Brass Thompson Receivers - Any Interest?

[buzz]

Facts are slippery

 

 

Metals have a quality called "toughness" which is a measure of how much impact energy they will absorb before they form a crack.

 

The kind of steel that reconbob would use for his receivers would have a notch toughness of about 30 times higher than brass.

 

Brass has a toughness about on par with cast iron.

 

Whether or not that would make a good receiver is unknown until it's tested.

 

 

I would guess that metal fatigue would be the limit state that would kill most Thompson receivers.

 

That's true of most firearms. They don't get overstressed in normal use but they do develop fatigue cracks at points of geometric discontinuity (like corners).

 

 

But that's all irrelevant since the buyers are looking for a novelty or an artistic display instead of efficiency.

[Paladin601]

"Brass has a toughness about on par with cast iron." ? really comparing those two, Brass has good mallability where cast iron does not. Brass can absorb shock where Cast Iron fractures and very poor.

 

The Brass Thompsons were made in improvised machine shop, my best guess is that Brass was picked do to the type of machinery they had, probably a blacksmith shop. Something like those "gentle men" in Pakistan who make copies of guns.

[shadycon]

:Brass???? I've had no sign of cracking of my brass barreled black powder cannon!!! 4th July around the corner, time to dust it off and get it ready!

Edited June 4, 2016 by shadycon

[buzz]

Brass is pretty strong in terms of tensile strength, it's similar to mild steel.

 

You can make all kinds of structural parts out of it.

 

Whether or not brass is suitable for the part depends on the size and shape of the part and type and magnitude of loading.

 

Can you hold a wood crate together with brass bolts? Sure, why not.

 

Can you substitute 1" diameter brass bolts for 1" steel bolts on a bridge? No you can't.

 

 

I don't have any idea how long that a brass Thompson receiver would hold up.

 

A million rounds? 100,000 rounds? 10,000 rounds? Who knows?

 

I don't even know how long a steel Thompson lasts.

 

 

You could write 50 books about this topic and not be finished.

[TSMG28]

Bob,

 

To answer your earlier question, we don't know how many were made because there are no records that have been discovered. I know of only two examples of the Turkish brass Thompsons. Both are in Canada, including the one displayed in this thread. If the serial numbers started at 100, probably only a few hundred were made before they discovered the weakness of the brass receivers. The serial numbers for the steel version used four digits instead on three, but again, we have no record of how many were made.

 

Roger

[Operator18]

>I do not believe a brass receiver would affect the function of the blish lock because

the bronze/steel surfaces are in the bolt, H-lock, and actuator, not the receiver.<

The ears of the lock cam into the sides of the receiver, so in a brass/bronze receiver there would be bronze-to-brass contact on these cam surfaces. Blish locks seem to be incredibly durable where the bronze ears run on steel surfaces, so any bronze-to-brass contact wouldn't be an issue in my view.

A brass Thompson would be fun to have.....

the blish lock relies on dissimilar metal contact

[reconbob]

The dissimilar metals - the surfaces that the Blish principle acts on are the

angled slot in the bolt for the H-lock (steel) and the H-lock itself (bronze0. The

function of the angles slots in the receiver is to lift and lower the H-lock when

to bolt moves back (lift) or forward (lower), not as the acting Blish surfaces.

 

Bob

[Black River Militaria CII]

The alleged frictional effect of the bronze in sliding contact with steel has proven to be mostly hypothetical. The dissimilar metals argument is not valid. Too bad.......

[Paladin601]

You don't have electrolysis with Bronze and that steel. The Bronze will wear faster then the steel as well. But a Bronze lock and Brass Receiver, the receiver will wear faster. How long, probably not in my lifetime. After that I would not care

[buzz]

If you want to know how well the blish lock works, shoot a 28 and record the ROF, then cut the ears off the lock and shoot it again and record the ROF

 

the difference in ROF will give you an idea of how effective the lock is

 

I think someone tried it and said it was 200 rpm difference

 

 

the interface between the bolt and h-lock is two big precision machined flat surfaces, it's perfect for using the blish principle

 

the lock has a long surface to slide against and drag on

 

since the blish lock and bolt stick together, the action is locked just by having the ears impinge on the receiver

 

it's a very clever design

[reconbob]

We occasionally hear that the Blish principle is hogwash and does not exist.

Yet the history of the principle is well known and Blish received a patent for it -

which he would not have if it was total BS. Where is the engineering data that

documents that the principle is "mostly hypothetical"? Now if you want to say

The Blish principle is not needed, ok. But you can't say it's a hypothesis that

does not exist in the real world.

 

Bob

[gijive]

Now if you want to say the Blish principle is not needed, ok. But you can't say it's a hypothesis that

does not exist in the real world.

 

Bob

 

Bob,

 

Well said!

[ron_brock]

The blish principle is proven. Ref page 14-15 of TUTB. The principle is not tomfoolery, but the necessity of it being needed for the Thompson could certainly be questioned.

 

Ron

[Black River Militaria CII]

Yes, its use in the Thompson is severely marginal at best. I am familiar with the principle and the ideas behind the use of the principle in the gun, having owned and been shooting and enjoying Thompsons since 1970. We are talking in the context of the Thompson, so my comment was that the principle was hypothetical with its application in the Thompson. Many years ago when the Gunmachines hot rod parts for Thompsons were all the rage, a number of us Thompson owners experimented with disabling the Blish. Firing five different guns, one Colt and the rest '28s with a Blish and with the disabled Blish and recording the rpm, the results showed that only one gun showed a significant increase in rpm, and the others revealed minor increase in rpm. We swapped unaltered locks between the guns and the guns still fired about what they had with their original locks in them with some variation. Variation with and without the effect of the lock was well within variations between guns as well as allowing for other variations such as chamber/barrel wear, ammo, more or less lube, shooters control of the gun, etc etc. When inserting a rod down the barrel and pushing against the bolt to feel the effect of the Blish on initial movement of the bolt, each gun was quite different. Cleaning and lubing the Blish reduced the slight effect or feeling of disadvantage in initial bolt movement significantly.

Also, the Blush device is not a "lock" as it does not lock the breech in any manner at all but acts as a very slight momentary disadvantage in opening the bolt, which is clearly not needed with the gun, in any case.

Not scientific clearly but in my opinion the Blish produces no beneficial effect on the function of the gun and there was no need for it as evidenced by the MP18, Villar-Perosa and other MGs of the era. FWIW

[buzz]

The blish lock mechanism is both good and bad.

 

 

The good part is that it was well designed.

 

It’s a straightforward and logical mechanism and it never seems to cause any problems.

 

So you could say that the designers of the gun did a very good job at incorporating the lock into the Thompson.

 

 

The bad part is that the lock is completely unneeded in a pistol caliber SMG.

 

That fact is 100% proven by the M1 and M1A1, which work perfectly and do not have the blish lock.

 

The blish lock mechanism was just a needless expense and complication.

 

 

The other bad aspect of the blish lock is that the effectiveness is questionable in actual use.

 

People have tested it and said that it slows the gun down by about 200 RPM, which I guess indicates that it is working pretty well.

 

Other people have reported that it is not slowing the gun down at all or was defeated by lubrication.

 

So you could say the effectiveness of the lock was probably not consistent.

 

 

That’s something that could be easily tested further with different ammo, different guns, with and without lube, etc. by using a shot timer.

[buzz]

I don’t think that pushing on the bolt with a cleaning rod is a good test of anything, because the blish principle is based on the metals sticking together during high pressure and then releasing when the pressure drops.

 

The highest load generated by the 45 acp on the bolt face is going to be around 21,000 psi x 0.45”^2*pi/4 = 3,000 lbs

 

That 3,000 lbs is going to be present for just a few thousandths of a second but it really is there and it will get the bolt moving backwards, which will make the bolt slam into the blish lock surfaces.

 

When you push the bolt back with a cleaning rod, you’re just matching the recoil spring force, maybe 10 or 15 lbs.

 

If you look at the interface between the bolt and the lock, the surfaces are smooth as glass. They are supposed to slide real easy when the gun is not being fired.

 

A better test would be to give the cleaning rod a mighty thump with a hammer and see if the lock grabs and locks up the action.

 

 

I kind of suspect that the 45ACP might not create enough recoil to make the lock work consistently.

 

If you’re shooting heavy loads in a gun with a loose, clean, well lubed chamber, the blish lock might work great.

 

If you’re shooting softer loads in a gun with a tight, dry, dirty chamber, the case might be grabbing the chamber so hard that the force on the bolt is reduced enough to keep the lock from operating properly.

[reconbob]

Yes, pushing on the face of the bolt doesn't prove anything. Remember that Blish

discovered this principle when firing heavy navy cannon. Full loads the breech would

stay locked. With light loads the lock would sometimes unscrew by itself. Now if you

were to turn the breech lock by handle to try to gage the effect of the principle of course

you would learn nothing. When I get a chance I will take one of the M1928A1's here,

put a brass or steel rod down the barrel against the face of the bolt and hit it real hard

with a 2 lb. hammer. (One of the precision tools we have here.)

Also the diameter of the base of the .45 cartridge is 0.475" so you might want to

recalculate...

 

bob

[Paladin601]

what markings will be on the recievers, if any?

[reconbob]

I can do M1921 Colt markings, M1928 and M1928A1 markings, and M1/M1A1 markings.

Its up to the customer what they want.

 

Bob

[buzz]

Bob,

The net force on a pressure vessel with a hole in it is the area of the hole projected onto the opposite side and multiplied by the pressure.

Only the area of the hole counts, because the pressure is cancelled out everywhere else.


This is one of those counter-intuitive things that most people get wrong.

Most people think a rocket shoots upward because it is venting a jet of high pressure gas.

But actually, the rocket is zooming upward because the gas is pressing really hard upward on the closed end of the engine bell and NOT pressing on the big hole at the bottom.

Edited June 13, 2016 by buzz

[Paladin601]

I can do M1921 Colt markings, M1928 and M1928A1 markings, and M1/M1A1 markings.

Its up to the customer what they want.

 

Bob

Thanks, I was wonder if it was Turk marked, what ever way they marked it

[reconbob]

Buzz - I disagree. At the instant of firing the case is like a balloon with the pressure acting

equally on all surfaces. The case expands in all directions under this pressure.

On the sides the force is transmitted to the walls of the chamber. At the front the pressure forces

the bullet down the barrel, and at the rear the base of the case pushes on the face of the bolt.

There is no "cancelling" out. The only thing is that the pressure drops as the bullet moves down

the barrel and exits the bore. The most destructive failures of firearms are when the case -

the high pressure balloon - ruptures. It could be defective brass where the head and/or rim is

too soft, it could be a defective primer or firing pin, or it could be an over loaded or improperly

loaded cartridge, it could be a plugged barrel But when any of these cause a rupture of the case

high pressure gases are vented into the action and the results are catastrophic.

Now, you are not going to see this with pistol cartridges in an open bolt submachine gun,

but when you get into "hi-power" rifle cartridges its a different story.

 

Bob

[buzz]

Bob,

 

I understand what you wrote and you're 99% correct.

 

Explaining physics on the internet is practically impossible.

 

If I write 10 sentences, it's nowhere near enough. If I write a huge wall of text, then it's too complicated for anyone to follow well.

 

If we were in front of a chalkboard, I could make everyone understand perfectly in 30 minutes.

 

 

Anyway, I promise you that the net thrust on the gun is the bore area times the pressure, NOT the cartridge base area times the pressure.

 

I did not make that up, it's a known fact and it makes perfect sense if someone explains it properly.

 

 

Look at it this way -

 

If the bullet leaves the bore at say 2800 fps, the gun will have a rearward recoil velocity of maybe 20 fps.

 

If you made the base of the cartridge 50% bigger, and that made the net thrust on the gun increase by 50%, then the gun recoil velocity would go up to 30 fps

 

At that point, the law of conservation of momentum would be violated

 

So that means that the area of the base of the cartridge is not what determines the net thrust on the gun. The bore area is.

 

that's the macro proof, you could also prove it other ways.

Edited June 11, 2016 by buzz

[reconbob]

At the instant of combustion the cartridge is - to use your terminology - a sealed pressure

vessel. The bullet has not started to move yet so there is not yet a hole to have an area to

project to the opposite side, etc. So for an instant the pressure presses on all surfaces - it

doesn't pick and choose where to push. Milliseconds later the bullet will start to move down

the barrel and perhaps your cancellation theory would come into play. When a gun blows up -

when the pressure vessel fails - it will happen instantaneously - before the bullet moves. These

are the events that are catastrophic to a gun - sometimes literally blowing them to pieces. A

different type of failure would occur if its due to a plugged bore. Then the bullet will travel down

the barrel some distance. In this case the failure - which is the failure of the brass - does not

occur within your area of the bore projected to the face of the bolt, the failure is outside of this

area. The gas does not realize that its supposed to cancel out and finds the weak

spot outside of the bore area and kaboom.

You lost me with the recoil. The recoil of any gun is a relation of the weight of the gun vs.

the weight and velocity of the bullet, not the area of the base of the cartridge. There is a difference

between the forces transmitted to the gun mechanism via the bolt and locking surfaces and

recoil. So increasing the base of the cartridge would disperse the forces on the bolt over a

greater area, but it would have no effect on recoil assuming the velocity and weight of the

bullet were the same.

 

Bob

[Black River Militaria CII]

My point about pushing the bolt through the barrel is to demonstrate that the Blish "lock" is NOT a lock. The bolt is never locked. Timing the rpm of the guns with and without the function of the Blish indicated some but really negligible increase in rpms. It is a solution to an imagined problem, which was not uncommon in the days of full auto invention and experimentation.

I understand the principle of the Blish lock having pursued the history of the novelty many years ago and found it interesting but not convincing in the application to the Thompson. Belief in the Blish is an article of faith, in my view, and I don't subscribe. That's about it…...

I have some very early MGs with very odd mechanics: 12/18 Dreyse, 07/12 Schwarzlose, 07 St Etienne, all of which have various design ideas that are imaginative but not effective and they are very interesting, and all were successful for a period of time. And, there are lots of other really strange MG designs that didn't reach production and issue and unfortunately are not in the US registry for us to play with.