over on www.weaponsguild.com, real cool!

Thanks,

Doug

[emmagee1917]

Was there not a time ( late 80s to early 90s ) when cutting the ears off was "fasionable " ? I believe that many people who did so for reliability issues ended up with cracks in the rear of thier recievers . I was going to do that to my WH but never got around to it , trying the gunmachines retractable pilot and springs instead. But , cracked recievers if you do and not so much when you don't leads one to think the Blish lock does it's job , but one wonders if it does so by the principle it was patented on or by other means.

Chris

[winbar]

Chris,

Gunmachines had just folded when I acquired my '28. After reading about the pros and cons in MGN and SAR, I went with an aluminum buffer.(It definitely left an impression)

Doug

 

Anybody have .400 Corbon chamber reamers? Time to truly test the blish lock.

[T Hound]

I can say from personal experience that the blish lock does work. After firing my WH1928 once at 650 rpm, I removed the ears from the blish lock which raised the cyclic rate 200 to 250rpm. I also put a 3/16" thick aluminum plate in the back of the receiver as an extra buffer, which also shortened the recoil stroke.

 

Joe ,

I found your build over on www.weaponsguild.com, real cool!

Thanks,

Doug

Interesting Doug, very interesting. And Chris, in regards to cracked receivers, I think they did have to increase the metal at the back of the M1 if I am not mistaken. That would certainly be an indication that they knew there would be increased pounding.

 

John

Edited March 7, 2012 by T Hound