Idea For Reducing Recoil Spring Pull Or Open Bolt?

[Merry Ploughboy]

See, Mendenhall, M., "Blish Lock Mystery Revealed", Small Arms Review, Vol. 9, No. 10, July, 2006, pp. 50-56.

[T Hound]

See, Mendenhall, M., "Blish Lock Mystery Revealed", Small Arms Review, Vol. 9, No. 10, July, 2006, pp. 50-56.

That's gonna be a long wait for me brother. I'll have to order that SAR issue first. Anyway you could shot us a few details and spoilers?

[mnshooter]

Not seeing what the aluminium can do as a buffer, maybe worth a few microns as it gets battered.

I can say that my limited tests seem to show the GI M1 buffer does no buffering whatsoever.

I used a dial mic and arbor press to compress one, and got no compression until I started to deform the rivets holding it together.

I beieve the rivets acted as rigid posts preventing the red composition material from doing even what little buffering it could offer.

My limited test seems to reflect this, but I'll be the first to welcome more supported information.

So far, this seems to show the additional strength of the M1 receiver design does it's job extremely well.

[winbar]

MNShooter,

I'm pretty much like Chris(emmaggee1917). Taking pictures is a real pain. You could copy your M1 buffer with some AL plate with very little time, put it in your gun and see what you get. I left the plate in the back of my '28 to help dissipitate the force over a larger surface area as opposed to just the little round buffer disc contacting the ridiculously thin rear of the 1928 receiver.

(I should have stayed over on the BM59 board.)

Doug

 

[mnshooter]

MNShooter,

I'm pretty much like Chris(emmaggee1917). Taking pictures is a real pain. You could copy your M1 buffer with some AL plate with very little time, put it in your gun and see what you get. I left the plate in the back of my '28 to help dissipitate the force over a larger surface area as opposed to just the little round buffer disc contacting the ridiculously thin rear of the 1928 receiver.

(I should have stayed over on the BM59 board.)

Doug

 

Although the aluminium plate might help as it could spread the force over more of the receiver, the rear mid section behind the buffer disc is already the strongest area. The total time and mass of impact (energy) would seem to be be unchanged with a solid aluminium plate.

Dissipation of force (on the 28's) by extension of time via compression of a softer buffer material seems widely accepted as the best way to minimize damage. PK and others have excellent buffers available. I improvised something similiar for the M1, even though it seems strong enough to not need it.

 

As always, I look forward to learning from anyone with more information.

Never been to BM59. Have to check it out.

[T Hound]

MNShooter,

I'm pretty much like Chris(emmaggee1917). Taking pictures is a real pain. You could copy your M1 buffer with some AL plate with very little time, put it in your gun and see what you get. I left the plate in the back of my '28 to help dissipitate the force over a larger surface area as opposed to just the little round buffer disc contacting the ridiculously thin rear of the 1928 receiver.

(I should have stayed over on the BM59 board.)

Doug

 

Although the aluminium plate might help as it could spread the force over more of the receiver, the rear mid section behind the buffer disc is already the strongest area. The total time and mass of impact (energy) would seem to be be unchanged with a solid aluminium plate.

Dissipation of force (on the 28's) by extension of time via compression of a softer buffer material seems widely accepted as the best way to minimize damage.

 

I agree totally. The physics involved is on sound footing. The goal is to definitely slow down the bolt; and,

therefore, compression of the spring AND buffer is the goal. Metal buffers will compress little and transmit more

force. So although aluminum is definitely a better choice than steel, strong rubber would have been a better buffer.

 

John

[jim c 351]

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

 

Bob,

No doubt that the Blish locked worked. The M1 TSMG is proof of that. The 28 and M1 bolts weigh the same and the M1 fires faster. According to your tests the semi seems to eject more violently than either the M1928 or the M1.

But isn't something missing in this discussion? Doesn't the semi bolt weigh less than the SMG bolt. And isn't the bolt channel shorted on the semi, making a shorter bolt travel?? This is like putting a plastic buffer behind a Uzi bolt to increase the ROF.

Lighter bolt ---shorter travel= stronger spring!!

Has this been considered, or am I wrong about the semi bolt being lighter?? (Or did I fail to read something in this discussion??)

Jim C

[T Hound]

Bob,

No doubt that the Blish locked worked. The M1 TSMG is proof of that. The 28 and M1 bolts weigh the same and the M1 fires faster. According to your tests the semi seems to eject more violently than either the M1928 or the M1.

But isn't something missing in this discussion? Doesn't the semi bolt weigh less than the SMG bolt. And isn't the bolt channel shorted on the semi, making a shorter bolt travel?? This is like putting a plastic buffer behind a Uzi bolt to increase the ROF.

Lighter bolt ---shorter travel= stronger spring!!

Has this been considered, or am I wrong about the semi bolt being lighter?? (Or did I fail to read something in this discussion??)

Jim C

 

Jim, if I may answer you, mindful that Bob is the tech expert here, you are not wrong about the bolt. The semi-bolt definitely has less mass than the full-auto bolt. I have both for comparison and I was just studying how to machine my FA bolt into a semi last night (for a striker M1 Semi build). I had both bolts in my hands and the felt weight is noticeably different between them. The Full auto bolt is much heavier by felt weight. And of course there is the fact that the semi is 1/10th of an inch less high than the FA which obviously means it has less of that metal there. But also there are the two recoil spring holes that the FA bolt does not have.

 

As for the Bolt channel, I'll have to compare a semi-measure to the full-auto blueprints. But I thought they were essentially the same.

John

Edited March 10, 2012 by T Hound

[Merry Ploughboy]

My thoughts and humble opinions on the Blish device. (1) The Blish device is not a "lock". At no time is the bolt locked in position by the Blish device. (2) There is little or no friction or sticking of dissimilar metals between the Blish device and the bolt. Friction causes wear. After tens of thousands of rounds through a 21 or 28 Thompson, has anyone seen a worn out Blish device? Pictures? (3) The Blish device, in conjuction with the slots in the receiver and bolt, slows the rate of fire by way of effectively lengthening the fore and aft path that the bolt travels - nothing more. As the Blish device moves through its angular and up/down "lost motion", about 5/8" or so of effective linear travel is added to the bolt path. As noted by others, longer bolt path usually equals lower rate of fire.

 

These opinions are not typically shared by others, and I've long given up trying to convince anyone to accept my beliefs. Likewise, I have not been nor do I entertain arguments that attempt to sway my beliefs.

[jim c 351]

My thoughts and humble opinions on the Blish device. (1) The Blish device is not a "lock". At no time is the bolt locked in position by the Blish device. (2) There is little or no friction or sticking of dissimilar metals between the Blish device and the bolt. Friction causes wear. After tens of thousands of rounds through a 21 or 28 Thompson, has anyone seen a worn out Blish device? Pictures? (3) The Blish device, in conjuction with the slots in the receiver and bolt, slows the rate of fire by way of effectively lengthening the fore and aft path that the bolt travels - nothing more. As the Blish device moves through its angular and up/down "lost motion", about 5/8" or so of effective linear travel is added to the bolt path. As noted by others, longer bolt path usually equals lower rate of fire.

 

These opinions are not typically shared by others, and I've long given up trying to convince anyone to accept my beliefs. Likewise, I have not been nor do I entertain arguments that attempt to sway my beliefs.

 

Merry,

I think you are getting all hung up on words, much like debating the definition of what the word "IS" is.

If the Blish Lock was truly a lock by your definition, one would have to pull the actuator back by hand after each shot.

The Blish was intended to slightly delay bolt opening to allow pressure to drop before ejection. You have just illustrated that it accomplishes this.

Would it make you happy, if, from this day foreward, we changed the term from Blish Lock to Blish Delay??

Anything to make you happy.

Jim C

[Merry Ploughboy]

Jim C., and anybody else who cares,

 

The Blish device does not delay the bolt opening. Were there such a delay (i.e., lock), the Blish device would break or wear. The Blish device does neither. Such breakage or wear would be very apparent in closed bolt, semi-auto versions that have been built having Blish devices still present, e.g., the Polston design. The Blish device adds overall length to the bolt travel path - nothing more.

 

Again, my opinion, and yet, another failed attempt on my part. I give up, and this time I really, really mean it.

[jim c 351]

. As the Blish device moves through its angular and up/down "lost motion", about 5/8" or so of effective linear travel is added to the bolt path. As noted by others, longer bolt path usually equals lower rate of fire.

 

Mary,

Here are your own words that the "Blish Delay" slows down bolt opening. If you can't read and understand your own words then no one in the world can explain it to you.

And yes I have seen worn and broken "Blish Delays".

Are you aware of the fact that the Reising SMG also uses the Blish Principle to construct a lighter weight Bolt/op- rod firearm?

Jim C

[reconbob]

You can't evaluate the blish lock by opening and closing the bolt by hand and concluding

it doesn't "stick" or "lock". The lock functions during the instantaneous millisecond peak of high

pressure. Annnd I agree that - just like advanced primer ignition - there are many who simply refuse to

believe this.

 

Bob

[Joe H]

Bob,

 

Anybody that doesn't believe in advanced primer ignigtion should read Engineering Design Handbook, Gun Series, Automatic Weapons. it's a phamphlet (348 pages) put out by the Federal government and its free. It's AMC Phamphlet 7C6-260. Just Google 7C6-260, it's about the third hit.

 

The first example is a straight blowback open bolt .45 cal. Based on a very complicated design ( including calculus) the bolt needs to be 3 lbs and the firing rate would be 400 rpm. The force of the return spring is ignored but used only in the calculation of the cyclic rate. Using the advanced primer ignition concept the bolt becomes 1.3 lb and the rate inceases to 800 rpm, sound familiar?

 

Assuming the calcs are reasonable approximations, a semi would require 3 lb bolt to keep the action closed, actually allow the bolt to move no more than the case head thickness, as the pressure drops to a safe level. If the bolt is slightly over 1 lb and the return springs are over 2 LB. the semi should work. I definitely wouldn't cut it that close but the 7 lb (my guess) from the ez pull springs plus the bolt weight have been proven to work. I just can't see any reason for the Kahr "gorilla" springs.

 

As far as the Blish Lock if you don't believe in "metal adhesion" just the geometry of how it works provides a "delay".

 

Joe

Edited March 11, 2012 by Joe H

[reconbob]

Yea...I was wondering who had the other copy. There are not too many of these books

around. The calculus gets real heavy real fast. But nevertheless you are (as usual)

correct. The example in the handbook for advanced primer ignition is basically a generic

rendition of the M1921, M1928, M1928A1, or M1 Thompson firing (not locking) mechanism.

I salute you as a true student of small arms...

 

Bob

 

http://i51.photobucket.com/albums/f392/reconbob/IMG_2774.jpg

 

http://i51.photobucket.com/albums/f392/reconbob/IMG_2775.jpg

[T Hound]

Bob,

 

I just can't see any reason for the Kahr "gorilla" springs.

 

As far as the Blish Lock if you don't believe in "metal adhesion" just the geometry of how it works provides a "delay".

 

Joe

I once called Kahr, a couple of years ago, and just after I go my 1928 models and asked them if they had a spring requiring less pull. The salesman kinda went

"ape nuts" on me trying to tell me how dangerous it was and that I shouldn't do that. Also went off about "putting on old barrels" which I didn't even mention. I figured the guy was having a bad day and so I excused myself. Don't recall the guys name, but I didn't believe him anyway. I figure it has to do with the usual thing most corporations worry about----law suits.

 

I agree with you whole heartedly about the geometry of the path. That is what I have been trying to say all this time. It is very true that you cannot gauge the reaction

of the recoil forces along that path simply by pulling it back or back and forth. I would stake my physics degree upon it. As for wear issue mentioned before: there isn't anything, including diamonds, that are indestructable. You fire the gun enough then you are going to get wear. It is a fact. It may start microscopic, but with continued pounding and sliding motion comes greater wear.

Edited March 11, 2012 by T Hound

[mnshooter]

I don't have the equipment to do this, but it would seem that putting the 28 in a fixture, with a strain gauge between the rear of the receiver and a stationary rear block -could be used to record a recoil time line. If there is any delay beyond the additional angle travel of the blish mechanism, a delay which should also show as a change in recoil impulse, -this should finally settle the issue.

Anyone have the machinery and desire to try this?

 

Mythbusters: Where are you?

[reconbob]

Mnshooter and others - I have thought about this for some time. My idea was not to have

a strain gage, but to use a sliding weight. Set up a receiver with an extra hole in the back

thru which a rod passes which is connected to the buffer. When the gun is fired the bolt

strikes the buffer and the force is transmitted to a small sliding weight. in this fashion the

force and velocity with which the bolt strikes the buffer can be determined.

You could test M1921 vs. M1928/A1, vs M1/M1A1. You could test blish lock vs. blish

lock with the ears cut off.

Now, you obviously don't want to wreck a transferable gun, but you could test all of the

bolts in one receiver. Since I am able to make a receiver, maybe someday soon I can find the

time to do this.

 

Bob

[jim c 351]

Mnshooter and others - I have thought about this for some time. My idea was not to have

a strain gage, but to use a sliding weight. Set up a receiver with an extra hole in the back

thru which a rod passes which is connected to the buffer. When the gun is fired the bolt

strikes the buffer and the force is transmitted to a small sliding weight. in this fashion the

force and velocity with which the bolt strikes the buffer can be determined.

You could test M1921 vs. M1928/A1, vs M1/M1A1. You could test blish lock vs. blish

lock with the ears cut off.

Now, you obviously don't want to wreck a transferable gun, but you could test all of the

bolts in one receiver. Since I am able to make a receiver, maybe someday soon I can find the

time to do this.

 

Bob

 

Bob,

For calculating chamber pressure, they, at one time used lead or copper crushers for comparison.

I would think the same could be done to compare bolt recoil force. A lead or copper crusher could be installed in the flange in front of the buffer. Then compression measurements could be used for comparison. Its probably a waste of time. Those that want to believe that the Blish has no effect will be unconvinced.

As far as the Blish is concerned its effect is there for anyone who wants to see. Paul K. at "Diamond K" has made a nice business out of correcting the West Hurley Blish slots in the receivers. This is necessary to prevent cracked receivers, broken actuators, and broken Blish locks . At my club I have personally seen one broken actuator and one broken Blish lock. This is at a club that fields only 4 or 5 M1928's.

The first AO m1's were rejected by ordnance inspectors because they fired at too high RPM. Why?? No Blish locks. They later let them pass because the needed guns.

Since the 28 Bolt/actuator and the M1 bolt weigh the same, I think ROF is the best indicator of the Blish working.

 

Now, as far as advanced primer ignition is concerned , more discussion would be required. If you saved the data that I sent to you several years ago, you'll remember the results were hard to predict.

I list some data so you won't have to look it up.

Savage M1 SMG #135,XXX

Rainier 230 gr plated bullet , 5.5 gr unique, CCI primer.

M1 bolt -----904 RPM

M1A1 bolt--771 RPM

M1 bolt with firing pin blocked into forward position to create a M1A1 bolt,--909 RPM

Theoretically test 2 and 3 should have had the same RPM ,but they did not.

My AO 28a1 clocked at 795 RPM with you weighted actuator.

 

But all this gets away from my previous question concerning the weight of the Kahr semi bolt an length of travel in the receiver.

Jim C

[Joe H]

Jim,

 

I used to have a Kahr semi so I don't have a bolt to weight. It is 0.1" less in height than a TSMG so I would estimate it at 1lb 8oz or less. It travels 5/8" less than the TSMG, at least mine do.

 

I've built a couple of semi's. The 1928A1 maintained the blish lock. I also built an M1. On the 1928A1 the force of the return springs on the closed bolt is 7#. On the M1 the force is about 9#. Both of my semi's use the reduced bolt travel. On the 1928A1 the reduced travel is a geometric necessity due to the length of the return spring guide rods. My semi bolt weights are below.

 

BOLT WEIGHTS (recoiling parts):

 

TSMG 1928A1 1 lb 9 oz

SEMI 1928 1 lb 8 oz

TSMG M1 1 lb 13 oz

SEMI M1 1 lb 10-1/2 oz

 

I have noticed any unusal ejection of the spent cases but then I really have no basis of comparison.

 

Joe

Edited March 11, 2012 by Joe H

[T Hound]

Copper/lead crushers are traditional but not as accurate as pressure transdrucers. I have already imagined a system in which the only

sacrifice necessary would be the buffer pilot (And these are still readily available from Numrich). You could mill a slot for the transducers

two (or three) wires along its rear head (the part that sticks through the receiver), and then place the transducer between the receiver and

the buffer assembly. This would give you a continous real time read out of the pressure that could be compared to all the theory I just

read in the Engineering Handbook you guys posted yesterday. Comparing the pressure of recoil curves between the M1/M1A1 and the

blish lock models would be definitive.

Edited March 11, 2012 by T Hound

[T Hound]

Okay guys, I measured the M1 TSMG bolt and the Kahr semi-auto bolt on a digital scale.

 

Here are the results:

 

Kahr Semi-auto Bolt: 1 pound 6.75 ounces or 22.75 oz

 

TSMG M1 Bolt: 1 pound 12.25 ounces or 28.25 oz

 

Both bolts are factory made with no modifications.

 

The difference between them is this: 5.5 ounces So the factory TSMG bolt is significantly heavier and would be noticeably

heavier by felt weight.

 

Question for JoeH:

I see your bolt weights are comparable, but it wasn't clear to me if the semi-bolt

weights you mentioned were factory stock or modified TSMG bolts you used in your

excellent SA builds.

Edited March 11, 2012 by T Hound

[Joe H]

T-Hound,

 

They the actual weight of the bolts in my builds. They are modified TSMG bolts.

 

Joe

[T Hound]

Well there we go. Since I now have the Engineering Handbook, I will use the info to calculate the required recoil spring strength for the Kahr semis. Maybe we can get a range of required spring strengths for any Thompson semi going. That would be useful for builders. I have been looking for a firearm project to use my underutilized math skills upon anyway. I will post the results (and calculation details) as a separate thread when I get them. Edited March 11, 2012 by T Hound