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Watched this video today , it is sobering . New people have been posting lately with less then standard plans for cannons.Under normal circumstances their plans would be fine with modern steel , it's when the freak stuff happens that there is a problem. Black powder can reach very high pressure under the right circumstances. With the rule of wall thickness equal to bore you have a fighting chance of surviving . In my years of shooting I have had three freak things happen while shooting, built in safety factures covered all three.
The video is about a 50 cal rifle with a whitworth style breach , it shows the inherent weakness of that design. With making cannons a 2x safety factor is none to much.If they had made the breech cap the full barrel diameter with more thread contact it would most likely have held.My 40mm whitworth cannon has been bothering me for a while now ,I think I will repurpose the barrel to a safer design.

 

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Geez, probably a good idea. I haven't seen the video and am working on a project at the moment, but I'll watch it when I can. With a Barrett, I'd consider him lucky with only the injuries he seems to have. Again, haven't watched the video so I don't know for sure what injuries happened, but at least he's relatively okay.
 

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If you watch the video they will show the breech with the threads wiped right off the barrel. They appear to be standard v thread. The blame it on bad ammo.. Watch the video. This is one lucky Dude, This gun is an RN 50 by Serbu. RN-50

The end of the barrel is threaded and cap screwed on to close the breech.

252243


The is off topic on this board, but the construction is very much on topic.
 

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The manufacturer says normal pressures are 55K PSI while the breach threads will let go at 85K PSI.

Surprised at the very small safety factor in the design.
 

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How are these even legally sold at all? Looks like an absolute disaster in the making, from the start.
 

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I guess it was some sort of cost-cutting option, I would assume, and it might've just been bad threads that finally broke, Matt from Demolition Ranch has one in .50 Beowulf and it hasn't broken... yet.
 

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I doubt I ever get around to building my Long Cecil, but if I do, I am going have to do serious study on thread strength. My barrel was going to be more traditionally-internal barrel threads and screw in breech plug. And. my gun is going to be black powder bag load.

Guessing now, but looking at the picture of the Serbu, I would guess that tread pitch is some where around 1"-6TPI.

Those of us building these antique breech loaders can learn from this accident.
 

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Guessing now, but looking at the picture of the Serbu, I would guess that tread pitch is some where around 1"-6TPI.

Those of us building these antique breech loaders can learn from this accident.

1.5"x12TPI is what the breech cap threads are on the RN-50.
 

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Sometimes you get what you pay for. The RN50 looks like part of a pistol attached to a barrel with an AR buttstock attached; $1200. A Barrett is $11,000. Guess you get what you pay for. And this applies to cannons. A well made tube is going to be costly as well as the carriage.

Know what you are shooting, what it is loaded with. Not sure why I needed to say this as it is preaching to the choir. Thanks for this post as it highlights to need to stay vigilant.

252294
252295
 

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Lets keep this relevant to breeching systems and not turn this into s discussion on 50 BMG rifles okay Remember here we are about Blackpowder mortars and cannon as were made in or before 1898- Antiques.


Both Scott and now Serbu have publicly claimed this gun fail because of the use of some spurious ammo.

Does anyone know how calculate shear strength of threads?

I was going to do 1-8tpi on the Cecil and that was using a surplus WWII Hispano Suiza 20mm barrel with black powder. Barrel threads were going to be internal.
 

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An appropriate buttress or modified buttress thread with large radii in the root of the thread would probably have prevented the threads from being wiped clean.
 

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Thanks Moose, that is interesting.

They don't define the type steel used in fasteners only say steel. (1018?, 4140?, etc) That would have an bearing I would think on the factors in table 10.4. The also don't say how the greater numbers are achieved-heat treating I think.

It was interesting in the video, the threads stripped off the barrel (the bolt). They did not show the interior of the cap. (the nut).

It was pretty obvious the threads failed, but why? Weak threads design or high pressure load? I hope we find out.
 

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Some have postulated shooting a sabot round with a muzzle break isn’t a good idea and may explain the slam flyers. IDK other then IMO that breech design is insufficient.

The muzzle loading muskets of the era had internally threaded breech plugs (1.5 - 2.0 calibers in length?) but with a lot less pressure.
 

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One thing that this brought up was the difference in surface area between bolt thrust of the (ID) of the case head and the total thrust of the breech cap in the event of a case head failure. For a home build cannon you'd want to look at the surface area of the ID of the breech cap and use that for calculating your safety factor number. There is a big difference in containing a .804" case head and a 1.392"ID breech cap being acted upon by the same PSI.
 

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Bolt thrust is an engineering concept used in design work to establish the specifications for the type of steel to use and the type fastening is needed. In this case it's not relevant. The pressures developed by a 50 MG are known. Strength of standard steel is know. What is relevant is the characteristics of the specific steel used in the gun and the pressure developed by the specific cartridge fired.

Stripped threads is a very big clue. But a lot more information is needed. Stripped threads them selves are not conclusive. They are pointer however.

Sabot in muzzle brake causing barrel obstruction would have shown damage between bullet and muzzle brake. This damage is behind bullet. As a bullet travels down the barrel towards the barrel obstruction the pressure would increase between obstruction and bullet. Most likely that would result in muzzle break failure as it would be the weakest point.

As the bullet travels down the barrel pressure behind the bullet would decrease. Barrel obstruction might slow the decrease, but wouldn't raise pressures unless obstruction was absolute. How absolute or solid would a sabot be, stuck in a muzzle break?

Muzzle break obstruction does not seem a logical possibility.

Peak pressure would be upon ignition. The threads and cap are the breech locking system. They failed. They claim is it would take 85,000 psi for them to fail. Seems low to me.

Examiners will look at this gun and determine the type steel used in the barrel and determine the strength of the threads. They will also look at the ammo, it's source and type. I would think that is where examiners will focus their exam-the threads failed, why? Bad ammo? Weak design? Anything beyond this is just speculation
 
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