More on Kentucky Ballistics - Serbu

[JohnKSa]

We don't know that the barrel tenon was or wasn't tempered, or what alloy was used. Mark didn't cover that in any of the videos I've seen.

I quoted Mark when he said, in the video, that the breech cap was "the hardest, strongest part in the whole gun" which is clearly the same as saying that the barrel (and every other part on the gun) was not as hard/strong as the breech cap.

So we don't need to know what alloy was used for the barrel or how it was heat treated to know that it was not as hard/strong as the breech cap. Besides, even if he hadn't made the statement, the fact that the barrel threads sheared instead of the breech cap threads would be pretty telling evidence of which was harder/stronger.

Having said that, sometimes there is a good reason to use a softer or weaker material or lower temper on one side of a threaded connection.

Sometimes, there could be, I agree. But what I said was very plain and also true. ..."hardening only one side of a threaded connection can't make it any stronger than the threads on the other side of the connection." Is it possible that the barrel tenon was designed so it was just only very slightly less hard/strong than the breech cap? It's a possibility, of course, but I don't see how making up all these possible explanations fits with your repeated assertions about how we shouldn't speculate.

Without knowing the chamber pressure that was actually generated and if there was or wasn't a barrel obstruction, it really is all speculation.

We don't need to know the chamber pressure, nor do we need to know if the overpressure was caused by a barrel obstruction to know what happened and to see how the gun failed.

1. We know that an overpressure event resulted in three parts of the gun being driven towards the shooter with lethal energy levels. (In spite of the fact that it is pretty clear that this is due to design choices, we could speculate that isn't because of the design but is due to some unknown factors, but I believe your position is that we shouldn't speculate.)

2. We know that the breech design allowed gas to act on an area more than 3x larger than a breech plug design would have. (It's true that we can speculate about whether or not the force applied was really 3x larger due to assumptions we could make about how the gas escaped around the head of the case, but I believe your position is that we shouldn't speculate.)

Which I guess begs the question if any manufacturer does such testing, and should they?

Why would it? Is there any reason to believe that this failure calls the strength or safety of other firearm designs into question?

Even if he did, I can't imagine having tested loads above the pressure that caused failure because you'd have already established that failure point.

Are you speculating that perhaps he tested to failure but found the failure benign and then further speculating that had he tested with even higher pressures he might have discovered a different failure mode similar to the one that produced this video?

It looks like what you're really saying is that no one but you should speculate, or perhaps that speculation is only acceptable if it makes Serbu look better. Similarly, it also seems that what we know is only what we know if it puts Serbu in a positive light, and everything else we know isn't really what we know.

 

[MachIVshooter]

I don't see how making up all these possible explanations fits with your repeated assertions about how we shouldn't speculate.

Answering assertions you made, such as "Making the breech cap the hardest, strongest part of the gun without giving similar attention to the barrel that it threads onto is (obviously) problematic."

I've been very clear that I do not have the answers, as I'm not the weapon designer. What I offer is a perspective on why it might be deliberate rather than the oversight you imply.

Manufacturers do make mistakes, and I haven't dismissed that possibility at all. But with hundreds of these rifles in the wild and no other problems, a single failure that resulted from overpressure ammunition that may have been remanufactured renders calling it a flawed design a bit premature.

It looks like what you're really saying is that no one but you should speculate, or perhaps that speculation is only acceptable if it makes Serbu look better. Similarly, it also seems that what we know is only what we know if it puts Serbu in a positive light, and everything else we know isn't really what we know.

At what point did I say anyone shouldn't speculate?

Calling it what it is ain't the same as saying don't do it. You read something into my statements that I never articulated.

What I did say is that a lot of people are postulating without a solid grasp on what we're dealing with or due diligence in considering the information that is available, and I don't just mean on this board. There's a difference between conjecture about the possibilities and making statements that it's a design flaw without having the data to support that assertion.

 

[MachIVshooter]

I have a dismal view of destructive tests, I consider them the same as blowing up stuffed animals. All fun for the children, but pretty useless about understanding failure modes.

I agree that it makes no sense beyond entertainment value for an end user, but for manufacturers, it provides important data. We need controls and often precision instruments to obtain that data, though. Otherwise it serves no useful purpose.

However, like I said, it's not a normal practice to "up the ante" once the failure point has been determined. If your 20,000 pound crane fails at 37,000 pounds, there's not much reason to destroy another testing at 50,000 pounds. Likewise, if I'm testing cans and get a failure with a .300 RUM that has a 12" barrel, I'm not gonna try another of the same can on an 8" barrel.

But......as I also said, the failure mode can change when the forces that caused it are amplified. Maybe that crane only buckled at 37,000 pounds, and would fracture with shrapnel at 50,000. On that note, though, failure is failure, and if you exceed the limitations of a design, what happens is on you, so I wouldn't expect any manufacturer to have data for failures at different degrees of excess, just a clear line of what is safe and what isn't.

 

[Kano383]

Sometimes there's a lot to say.

Your (non-rhetorical) questions were addressed, but you oversimplify. Sometimes a yes/no answer is insufficient. Try reading.

I asked 11 questions, you ignored them all.

My questions are not oversimplified, they go straight to the core of the problem.

Answer thoses, and all you need to do then is to get the exact dimensions and material data to calculate how much and what failed, but the HOW/WHY the design does not handle overpressure/ruptured cases safely is clear.

I said it before, you talk a lot. It seems that the only reason you engaged in the discussion is not to assess the problem or participate constructively, but to show how much you know about threads and argue for the sake of arguing...

As far as I'm concerned, there are very simple principles to apply in a design. When the principles are correct, one can spend his heart content calculating all the tiny complicated details and refining everything.

In this case, the principles are wrong. This is my stance, and this is the discussion.

 

[JohnKSa]

Answering assertions you made, such as "Making the breech cap the hardest, strongest part of the gun without giving similar attention to the barrel that it threads onto is (obviously) problematic."

I conceded that it was a possibility that the barrel was intentionally left softer, but since speculation is ok, let's hear your honest assessment of how easy it would be to design the gun and tailor the strength/hardness of the materials so that it is reasonably likely that an overpressure event would stretch the barrel threads just enough to lock up the gun but not actually cause the threads to fail. You'd have to get the threads to deform but not shear--what's the range of force where that could be accomplished with a typical barrel steel?

Then again, your speculation was that this could have been done to avoid a "catastrophic failure when the strength is exceeded" without "warning signs". As it was, the gun failed catastrophically when the strength was exceeded and without warning signs, so I'd say if that was the goal, the goal was not achieved.

However, like I said, it's not a normal practice to "up the ante" once the failure point has been determined. If your 20,000 pound crane fails at 37,000 pounds, there's not much reason to destroy another testing at 50,000 pounds. Likewise, if I'm testing cans and get a failure with a .300 RUM that has a 12" barrel, I'm not gonna try another of the same can on an 8" barrel.

This is speculation on top of speculation. We don't even know if there was any failure mode testing done at all. It's not required by law and Serbu hasn't made any claims or even hints that they did that kind of testing. Why does it make sense to speculate that there was and then further speculate how the hypothetical testing was done and even further speculate what the limitations of the hypothetical testing was? This is what I'm talking about.

Someone points out a fact--the breech cap is harder than the barrel steel which means that the extra hardness didn't provide any extra strength since the weaker threads are the ones that will fail in that case--and you label that speculation (which it absolutely is not) and then start speculating about possible causes why that might have been intentional.

Someone points out a fact--the ears weren't failsafes, they instead became potentially lethal projectiles--and you label that as speculation which it absolutely is not. Even if they were intended to be to be failsafes, they clearly weren't in this situation. And then you speculate about reasons why they could have failed as failsafes that don't include their being part of a flawed design.

What I did say is that a lot of people are postulating without a solid grasp on what we're dealing with or due diligence in considering the information that is available, and I don't just mean on this board.

If that's a problem, it's a problem whether the speculation leads to an assertion of a design flaw or whether the speculation leads to exoneration of the manufacturer. But that's clearly not the issue you're addressing.

There's a difference between conjecture about the possibilities and making statements that it's a design flaw without having the data to support that assertion.

And that's the crux of it. You're willing to speculate levels deep to argue against the idea of a design flaw, but anytime someone makes any assessment that there could be, you label it as speculation.

All I'm saying is that maybe we could have some sort of an even keel.

 

[MachIVshooter]

I conceded that it was a possibility that the barrel was intentionally left softer, but since speculation is ok, let's hear your honest assessment of how easy it would be to design the gun and tailor the strength/hardness of the materials so that it is reasonably likely that an overpressure event would stretch the barrel threads just enough to lock up the gun but not actually cause the threads to fail. You'd have to get the threads to deform but not shear--what's the range of force where that could be accomplished with a typical barrel steel?

If that were a component in the design, then I'd say that would be the function of the ears to stop the cap from leaving after the threads stretched, and possibly prevent the weapon from opening due to lack of clearance so it couldn't be fired again after it was comporomised.

But this would be a question for Mark, as I have never designed or built a firearm with a screw on breech plug, nor would I. I've done multiple break actions with different breech locks, but they were more conventional in terms of breech face.

Then again, your speculation was that this could have been done to avoid a "catastrophic failure when the strength is exceeded" without "warning signs". As it was, the gun failed catastrophically when the strength was exceeded and without warning signs, so I'd say if that was the goal, the goal was not achieved.

Again, that would be for Mark to address, but operating under this supposition, you know as well as I that exceeding the margins of any design by enough can result in total failure. A truck has a weight rating, and then it has overload springs, and then bump stops. But if you put enough weight on it, you're going to exceed those built in overload mechanisms and break stuff. I feel like I really shouldn't have to explain this.

The real question is whether the forces generated were adequate to exceed all those margins, or if the design did not have enough margin. The information required to answer that question accurately we simply are not privy to.

This is speculation on top of speculation. We don't even know if there was any failure mode testing done at all. It's not required by law and Serbu hasn't made any claims or even hints that they did that kind of testing. Why does it make sense to speculate that there was and then further speculate how the hypothetical was done and even further speculate what the limitations of the hypothetical testing was? This is what I'm talking about.

I was addressing destructive testing in general, in response to a statement about the relative merits of destructive testing in general. Already covered that we don't know if that has ever been done by Serbu. Please pay attention, and stop attributing to me things that I haven't said.

Someone points out a fact--the breech cap is harder than the barrel steel which means that the extra hardness didn't provide any extra strength since the weaker threads are the ones that will fail in that case--and you label that speculation (which it absolutely is not) and then start speculating about possible causes why that might have been intentional.

I said we don't have the information on the alloy or temper of the breech tenon. That's a fact unless you have new information. If you like, I'll be happy to ask Mark about that, but I haven't, and you clearly don't know either. I can also ask what the receiver material and temper is. But I'm not going to message him at 2AM EST, so ain't happening tonight.

Someone points out a fact--the ears weren't failsafes, they instead became potentially lethal projectiles--and you label that as speculation which it absolutely is not. Even if they were intended to be to be failsafes, they clearly weren't in this situation. And then you speculate about reasons why they could have failed as failsafes that don't include their being part of a flawed design.If that's a problem, it's a problem whether the speculation leads to an assertion of a design flaw or whether the speculation leads to exoneration of the manufacturer. But that's clearly not the issue you're addressing. And that's the crux of it. You're willing to speculate levels deep to argue against the idea of a design flaw, but anytime someone makes any assessment that there could be, you label it as speculation.

I have maintained that I am speculating about any and all of the details we don't have hard facts on, just as is everyone else.

On that note, the ears are absolutely a safety mechanism. I thought you would have gleaned as much as you could about this before debating further, especially after the thread discussion, but here ya go @ 1:30

As far as I'm concerned, there are very simple principles to apply in a design. When the principles are correct, one can spend his heart content calculating all the tiny complicated details and refining everything.

In this case, the principles are wrong. This is my stance, and this is the discussion.

And your qualifications for this judgement are what, exactly? Are you a gun designer? How many weapons have you scratch built?

I'm sure you'll ignore those questions just as you did my others, but I'm putting them out there anyway.




What everyone here needs to get is that this is not about defending Mark Serbu's design. If it's flawed, it's flawed. But unless someone here has the TDP on the weapon and has done destructive testing of one or more under laboratory conditions, not a single one of us is in a position to make that determination one way or the other. This is about employing the objectivity we would owe any maker under these circumstances and admitting that we don't know what we don't know rather than putting forth opinions as facts, especially when one lacks the requisite understanding of metallurgy and mechanical design to properly assess a catastrophic firearm failure. When one says "is a design flaw" rather than "may be" or "I think", it changes the context entirely. You'll notice that I emply "maybe", "possibly", "might have", "looks like", etc. very frequently here. That's because I don't know, so I'm careful to denote it as opinion or conjecture. When you assert something as a fact, it's no longer subjective and bears a burden of proof.

 

[DeepSouth]

May I interject:

I think I have a perfect illustration that may resolve some of the tensions.

Let’s recall “Project Eldest Son”
During the Vietnam war we (Americans) sabotaged Vietnamese ammo.
according to Wikipedia, (and some first hand accounts of a friend of my fathers who was actually involved with the project)

“Captured ammunition was partially disassembled and reassembled with substituted components. Rifle and machine gun cartridges had the smokeless powder replaced with a high explosive of similar appearance which would generate approximately five times the design pressure of firearms. The bolt and pieces of an exploding AK-47 receiver would typically be projected backward into the head of the individual firing the rifle.”
-quoted from Wikipedia-


So, do we conclude th AK design is flawed?
I certainly don’t, I love my AK’s.

So what if (speculation warning) the SLAP round was 5 times the pressure of a standard .50 big round?

do you conclude the design is flawed?



Personally if I do not know the pressures, (and no one does at this time) I cannot in good concise make a call either way.

All I ask for is consistency.

 

[Kano383]

If that were a component in the design, then I'd say that would be the function of the ears to stop the cap from leaving after the threads stretched, and possibly prevent the weapon from opening due to lack of clearance so it couldn't be fired again after it was comporomised.

But this would be a question for Mark, as I have never designed or built a firearm with a screw on breech plug, nor would I. I've done multiple break actions with different breech locks, but they were more conventional in terms of breech face.



Again, that would be for Mark to address, but operating under this supposition, you know as well as I that exceeding the margins of any design by enough can result in total failure. A truck has a weight rating, and then it has overload springs, and then bump stops. But if you put enough weight on it, you're going to exceed those built in overload mechanisms and break stuff. I feel like I really shouldn't have to explain this.

The real question is whether the forces generated were adequate to exceed all those margins, or if the design did not have enough margin. The information required to answer that question accurately we simply are not privy to.



I was addressing destructive testing in general, in response to a statement about the relative merits of destructive testing in general. Already covered that we don't know if that has ever been done by Serbu. Please pay attention, and stop attributing to me things that I haven't said.



I said we don't have the information on the alloy or temper of the breech tenon. That's a fact unless you have new information. If you like, I'll be happy to ask Mark about that, but I haven't, and you clearly don't know either. I can also ask what the receiver material and temper is. But I'm not going to message him at 2AM EST, so ain't happening tonight.



I have maintained that I am speculating about any and all of the details we don't have hard facts on, just as is everyone else.

On that note, the ears are absolutely a safety mechanism. I thought you would have gleaned as much as you could about this before debating further, especially after the thread discussion, but here ya go @ 1:30





And your qualifications for this judgement are what, exactly? Are you a gun designer? How many weapons have you scratch built?

I'm sure you'll ignore those questions just as you did my others, but I'm putting them out there anyway.




What everyone here needs to get is that this is not about defending Mark Serbu's design. If it's flawed, it's flawed. But unless someone here has the TDP on the weapon and has done destructive testing of one or more under laboratory conditions, not a single one of us is in a position to make that determination one way or the other. This is about employing the objectivity we would owe any maker under these circumstances and admitting that we don't know what we don't know rather than putting forth opinions as facts, especially when one lacks the requisite understanding of metallurgy and mechanical design to properly assess a catastrophic firearm failure. When one says "is a design flaw" rather than "may be" or "I think", it changes the context entirely. You'll notice that I emply "maybe", "possibly", "might have", "looks like", etc. very frequently here. That's because I don't know, so I'm careful to denote it as opinion or conjecture. When you assert something as a fact, it's no longer subjective and bears a burden of proof.

I've been working on guns from .17 to 40mm in caliber in all sorts of places for decades, including operation areas. This said:

1. No vent holes
- Are there vent holes? Yes/No
- Corollary to the question above: Would vent holes help reducing the pressure in the breech in the event of a case failure? Yes/No

The answer to this question is exactly the same whether I design ICBMs, or lace thinghies for Victoria's Secret, so please stay on topic.

2. Moved at bottom

3. No fail-safe protection for the shooter, preventing the breech cap from flying directly in his face
- Is there anything preventing the breech cap from injuring the shooter in case of catastrophic failure? Yes/No, if yes: What?

You say these "ears" are a failsafe measure to protect the shooters, the FACTS say they aren't. Not only they did not protect anybody, but they turned into missiles that almost killed the shooter.

4. Breech cap design with threads external to the chamber is in itself a poor choice, since it multiplies any stresses on the threads by a factor of 3-4 if the case gives up.
- Is the screw-on-the-outside design multiplying the total thrust by a factor (ETA: roughly) equal to the ratio between the case head and the cap's inner area, Yes/No ?

Again, the answer does not correlate to the political affiliation of the asking party, nor to his tastes in matters of wines.

2. - Overpressure/case separation not sufficiently considered in the design - What were the considerations, and the calculations showing the relationship between thrust on the cap and threads shear limit, in case of a case failure?

The above is speculative and can only be answered by the designer himself, so we'll disregard it from now on.

3 simple questions with simple answers. Whether a hole is there or not doesn't involve Mr Boyle, Mariotte, Avogadro, and their laws...

 

[JohnKSa]

On that note, the ears are absolutely a safety mechanism.

I think it's pretty clear what I mean when I say they aren't failsafes from my previous comments.

"I have a hard time calling them failsafes when not only didn't they prevent the cap from hitting the shooter, but they, themselves actually became projectiles with enough energy to be lethal."

"Even if they were intended to be to be failsafes, they clearly weren't in this situation."

In fact, the most serious injuries in this incident were caused by the ears--the secondary safety features. He can call them whatever he wants, but when they turn out to be the most dangerous things on the gun they clearly don't fit the definition of failsafes.

His comment about them being "almost enough to handle the normal bolt thrust on their own" is interesting. Presumably in a case where something has gone so wrong that the cap is ejected from the barrel, one would reasonably assume that much more than normal bolt thrust is involved...

...but, did you notice that Serbu points out that someone else came up with the pressure numbers for him in that video?

Given that Serbu had the material properties and thread dimensions when he designed the gun, it's kind of interesting that he didn't already have the information about what pressure would be required to shear the threads from the design process he should have gone through when he decided how hard to make the breech cap.

There are online calculators that will provide sufficient information to calculate the minimum force acting on the cap required to cause the threads to shear if the material properties and the thread dimensions are provided.

But he apparently didn't know that number when the incident occurred and instead quoted a number that he now says he doesn't know the origin of.

It's more interesting that even after this incident occurred and some time has passed, he apparently still doesn't do the calculation himself and has to provide the result of someone else's calculation in the video.

I said we don't have the information on the alloy or temper of the breech tenon.

Yes, you said that in response to my comment that the breech cap was harder than the barrel tenon. You said it as if we needed to know that information to know which part was harder when it was already obvious from which part had the sheared threads. When Serbu had stated which part was harder/stronger in the video. Your comment implied it was speculation to make the assertion that the breech cap was harder/stronger than the barrel.

So I responded to your comment based on the context.

I admit that if you intended your comment to be agreement with the observation I made, or if you didn't intend for it to relate to the observation from my post that you quoted, then I completely misinterpreted what you posted.

That's a fact unless you have new information. If you like, I'll be happy to ask Mark about that, but I haven't, and you clearly don't know either.

But I don't think so because you are still talking about having to get that information from Serbu.

It is not speculation that the breechcap is harder than the barrel tenon. Period. Makes no difference what the barrel alloy is or how it was heat treated. Serbu says the breech cap is "harder/stronger than any other part in the whole gun". We can see which part has the sheared threads.

This is what I'm talking about. It looks like you're grasping about for any possible pretext to label things as speculation even when it's clear that they are fact.

Rifle and machine gun cartridges had the smokeless powder replaced with a high explosive of similar appearance which would generate approximately five times the design pressure of firearms.

So far, no one has made any allegations that the rounds were sabotaged with high explosive or that the pressures generated were anything more than 2x-3x above normal. Which, don't get me wrong, are horrendously high pressures, especially if the gases can act on the whole interior of the breech cap. But they are nothing like what would be generated from a comparable volume of high explosive.

 

[DeepSouth]

So far, no one has made any allegations that the rounds were sabotaged with high explosive or that the pressures generated were anything more than 2x-3x above normal. Which, don't get me wrong, are horrendously high pressures, especially if the gases can act on the whole interior of the breech cap. But they are nothing like what would be generated from a comparable volume of high explosive

Numerous people have suggested they were improperly reloaded, not necessarily with explosives but with pistol powders, etc… which would creat an unknown (that’s the key) pressure.
Many people have also indicated the type of powder they were originally loaded with deteriorated in a manner that made an unknown (again) pressure spike.

the point is we don’t know what we don’t know, if the pressure was 5x more than it should be, or possibly more… who knows, (that’d be no one) and with zero other reported failures it’s unreasonable to assume a product fault.

similarly, if the pressure was lower 1.5 maybe 2x normal. Then it’s obviously a tremendous failure…. No multiple tremendous failures, in the gun.

I don’t know where the line is that would make that an understandable failure, and no one knows how far the pressures spiked.

 

[mcb]

For minimal new information you guys sure have made some mountains out of mole hills.

From my perspective (what very little that is worth) the venting or lack there of in the event of a case failure or over pressure event seems a salient point. I wonder if Surbu ever did an obstructed bore test with this model? There are standards for doing an obstructed bore tests and often required for many guns going to government entities. The gun does not have to survive the test (they rarely do) but it must fail gracefully enough to not endanger the uses life. This gun clearly would not have passed that test.

 

[someguy2800]

On the subject of the receiver ears being sheared off. Its entirely possible that had those ears not been present that the breach cap could have come back at a velocity and in a direction that the injuries could have been far far worse. So even though they did not prevent the rifle from self disassembling, it is entirely possible that they did prevent this from becoming a fatal accident. Also wanted to note that the force on the breach cap would be the area of the major diameter of the threads, not the minor diameter, and its predictable that the barrel threads would shear rather than the cap threads even if the two parts had been the same material and hardness just because the root area of an internal thread is much larger than the root area of an external thread of the same size.

 

[MachIVshooter]

I think it's pretty clear what I mean when I say they aren't failsafes from my previous comments.

"I have a hard time calling them failsafes when not only didn't they prevent the cap from hitting the shooter, but they, themselves actually became projectiles with enough energy to be lethal."

"Even if they were intended to be to be failsafes, they clearly weren't in this situation."

In fact, the most serious injuries in this incident were caused by the ears--the secondary safety features. He can call them whatever he wants, but when they turn out to be the most dangerous things on the gun they clearly don't fit the definition of failsafes.

His comment about them being "almost enough to handle the normal bolt thrust on their own" is interesting. Presumably in a case where something has gone so wrong that the cap is ejected from the barrel, one would reasonably assume that much more than normal bolt thrust is involved...

I only addressed the matter of if those are or aren't failsafes, not whether or not they serve that purpose. The designer has stated that's what they are, so that's what they are. Whether or not they are sufficiently strong we don't yet know. As I've said before, how much overpressure occurred is a critical factor in determining whether or not the design is sound.

So I responded to your comment based on the context.

I admit that if you intended your comment to be agreement with the observation I made, or if you didn't intend for it to relate to the observation from my post that you quoted, then I completely misinterpreted what you posted.

The point was still not knowing what we don't know. It's the same reason I got into detail about threads that should be strong enough potentially being a problem if not properly executed. Knowing that the cap is stronger and harder doesn't actually tell us anything about the strength and hardness of the tenon except that it's not as hard or strong as the cap. It could be a little less or a lot less. I'm not a blacksmith who works with mystery metals, I don't deal in "not as strong" or "harder than". I use certified materials, machine to proper dimensions and surface finishes, and heat treat with a precision oven to get exact mechanical properties. As such, if I'm evaluating a part or design, I won't take guesses or run with vague, possibly hyperbolic assertions by the designer or anyone else about the materials, and I will qualify my assessments with verbiage such as "should be strong enough assuming the dimensions and temper are correct"

That's why I get into the minutiae on subjects like this. It matters. When the material, the temper and the dimensions can make the difference between plenty of strength and not nearly enough, it would be foolish to say anything else about a part or assembly without knowing and considering those things. That's why statements like

When the principles are correct, one can spend his heart content calculating all the tiny complicated details and refining everything

are downright foolish. You can't have a solid foundation, "principles" in a mechanical design without having established what it will be made from and how it will be assembled. The alloys, the temper, and the specific features are everything. You can take the same design and execute one with tempered 4140 from solid billets or forgings, properly radiused transitions and correctly formed threads, the other from annealed material with butt welded connections, sharp corners and crappy threads and you will get completely different integrity, despite same design and material. As such, you cannot effectively create or evaluate a design without all of those details. That's why they're part of a TDP, that's why they're spec'd out on prints, and that's why I harp on it in this discussion and am dismissmive speculation about the design which doesn't consider these things.

 

[MachIVshooter]

Also wanted to note that the force on the breach cap would be the area of the major diameter of the threads, not the minor diameter,

True if you have an undercut at major diameter. These caps do not, the surface area of the inside of the cap is (visually) minor, could be a few thou one way or the other. We don't know if we have exposed internal threads that the flanks could be acted on, looks like there probably are. It's not a huge difference either way in this case with a diametral disparity of ~5-6% from minor to major.

(1.5)-12 UNF 2B
(Internal)
Diameters
Major diameter: 1.5204 / 1.5000
Pitch diameter: 1.4542 / 1.4459
Minor diameter: 1.4278 / 1.4098



It would be a more involved calculation to figure out the force increase resultant of exposed threads because the thread geometry, the flank angle, will cause that force vector to be different than a flat surface, generating both radial and axial, and we would need to know exactly how may are exposed to do it.

its predictable that the barrel threads would shear rather than the cap threads even if the two parts had been the same material and hardness just because the root area of an internal thread is much larger than the root area of an external thread of the same size.

That is true, provided that there is no expansion of a female threaded cap that weakens those threads.

 

[Dr T]

After reading (and rereading) this thread, several items have been left inadequately addressed.

The first is the usage history of the firearm that failed: Was there something in this history that could have caused the metal to fatigue through repetitive stress that contributed to the catastrophic failure? This has been danced around by the ME's and gun designers discussing the design elements, but what is actually known about the possibility that the gun has been repeatedly fed dangerously over-pressure rounds. A little digging indicates that this is not the first time the rifle was fed SLAP rounds.

The second is the nature of the SLAP ammo per se. Does anyone know why Barrett specifically prohibits the use of SLAP rounds in their (semi-auto) rifles and the military approves them for use in the M2 machine gun only? Do these rounds destroy barrels? Are they prone to over-pressure? Were these reloads? Given the military use experience, there will be some documented reason for the restrictions and prohibitions.

Does anyone actually know?

 

[mcb]

The

After reading (and rereading) this thread, several items have been left inadequately addressed.

The first is the usage history of the firearm that failed: Was there something in this history that could have caused the metal to fatigue through repetitive stress that contributed to the catastrophic failure? This has been danced around by the ME's and gun designers discussing the design elements, but what is actually known about the possibility that the gun has been repeatedly fed dangerously over-pressure rounds. A little digging indicates that this is not the first time the rifle was fed SLAP rounds.

The second is the nature of the SLAP ammo per se. Does anyone know why Barrett specifically prohibits the use of SLAP rounds in their (semi-auto) rifles and the military approves them for use in the M2 machine gun only? Do these rounds destroy barrels? Are they prone to over-pressure? Were these reloads? Given the military use experience, there will be some documented reason for the restrictions and prohibitions.

Does anyone actually know?

On your second point. Slap rounds are a discarding sabot round and thus not compatible with most muzzle device. The sabot segments colliding with the baffles in the brake is a bad plan and thus should only be shot in 50 BMG weapon with no break or suppressor. The Barrett M82 will damage itself (its hard on the shooter too) if fired without its muzzle device. The muzzle device slows the reward motion of the barrel, since the M82 is a recoil operated gun the reward motion of the barrel and bolt locked together gives the bolt the momentum to cycle. Without the muzzle brake the bolt velocity will be increased to damaging velocities.

 

[zdc1775]

After reading (and rereading) this thread, several items have been left inadequately addressed.

The first is the usage history of the firearm that failed: Was there something in this history that could have caused the metal to fatigue through repetitive stress that contributed to the catastrophic failure? This has been danced around by the ME's and gun designers discussing the design elements, but what is actually known about the possibility that the gun has been repeatedly fed dangerously over-pressure rounds. A little digging indicates that this is not the first time the rifle was fed SLAP rounds.

The second is the nature of the SLAP ammo per se. Does anyone know why Barrett specifically prohibits the use of SLAP rounds in their (semi-auto) rifles and the military approves them for use in the M2 machine gun only? Do these rounds destroy barrels? Are they prone to over-pressure? Were these reloads? Given the military use experience, there will be some documented reason for the restrictions and prohibitions.

Does anyone actually know?

My understanding/memory is that Barrett advises against there use in the M-82 due to the potential for the sabot to strike and possibly lodge in the muzzle brake.

 

[MachIVshooter]

The first is the usage history of the firearm that failed: Was there something in this history that could have caused the metal to fatigue through repetitive stress that contributed to the catastrophic failure? This has been danced around by the ME's and gun designers discussing the design elements, but what is actually known about the possibility that the gun has been repeatedly fed dangerously over-pressure rounds. A little digging indicates that this is not the first time the rifle was fed SLAP rounds.

A very valid question, but certainly not one any of us can answer. Determining that forensically would be nigh impossible, akin to trying to verify prior front bumper damage on a car that was totaled in a front end collision. Scott would really be the only person who could know, but I wouldn't expect that he does.

Best hope in this regard is that the unused SLAP rounds give an indication of what likely happened with the one that destroyed the weapon.

 

[Dr T]

A follow-up question: What would the result be if a SLAP round was fired in a barrel with a discarded sabot lodged in the muzzle brake?

 

[Mk-211]

Looking at everything, I'm wondering if vent holes would have even helped in this situation.

The SLAP round definitely looked like a reload, it had the correct components but the crimp on the neck wasn't correct for the ammo. It had a tamper crimp, something not seen on SLAP ammo. The crimp is a 4 or 5 part 1/16" ring that goes around the mouth of the case.

Mark has the last of the SLAP ammo the was bought by KB. Mark needs to send the powder out to have it inspected to find out if it's old and deteriorated. That and what type of powder it is.

KB said that the rifle had a "Military Chamber" and all that means is that it's looser than a match chamber. SLAP ammo needs extra freebore to allow for the sabot. If this isn't done, the sabot could be jammed into the muzzle end of the chamber and rifling. That alone will drastically increase the pressure, add the wrong powder to the tune of 200+ grains and you have a recipe for disaster!

I'll wait to see what Mark comes up with on the ammo in question. Everything else is a moot point until then.

 

[Jim Watson]

Me, too. Everybody has been focused on the rifle, how it was not failsafe or no-fail with gross overload.
I am interested in what caused the gross overload.

What would the result be if a SLAP round was fired in a barrel with a discarded sabot lodged in the muzzle brake?

I think it would blow the muzzle brake off, I do not see it propagating pressure back to the breech and wrecking the action.

 

[Mk-211]

What would happen if a sabot round hit the muzzle brake?

I would hope that the penatrator would go through the brake and send it down range and take the sabot with it.

What would probably happen, the brake would be grenaded by the penatrator and sabot. Pieces of the brake would go in different directions as would the sabot.

Another issue, if anyone was hit by these pieces and injured. Be ready to be sued by their attorney!

 

[Jim Watson]

I think Mr Serbu ought to chance expending a similar rifle with abuse by SLAP, KB's remaining maybe faux SLAP, and HPT. Very high speed cameras, Pressure Trace, and a long string.

 

[Mk-211]

Thought I would post a couple pics of a SLAP round and an AP round to give everyone an idea of the ammo.

 

[DeepSouth]

A follow-up question: What would the result be if a SLAP round was fired in a barrel with a discarded sabot lodged in the muzzle brake?

just so people know the round before the the kaboom was not a SLAP round, and the projectile was recovered from the target.

Again, the round before the slap kaboom.