Jim Watson
Member
Not much more. There is an unboxing look at the demolished gun by Mr Serbu.
His pictures show it more comprehensively wrecked than I thought.
No 30
His pictures show it more comprehensively wrecked than I thought.
No 30
More on Kentucky Ballistics - Serbu
- Thread starter Jim Watson
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someguy2800
Member
I was watching that the other day and my immediate thought as an engineer is that there is a critical design flaw in this rifle that probably caused it to blow up. There does not seam to be (at least as far as I have seen) any gas vent port under the threaded breach cap. In one of these videos they mentioned they had calculated the chamber pressure that would be required to strip the threads off the back of the barrel, I think they said it was 160,000 psi or something like that. I think they are calculating that off the base diameter of the cartridge case, which would make sense if you assume the case head will stay intact and contain the pressure. The case head will act as a piston pushing on the breech face with the total force being pressure x surface area of the case head. But if the case head were to separate or split, or blow out the side, or whatever and pressurize the inside of the breach cap, then the force on the threads will be the pressure x the surface area of the back of the barrel including the major diameter of the threads.
The case head of a 50 bmg is .804", which is a surface area of .507 square inches. So a 60,000 psi load will be 30420 lbs of thrust on the breach cap.
Serbu said in one of the videos that the thread size of the cap is 1.5" which is an area of 1.76 square inches. If you had a case head blow up and pressurize the inside of the breach cap to 60,000 psi, that would be 105,600 lbs of thrust on the breach cap.
I think the most plausible explanation is that there was a case head separation of the case which pressurized the inside of the breach cap, and because there is provision to vent that gas it blew the breach cap off.
The case head of a 50 bmg is .804", which is a surface area of .507 square inches. So a 60,000 psi load will be 30420 lbs of thrust on the breach cap.
Serbu said in one of the videos that the thread size of the cap is 1.5" which is an area of 1.76 square inches. If you had a case head blow up and pressurize the inside of the breach cap to 60,000 psi, that would be 105,600 lbs of thrust on the breach cap.
I think the most plausible explanation is that there was a case head separation of the case which pressurized the inside of the breach cap, and because there is provision to vent that gas it blew the breach cap off.
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CapnMac
Member
It was an interesting video, if less informative than I might have liked (but, it's early days, too)
25-20 WCF
Member
As an engineer you should have noticed the large flaw in your calculation. The force applied by the chamber pressure is based on the maximum internal area, not the external area of the case head. A 60,000 psi load would actually exert considerably less than 30,000 pounds of thrust. I don’t have a .50 BMG case that I am willing to cut open to measure the actual maximum internal diameter.
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mcb
Member
Actually bolt thrust is calculate using the largest diameter of the chamber as the piston area and a friction model treating the case like a piston in the chamber. The normal force for the friction being proportional to chamber pressure. This model only works with stresses in the brass below the yeild strength of the brass.
That said once you reach pressures that likely occurred here the brass likely is flowing like a plastic. The gas pressures alone were well above the yeild strength of brass, let alone the actual load paths of the geometry. Brass yeild strength is only about 80-85ksi.
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someguy2800
Member
incorrect because the chamber pressure is pushing the entire case backwards. That’s just not how the mechanics of pressure works.
Wreck-n-Crew
Member
Engineers never make mathematical mistakes they just use the wrong equation...im not talking about A guy named mark either.
To err is human.
To err is human.
Kano383
Member
Exactly.
As mcb says, bolt thrust is calculated on the assumption that the case acts as a piston, disregarding other variables for simplicity.
The inner area of the breech cap is more than 3 times the case head area. If, for any reason, the case ruptures you have 60kpsi or more ( you're most likely in overpressure already) on 1.8 square inches, which quickly takes you above the shear strength of the threads - and you get EXACTLY what happened here.
- Case separation
- No vent holes
- Pressure acts on the entire breech cap inner surface
- Threads sheared
- Breech cap flew in the face of the shooter
The obvious faults I find (personal opinion, can be changed if facts show that it's wrong):
- No vent holes
- Overpressure/case separation not sufficiently considered in the design
- No fail-safe protection for the shooter, preventing the breech cap from flying directly in his face
- 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
MachIVshooter
Member
I'll get deeper into the issues with some of the arm chair critiquing when I have time later, but for now, I'll just leave it at things break when safety margins are exceeded, and injury sometimes results.
If we designed all small arms to withstand the kind of overpressure this rifle seems to have been subjected to, you would not be able to buy a pocket handgun or lightweight rifle.
We'll have to wait for more information to critically analyze this particular failure, but saying things like "no vent and no failsafe to prevent the breechcap from flying into the shooter's face" describes virtually every bolt action rifle made in the last half century if you just replace the words "breech cap" with "bolt". Again, we don't (and shouldn't) design mechanical things to withstand forces they should never be subjected to because nobody would accept the weight and/or cost. Blaming a firearm design for failure under extreme conditions that far exceed responsible design parameters is like saying a truck's suspension was flawed because the vehicle took damage after the driver launched it 12' in the air.
Again, need to have more information than we do, but the fact that not only were these large threads sheared but it also ripped off a pair or 1/4" wide x ~1" long steel ears (there's your failsafe, @Kano383 , BTW) that were properly radiused at the transition is very telling.
I haven't every physically examined one of these, and if Mark erred in the design, he'll own it, but I don't see evidence of a faulty design, just an unlucky shooter who got a bad catridge.
On that note, maybe some of our members here who have been so cavalier about headspace and overpressure will temper their flippancy.
If we designed all small arms to withstand the kind of overpressure this rifle seems to have been subjected to, you would not be able to buy a pocket handgun or lightweight rifle.
We'll have to wait for more information to critically analyze this particular failure, but saying things like "no vent and no failsafe to prevent the breechcap from flying into the shooter's face" describes virtually every bolt action rifle made in the last half century if you just replace the words "breech cap" with "bolt". Again, we don't (and shouldn't) design mechanical things to withstand forces they should never be subjected to because nobody would accept the weight and/or cost. Blaming a firearm design for failure under extreme conditions that far exceed responsible design parameters is like saying a truck's suspension was flawed because the vehicle took damage after the driver launched it 12' in the air.
Again, need to have more information than we do, but the fact that not only were these large threads sheared but it also ripped off a pair or 1/4" wide x ~1" long steel ears (there's your failsafe, @Kano383 , BTW) that were properly radiused at the transition is very telling.
I haven't every physically examined one of these, and if Mark erred in the design, he'll own it, but I don't see evidence of a faulty design, just an unlucky shooter who got a bad catridge.
On that note, maybe some of our members here who have been so cavalier about headspace and overpressure will temper their flippancy.
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Kano383
Member
Touched a nerve, did we?
Rifles do go kaboom. HOW they go kaboom, that's the difference between a good design and a poor design.
The "ears" are NOT a failsafe, since they sheared off and did not prevent anything.
A breech cap screwed tight on the outside of the chamber has nothing to do with a bolt of roughly the same diameter as the case head, and locking into a receiver's recesses.
There are NO vent holes on that RN-50 cap, which means NO way for excess gases to escape, and pressure to be relieved.
Hundreds of million rifles were built on various variants of the Mauser design, and if built with the proper materials, they DO NOT send the bolt in your face. The only design that does that and that I'm aware of, is the Blaser R93.
I said that my opinion can change if facts show that it is wrong, so:
- No vent holes - Are there vent holes? Yes/No
- 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?
- 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?
- 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 - Compare this with a threaded breech block on an artillery piece, which screws INSIDE the breech, and has an area similar to that of the case head. Is the screw-on-the-outside design multiplying the total thrust by a factor equal to the ratio between the case head and the cap's inner area, Yes/No ?
Now, when you answer to these question with simple facts, and prove my assertions wrong, as I said I'll change my opinion.
Meanwhile, don't get all worked up because someone points out areas that contributed to a clusterfcuk, because I'm only interested in facts, and not in emotions.
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MachIVshooter
Member
One, if you want to have intellectually honest, productive discourse with someone, do not make feeble attempts to invalidate their statements by accusing them of arguing with emotion.
I do not own an RN50, I do not have a stake in Mark's company, and I do not know Scott DeShields, had never heard of him before this incident. In short, it makes no difference to me.
What I do know is that we have people here doing hillbilly napkin math and accusing a veteran gunmaker and degreed ME of a design flaw based on a single incident, the salient details of which we know very little.
I can't answer your questions any better than you can, which is why I said we need more information. You cannot make a determination either way without knowing a great deal more about the weapon and the load that blew it up.
Do you know:
-The size of the chamber that would exist between the breech face and cap?
-The alloys & tempers used in the cap & breech tenon?
-The thread pitch & diameter of those parts?
-The number of threads engaged?
-The amount of thread flank engagement?
-The alloy used for the receiver that bears the ears?
-The distance from the breech cap to the ears?
-The size, alloy and temper of the hinge pin that sheared?
-The expansion rate of the powder used in this load?
-The time it would take for pressure to equalize between the chamber and the space in the plug/breechface gap, and how far down the bore the bullet would be at that point?
-Most importantly, the chamber pressure that was actually produced by this cartridge?
The time components and Boyle's law factor are not something that can be ignored in assessing a pressure-related mechanical failure.
A couple of the tech specs Mark does go over in one of his videos, but not enough for us to draw the conclusions being drawn.
There's really no comparison to be made between an interrupted thread long tenon breech plug and this cap.
All else being equal, larger threads are stronger than smaller threads. You don't need to be an ME to understand that. The question of whether they were proportionately weaker or stronger relative to the increased surface are than an internal thread we cannot answer with the information we have.
Would this load have blown up another rifle? Maybe. We do know that SLAP rounds are prohibited from use in military DMR/sniper rifles. There are reasons for that.
Without testing other RN50s to failure both with and without the ears present, that is simply not a determination you or I can make. A failsafe failing doesn't mean it wasn't one; it means the limits were exceeded by that much. There are innumerable examples of this in the world of mechanical designs.
I would have predicted the lugs bowing outward and sending the cap in a tumbling upward arc. It didn't happen in this case. Things sometimes fail in unpredictable ways. I've seen more surprising mechanical failures than I could easily count in my 2.5 decades of manufacturing and repairing mechanical things professionally. Which covers:
Sometimes shafts twist in half before a shear pin lets go. Sometimes gears are stripped before a slipper clutch slips. Double and triple redundant systems fail completely, resulting in deadly crashes. These are the exceptions, the statistical outliers. Just like we shouldn't change public policy based on one incident, we can't call a design faulty based on a single failure with unknown, extreme variables.
If a failure is repeatable, we can call it a flaw and address it. A one-time, freak thing, best we can do is analyze what happened to the best of our abilities and try to determine the casual factors, go from there. And that is what Mark is doing right now. Unless you know of another RN50 KB that sent a breech cap into the shooter, this is an isolated incident with a round that may have been reloaded and which, even if it hadn't, is explicitly not approved for use in some rifles. Barrett says no SLAP in his guns, military says M2 machine gun only. There are issues with both muzzle brakes and the (lack of) freebore with the sabot round.
Roughly the same diameter? When was the last time you pulled a measurement on a bolt head?
Your typical centerfire bolt action rifle has a bolt head of around .7" for rounds with a case rim diameter of .47"-.53", plus the area of the lug faces. Roughly double the surface area of the rim.
Bolt lugs can and do shear, and with the extreme majority of bolt rifles, there's nothing else to prevent the bolt going directly into the shooter's face. People have certainly been maimed and killed by failed bolt lugs.
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Kano383
Member
You talk a lot... I asked 4 questions, three of them having a simple Yes/No answer. You didn't answer any of these.
MachIVshooter
Member
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.
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. I think it's problematic that the cap, after shearing the ears off still had sufficient energy to put large dents in the side plates even though it hit them at only a glancing angle and then, after expending that energy and deflecting still hit the shooter with enough energy to cause injury. But clearly, it was the detached ears that caused the most serious injury to the shooter.
The ears had some kind of hole partially through them. I wonder what that was for--looks like maybe there was a pin across the receiver between them, perhaps to stop the hammer when dryfiring with the action open? At any rate, the holes didn't do anything to increase their strength.
Interesting to note that at one point in the video you can see the small indentation on breechcap that shows it was tested for hardness at some point before it was shipped to the user.
Based on the fact that there's brass coloration over the entire end of the barrel where the cap screws on (visible at 1:31 & 2:51 in the video--also to a lesser extent, brass coloration can be seen on the inside of the breechcap), it appears that vaporized brass and discharge gases filled the entire space and therefore the gases were able to act on the entire inner surface of the breech cap. Once the brass failed, the entire inner surface of the breechcap became area for the discharge pressure to work against.
Guessing at the diameter of the back inside surface of the breechcap, I'm going to say 1.75". That gives a surface area of about 2.4 square inches. We know the brass failed, so we know the pressure was at least 80kpsi. Those numbers suggest that the force against the breechcap was was over 190,000 pounds. Running the same numbers with 1.5" inner diameter gives over 141,000 pounds of force.
Had the breech been a plug of minimum diameter instead of a cap, the force against it, with the same 80kpsi pressure would have been around 41,000 pounds. Even using the smaller estimate at the inner diameter for the cap indicates that particular design choice resulted in a 3x-4x increase in force against the breech in the event of brass failure.
In any event, we know what caused the failure--there was an overpressure event of some sort or the brass wouldn't have failed. I think that was clear from the beginning. My concern was more about how the design handled the failure.
I think that the gun was designed properly to withstand normal use. I do think that the evidence suggests that it could have been designed with a better failure mode.
Jim Watson
Member
I await inspection and maybe dissection of the remaining ammo.
Perhaps Mr Serbu will shoot some. Preferably from a rest with a string.
A pre-failure video shows the sabot kicking up dirt out in front of the bench.
Perhaps Mr Serbu will shoot some. Preferably from a rest with a string.
A pre-failure video shows the sabot kicking up dirt out in front of the bench.
MachIVshooter
Member
You don't have to guess; Mark tells us the threads are 1-1/2 12TPI. There's no undercut visible in the images I've seen, so if we use class 2 thread specs at max minor diameter, that's 1.4278" for a surface area of 1.60112278 square inches, yielding 128k pounds of force if we assume 80 KSI chamber pressure and ignore Boyle's law.
The problem with assuming an 80KSI load alone was responsible for this failure is exactly that; it's an assumption. If there was a bore restriction or obstruction, the continued expansion of the gasses is going to result in much higher pressure. It's common knowledge that loads well under max pressure will blow up guns if there is a bore obstruction.
Detonation is another possibility.
We'll just have to wait for the answers, though. It's all speculation at this point.
I didn't make that assumption. I clearly stated we knew the pressure was at least 80kpsi. It definitely could have been much more than that. For the record, I also didn't mean to imply that I think that "load" (that particular cartridge) was definitely overpressure. It could have been an overpressure load, or there could have been some other factor (or factors) that led to the overpressure event. But we do know that the pressure (whatever caused it) was at least 80kpsi.
Anyway, the key is the area difference. Regardless of the pressure involved, the design choice to use the cap style that was chosen vs. a plug style breech resulted in more than 3x more force on the breech in the event of a brass failure.
Also, another interesting design choice was to make the breechcap very strong and hard but without similar attention to the threads on the barrel. The video states that the cap is the "hardest, strongest part of the whole gun". They might as well have been made of the same material, strength and hardness--there was really no benefit in making one of them stronger than the other since a failure of either one would have the same result--the breechcap coming off the back of the gun.
There's a lot known.
- The threads on the barrel failed.
- We know the dimensions of the threads on the cap and barrel.
- The brass failed. This means that there was an overpressure event and provides a minimum (though not a maximum) pressure for that event.
- The cap came back with enough energy to break off the ears.
- We know the inner surface area of the cap.
- The video provides evidence that the gas was able to act on the entire inner surface of the cap.
- The cap was not sufficiently deflected by any design feature to prevent it from hitting the shooter.
- The cap after coming off and breaking the ears had enough energy remaining to injure the shooter.
- The ears, after being broken off were propelled in the direction of the shooter.
- The ears acquired enough momentum to cause wounds that were potentially lethal.
Jim Watson
Member
I have seen a few cases of bore obstruction and a lot of pictures. All were barrel ruptures or bulges. Are there blown breeches out there that I haven't seen?
The only sort of obstruction I know of to affect the action is a tight neck, Clark "bullet pinch." Yet we are told that the wrecked rifle had a machine gun spec barrel for use with SLAP.
Looked like a session of casual centerfire plinking without spotting each hit on target. Oops.
MachIVshooter
Member
I was speaking generally; that has been a widespread assumption, the origin of which is anyone's guess
You're an engineer, you know the calculation is not that simple. Again, Boyle's law is one factor, but also how much gas was able to leak past the ruptured case head, and how quickly? We can't really nail that one precisely even if we had more details, but it would not be instantaneous. There would be a pressure differential for a time. A fractured piston in an IC engine causes excessive crankcase pressure, but the duration of the combustion event isn't long enough for the pressure to equalize between the cylinder and the crankcase. Same thing, but higher pressure and a much shorter event duration with a firearm cartridge discharge.
There's definitely a reason to make a (relatively) thin female threaded part stronger than the male part. Threads shearing is not the only failure mechanism of threaded parts; UN or metric threads have a 30° flank taper, which will create a radial force vector in addition to the axial one. This can and does cause female threaded parts to split before thread failure. Male threaded pieces compressing inward is far less likely, especially when they're thicker. The only way to avoid that completely is square threads, but then you also get stress risers unless you use a proprietary thread with radiused transitions at the thread roots and crests
We actually don't know that, and it's an important detail. We know they are 1-1/2 12 threads, but we do not know what thread class, the root radius, precisely how many threads were engaged, or of they were cut or roll formed. These are important details in evaluating the strength of a threaded connection. 5 threads engaged is 25% more strength than 4, tight class 3 threads with 85% engagement are much stronger than sloppy class 2 that are only 60%, properly radiused roots are less likley to shear than sharp V or flat bottom, and roll formed threads are stronger than cut. I wouldn't expect most barrel & receiver (or breech cap) threads to be roll formed, it's not a practical method for these larger sizes, and the least of the 4 considerations here. But it is one, nonetheless. There's actually quite a lot to threading, much more than most people realize. If it weren't critical stuff, I wouldn't have spent tens of thousands of dollars on thread micrometers, plug and ring gauges, and full profile carbide inserts for cutting both male and female threads in all the different UN and metric pitches, and then scrap parts and tweak a program (or manually feed on X while cutting on Z on the manual machines) to compensate for deflection until the threads are perfectly uniform over the entire bore or tenon. Believe me, it would be a lot more cost effective to do it backwoods with a caliper and AG60 inserts and settle for "it goes together". And many people do just that, even some who call themselves machinists or gunsmiths.
The point of all that is that the form and fit of the threads could actually be a flaw, even if the design itself is sound. I doubt Mark would get that wrong, but it's another detail we simply don't know.
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MachIVshooter
Member
An image search of failed bolt lugs turns up lots of carnage, but usually not much detail on how it happened.
Regardless, all it takes for the bolt lugs, breech cap/barrel threads or receiver to fail from a bore obstruction is for that obstruction to be at a point in the barrel which is still able to resist more force than the lugs, threads or receiver can handle.
The RN50 has a husky barrel.
Ok. So I should have said "potentially more than 3x more force". The point that the design choice results in tremendously more force applied to the breech in the event of a gas escape than in a plug design is not in question.
No, of course it's not. I suppose they could have made the cap so weak that it could have simply ruptured instead of causing the barrel threads to fail. However, the point remains that hardening only one side of a threaded connection can't make it any stronger than the threads on the other side of the connection. 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.
Fair enough. But you picked just one error out of an incomplete list of things we do know which was a response to your claim that it is "all speculation". Clearly we know enough that saying it's "all speculation" is ridiculously inaccurate.
I don't really get where you're coming from. It's one thing to say that we don't know everything about this accident and can't nail down all the factors that contributed to the outcome. It's another thing to argue that "it's all speculation", and to pick nits about minor factors that are uncertain or unknown while ignoring major factors that are known.
Everyone seems to be looking for a mechanical explanation, when the explanation is staring them in the face. The ammunition is from 2007. We don't know the storage conditions, but that ammunition is over 20 years old.
I am going to claim that what happened is old, deteriorated gunpowder. Combustion pressures rise with old gunpowder, and it does not have to be all that old, to be deteriorated.
since IMR 4007 came out in 2007, this would mean these lots are no more than
this went bad fast
RN-50 Blow-Up
Kentucky Ballistics
And the thing that jumped out to me was the immense fireballs, and finally the gun blew up. You can see, the breech locking threads are stripped. And there you have it. Old ammunition.
I am going to claim that what happened is old, deteriorated gunpowder. Combustion pressures rise with old gunpowder, and it does not have to be all that old, to be deteriorated.
since IMR 4007 came out in 2007, this would mean these lots are no more than
this went bad fast
RN-50 Blow-Up
Kentucky Ballistics
And the thing that jumped out to me was the immense fireballs, and finally the gun blew up. You can see, the breech locking threads are stripped. And there you have it. Old ammunition.
MachIVshooter
Member
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.
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. When you harden an alloy, you increase the yeild strength dramatically, but the ultimate tensile strength doesn't go up proportionately, sometimes very little. Some of the hardest and strongest alloys out there have a TYS and UTS that are practically the same, such as 7068 Tennalum. When you have materials like that, you get no warning signs, just catastrophic failure when the strength is exceeded. The way the RN is designed, it won't close unless the cap is fully seated, so it's entirely possible that it was designed in such a way that the male threads would stretch in the event of an overpressure condition and render the gun inoperable until it could be inspected and serviced. Just one possible reason that the male threads may have been less hard. But unless Mark has answered that question, I believe we are still in the dark about the alloy and temper of the breech threads.
Without knowing the chamber pressure that was actually generated and if there was or wasn't a barrel obstruction, it really is all speculation. If we see a second or third failure that sends the breech cap backward and shears the ears off, we can start to point to design. Even better if those failures are reproduced under controlled conditions where pressures and other variable are known.
I don't know if Mark has done destructive testing on these or any of his others. 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. But seeing the things I've seen, I would say with confidence that an overpressure condition well beyond the minimum failure point could produce a different type of failure. Which I guess begs the question if any manufacturer does such testing, and should they?
At any rate, short of that, all we really do know is the same thing we've known for a long time: It was a massive overpressure condition, and a massive overpressure condition can cause a catastrophic weapon failure that may injure or kill the shooter or bystanders. Receivers explode, chambers rupture, cylinders fracture and tear top straps. It's just that much worse at .50 BMG scale.
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Really good points. 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 am going to say, what's the point of a destructive blowup test, other than to have fun? Do you plan to take your weapon to destruction? Is that the reason you bought your 50 caliber, so you could be Kamikaze Joe and end yourself in a spectacular kaboom on the very first shot?
If so, you are too dangerous to be around. Say, here is something you can do, jump a motorcycle across a long time of vehicles, and we can watch the video. You will be famous.
I am more interested in endurance tests, which will lead to part failures, than the typical once off blowup test. I want to know how long the structure will last with maximum loads, to have an idea of its useful life. Anyone filling the barrel of their muzzleloader with smokeless powder has gotta know, they are not going to fire the thing twice. So, why do that? Anything made by man, can be unmade by man.
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