If the 1928 bolt had weight added to it, could the lock be omitted?

[buzz]

Why can't a weight be added to the 1928 bolt and then operate like an M1 bolt?

 

The 28 is a retarded blowback and the M1 is blowback

 

The delay in opening the bolt provided by the blish lock isn't actually needed.

 

So the concern with removing the blish lock would be that the rear of the receiver would be battered by the higher bolt velocity and energy.

 

So why not soak up some of that velocity with a heavy bolt, and reduce the rate of fire to M1 levels?

 

 

This option would eliminate the need to re-machine the blish slots on a west hurley.

 

Probably be a lot easier to hack the rear part of a bolt off and pin on a tungsten replacement.

 

[06AngusSG]

For that matter, what about doing the opposite of what we do with the SA guns. Would a heavier recoil spring do the same thing as a heavier bolt w/o the blish?

[Paladin601]

It would be the same a an M1 bolt in a 28 receiver, I believe.

[thirtyround]

To the true Thompson Lover... ABSURD,

But good and intriguing ideas of what if. The 28 modification added weight to the actuator to slow the rate of fire down to what the Navy wanted. The M1 Auto Ordnance Thompson had an issue with high rate of fire, and extra weight was added to the bolt handle in an attempt to slow it down. This wasn't practical, in the end the M1/M1A1's were accepted and speed along at 700 -750 rpm. Some on the board here have tested their guns at even higher rates.

Back to your idea on the adding weight to the 1928 to slow it down where you don't need the blish lock. The receiver is what it is in the back end so even if you put a synthetic buffer back there it still is going to kick the back end pretty good, probably with negative and damaging results.

As long as were "what if'ing" things here a bit, why not a rate reducer similar to what the rate reducer does in some later model AK 47's... that is slow down and impede the rearward momentum of the bolt by catching it with a pawl in a small recessed pocket in the bolt, and then release it under forced spring tension.

Humbly submitted.

[mnshooter]

No matter what else is done, the 28 receiver doesn't have the massive rear angular reinforcement of the M1.

In fact, the beautiful Art Deco curves of the 28 rear actually create a leverage effect that directs rear thrust against the weaker areas.

[gunhistorian]

This is just a further thought on the "no Blish lock" bolt in a '21 or '28: If an M1 or M1A-1 bolt was modified by filling the bolt handle hole (for weight) and drilled on top for an actuator type bolt handle, maybe adding a stronger recoil spring, would the rear impulse be sufficiently reduced to prevent rear receiver failure. Whoops! Would the receiver have to be modified for the M1 or M1A-1 bolt to fit? (Trying to picture the inside dimensions -- width to width -- of the receiver and the M-1 bolt. And, how about making the "oiler" a retarding device? Maybe lengthen it and add "something" to provide some friction. . . See! these questions are why I never could be an engineer!

[shadycon]

Don't f*ck with success!!!!!!!!!!!!!!!!!!!!!!!!

[Piep]

This is just a mental excercise.

No need to be concerned.

[buzz]

Suppose a 28 with the blish lock installed ran at 900 RPM

 

Then the lock was removed and it ran at 1000 RPM

 

Then you added enough weight to the bolt to bring the ROF back down to 900 RPM

 

At that point, wouldn't the extra weight be doing the exact same job as the blish lock?

 

 

With the blish lock installed, the recoil cycle goes like this -

First the gun fires.

 

Then there is a pause while the lock retards the bolt from recoiling

 

Then the bolt recoils backward with an initial velocity

 

the spring is compressed

 

the bolt thumps against the buffer

 

the spring is uncompressed

 

the bolt thumps against the stop at the front of the receiver.

 

the end

 

 

with the heavy bolt installed instead,

 

you have the exact same sequence except that the pause is eliminated

 

and the spring compression cycle is longer because the bolt velocity is slower

 

 

You'd have to come up with some kind of pressure test to prove that the bolt was thumping softer into the rear buffer and front bolt stop.

 

What you would need is an electric strain gauge mounted to the receiver to show that the load on the receiver was reduced.

[buzz]

Here's another idea -

 

Instead of making the bolt heavier, make the buffer disk thicker and more elastic.

 

The original buffers are just a resin washer, it's hard as a rock

 

The new urethane buffers aren't much softer

 

if you put a 1/2 thick buffer made of some squishy elastic material, like neoprene, you could greatly reduce the load on the rear of the receiver

 

 

the rear plate on the receiver picks up two loads -

 

the gradual load applied by the recoil spring as the bolt compresses

 

the sudden impact load of the bolt ramming the buffer disk

 

technically, the buffer disk acts as a spring with a real high spring load rate

 

lower the load rate and you'd greatly reduce the impact force

 

 

 

Suppose you keep the blish lock in place -

 

Has anyone ever experimented with using a softer buffer material along with the blish lock?

 

what I would try is use the normal urethane buffer disk and just use another softer and thicker neoprene disk in front of it.

 

that way if the softer material failed or was completely compressing, you'd still have the urethane to prevent metal to metal contact

 

you could use the ROF to check if it was working

 

as long as the ROF was the same or lower, that means the loads on the receiver are reduced

Edited June 3, 2015 by buzz

[Paladin601]

Richardson has already done it, he does not recommend using the M1a1 bolt. He also has a receiver modified for that.

 

Great idea, though, I like it. Put all the guts of a M1 in the 28 with a modified buffer, why slow it down, just learn good trigger control.

[buzz]

actually, we are all already sort of doing one of the above suggestions

 

urethane buffers are a lot softer than the resin buffers

 

and they are thinner

 

so you can conclude logically that the impact load is lower

 

how much lower would be the square root of the ratio of the two spring rates

 

 

for example, if the urethane was 75% as stiff as the resin, the load would be reduced by about 15%

 

I remember doing this calculation for my MAC10

 

the original MAC buffer was a real soft neoprene rubber but people use hard urethane buffers now

 

which is going to increase the load on the rear plate welds

 

so I calculated the theoretical difference out of curiosity

 

it doesn't seem to make much difference for those guns

 

people use real thick hard buffers to make the ROF go way up

 

that's going to hugely increase the loads but it doesn't seem to bother the gun any

Edited June 3, 2015 by buzz

[giantpanda4]

Blish lock retards some of the energy into the sides of the receiver. Without them, the rear of the receiver must bear all of the energy.

 

Hurleys have been known to crack... and M1s have signuificantly more meat in the rear than the 21/28 guns.

 

So - Not the same.

[buzz]

this problem is actually very complicated

 

I'm aware that the blish lock makes the bolt unlock at a lower pressure.

 

 

I can logically prove that the bolt could be made heavy enough to do the job of the blish lock:

 

 

Looking at the problem backward -

 

consider the fact that if you lightened the bolt and kept the blish lock, the ROF would increase

 

this is already been proved by experiment

 

the operation of the blish lock would not be affected by the lighter bolt

 

so the higher ROF could be attributed to the higher kinetic energy of the bolt

 

there's no other possibility - a higher ROF means a faster moving bolt

 

so the higher ROF means higher energy in the bolt, which has to be absorbed by the spring and buffer

 

 

But if you went the opposite way, made the bolt heavier, you'd get the opposite effect

 

In the extreme case you could make the bolt so heavy that it barely moved

 

 

the problem is that the bolt strikes the buffer, which makes the math real complicated

 

otherwise I would just do a little sample problem with assumed values and post it up

 

 

no matter what you came up with, you'd have to show a test that proved it worked

Edited June 3, 2015 by buzz

[buzz]

FWIW, I studied this topic (kinematics and deformation of elastic bodies) on the graduate level and I know it pretty well

 

so there's some actual knowledge behind this stuff I'm writing

 

on the other hand it's all just speculation until it's tested

[Motorcar]

If I remember correctly, the British in North Africa took out the Blish locks on some of the '28s they had and relpaced them with a small bolt and nut just to keep the bolt and actuator locked together. It didn't ride in the lock slots of the receiver or provide any retardation, and from pictures looks to be the same or just below the machined surface of the bolt. It was one of the field modifications they did like cutting an additional sear groove in the bottom of the bolt so it could be placed on "Safe" with the bolt closed. All was an effort to keep sand out of everything until needed for use. Keep in mind these were modifications done during war like human kind had never seen, where survival was imperative, because it worked then for a short time doesn't mean I'm throwing away my Blish lock today.

[ACBurke]

Not sure about bolt and nut in North Africa, but the British did file the ears off the Blish lock due to concerns with jamming in the sands of the desert.

[darrylta]

Buzz,

I may be thinking about this wrong, but the ROF doesn't effect the one time striking force of the bolt on the receiver butt.

It would increase the total gathered load in a finite time frame. The striking force is the same for each bolt cycle.

What am I missing?

Darryl

[LSU Tiger]

Didn't Eickhoff and Payne question the effectiveness of the Blish lock with the .45 ACP round?

[buzz]

you can tell the blish lock actually does retard the bolt because the ROF jumps up a lot when the ears are cut off

[buzz]

Darryl, a lot of physics is hard to explain without pictures or an example

a lot of it is counter intuitive


the only way for the bolt to cycle faster is for the recoil spring forces on it to be higher

there's no other possibility

the faster moving bolt causes the spring and buffer to deflect more backwards

and then the spring and buffer un-deflect and supply more force going forward

the more the spring and buffer deflect, the higher the force is


If you throw a tennis ball at a wall, it will flatten out a little on the wall, then the ball will un-flatten and spring back at more or less the same speed

if you throw the ball twice as fast, it will make the whole round trip twice as fast, flatten out twice as much and exert twice as much force on the wall

the thing that makes the ball come back is that it's exactly like a spring

if it was a ball of putty it would just deform and drop to the ground at the wall

it's not the exact same situation as the bolt but the principle is the same

 

higher initial speed = more spring deflection = higher force = faster round trip

Edited June 3, 2015 by buzz

[Paladin601]

Maybe, a Hydraulic buffer, such as used on the AR 15. The diameter of the spring, about the size of a ball point pen.

[gunhistorian]

I don't remember if Chinn, in one of the volumes of The Machine Gun, gave the math formulas that would applicable to blow back firearms. Like Bill Engval said about looking at his daughter's math homework: "Hey! When did they start putting letters with numbers?" My sentiment, exactly.

[buzz]

I actually just posted this thread up because I wanted to see if anyone had come up with this sort of stuff and tried it out in the past.

 

 

I would not use math to solve this problem even though it's actually pretty easy to set up, there is already a method in place.

 

What you would do is apply d'Alembert's Principle.

 

And then you use a computer program to perform a numerical solution.

 

The program would start at time = 0 and then increment the time in tiny steps

 

The deal killer is that you'd have to come up with all kinds of values like the spring rates and damping and the initial recoil velocity of the bolt.

 

To figure all that stuff out, you'd need to test the gun so much that it would be easier to just forget the math and just test the gun.

 

 

A similar situation would be if we were shooting off a rocket and you wanted to know how high it would go.

 

I could set up the d'alembert's equation for the rocket and use a computer to solve it.

 

But we'd have to shoot the rocket 20 times to figure out the drag on the rocket and the impulse from the engine and all that kind of stuff to plug into the math solution.

 

It would be a lot easier to just shoot the rocket once and see how high it goes.

 

 

This mathematical method I described is more useful for stuff that you cannot test.

 

Or where testing would be extremely expensive.

 

For example, you could model a building using assumed values for damping and so forth, and then see what the forces in the building are during an earthquake.

 

Obviously you couldn't do a real life test unless the building was already constructed and getting busted up by an earthquake.

 

Another example would be if you worked in a car company and you wanted to figure out how much an exhaust pipe on a car would vibrate from the engine. You'd use the computer to figure out where to hang the exhaust to keep it from resonating and humming.

 

 

If you want to know more about this, just google "d'Alembert's Principle" and "Euler's Method".

 

Also, I'm not playing Mr. Genius or anything, I did not invent this stuff, I just had to learn it for my degree.

Edited June 4, 2015 by buzz

[darrylta]

I thought about this a little last night. Your right, as the ROF increases, the velocity increases and or the impact load increases.

The only thing restricting the impact force from the bolt would the friction of the blish lock ears on the 21s & 28s.

To increase the velocity, I guess it just boils down to 2 things, either hotter loads or a lighter bolt.

It's nice that the AOC engineers got it figured out with a proven track record.

 

I own a 28 Hurley that was abused by it's previous owner with cropped blish lock ears and a speed bolt, resulting in a cracked receiver.

PK worked his magic on it with a U shaped collar reinforcement that is affixed in the rear of the receiver.

It must be reliable, since it's outfitted with one of his modified 21 actuator and buffer assemblies.

 

I haven't shot it yet, but I have tons of faith in PK's capabilities.

My head hurts,

Darryl