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Old May 2, 2021, 03:18 PM   #1
ghbucky
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Serbu RN-50 explodes

Youtube video going over a .50 cal MSR exploding on a guy.

He sustained life-threatening injuries, but he did survive and reports he will make a full recovery.

My reason for posting this is that I am very dubious of his claims that it was a 'freak accident' caused by old ammo that had been 'tampered' with.

I see he had a difficult extraction of the prior round and just tossed the case away with no inspection.

The gun is completely blown apart (at 4:31) so it must have been a very violent overpressure. He claims the manufacturer says it would take 30k psi over spec to cause the chamber to be blown out.

Any ideas as to what happened here? I'm having a hard time with that much overpressure out of surplus ammo.

[edit] oops... this should probably be in the rifle section. Apologies for the extra work, mods.

Last edited by ghbucky; May 2, 2021 at 03:30 PM.
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Old May 2, 2021, 05:20 PM   #2
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He was shooting SLAP rounds, possibly ultra hot and of dubious pedigree.
Anyone with a .50 should know you don’t shoot SLAP rounds if you have a muzzle brake, much less rounds you don’t know where they came from.

Mark Serbu has a good overview of it on YouTube.
(On a related note, that’s not an uncommon way to booby trap a weapon: leave rifle rounds filled with pistol powder for an enemy to find and destroy themself. Many armies have used this tactic over the years.)
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Old May 2, 2021, 07:01 PM   #3
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Quote:
He was shooting SLAP rounds, possibly ultra hot and of dubious pedigree.
Anyone with a .50 should know you don’t shoot SLAP rounds if you have a muzzle brake, much less rounds you don’t know where they came from.
One thing that I don't understand about this is that he had Mark Serbu make him a bespoke upper for SLAP some time prior to this. And he had used it before to shoot SLAP in the RN-50.
Not that it matters much. I don't think the muzzle brake played a part in this failure.
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Old May 2, 2021, 08:42 PM   #4
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I am not going with the notion that the round was ultra hot, but that he was shooting ammo from a rifle that was not designed for it, and was using a muzzle brake with SLAP ammo which you should never do. I agree with this analysis that the overpressure situation resulted from a barrel obstruction, likely caused by the build up of SLAP round sabot material, that and combined with a gun design not capable of handling the pressure. Unlike military guns, the Serbu is NOT over engineered.

One lesson I got from Barrett school was to never fire a SLAP round from our Barretts with the muzzle brake attached due to the possibility of catastrophic failure.

With that said, there is no way to know at this point if it was an obstruction event, hot round event, structural defect of the rifle, or some combination thereof.

https://youtu.be/ardw09S7GfE
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Old May 2, 2021, 09:24 PM   #5
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First thing I thought was that he was shooting a slap with a muzzle brake. I'm guessing the sabos were getting shredded causing the initial accuracy issues. I'm also guessing he would up with a bore obstruction from the Sabo debri, or the slap went sideways in the brake causing the over pressure.

Could have just been an over pressure round too, or a combo of the spicy round and the brake.

Just glad he made it out alive
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Old May 3, 2021, 05:31 PM   #6
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According to somebody on TheHighRoad,

Mark Serbu is peforming an inspection of the rifle.
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Old May 3, 2021, 07:11 PM   #7
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Which can't be unbiased.
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Old May 3, 2021, 07:36 PM   #8
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Quote:
Which can't be unbiased.
Right? Let me prognosticate: it was an ammunition problem and the rifle design is fine.
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Old May 3, 2021, 07:55 PM   #9
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According to Wikipedia (which, as we all know, is infallible), a .50 BMG proof round is 65,000 psi, which is 17% above the nominal maximum.

https://en.wikipedia.org/wiki/Proof_test

Quote:
An example proofing round for the .50 BMG (12.7 × 99 mm) is the "cartridge, caliber .50, test, high pressure, M1". This uses the standard-weight .50 BMG M1 round propellant (240 gr of WC860), but a bullet weighing 999 gr (+/- 11 gr). The M1 proof round gives a proofing pressure of ~65,000 psi, 11,000 psi (~17%) above the standard service pressure.
The question is, how can anyone state with any degree of certainty that the round that blew up the gun "must have been" 80,000 psi?

One comment I saw on this incident commented that the threads of the screw-on breech cap appear to have been standard 60-degree machine threads, and that Acme threads are usually employed in high stress/high load applications. That makes sense to me.
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Old May 3, 2021, 08:18 PM   #10
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The rifle design is no good because failure occurred catastrophically at the point of lock up.

I think bolt guns typically blow up by splitting the action around the barrel which vents the pressure with less severe results to the shooter.

IMO, they need to go out of business or develop a non-catastrophic bolt retrofit they can give to all their owners.
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Old May 3, 2021, 08:52 PM   #11
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Muzzle brake?
I would not have thought a muzzle brake strike would propagate back to the breech.
Why aren't we seeing pieces of muzzle brake?
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Old May 3, 2021, 11:14 PM   #12
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Double Naught Spy:

My comment about Mark Serbu's inspection wasn't to imply that a creator/builder is unbiased.

I stated what was mentioned on THR...just in case some people were unaware of it.
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Old May 3, 2021, 11:33 PM   #13
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The youtube film says this is Kentucky Ballistics. My nephew sent me a couple links to watch. The first one was shooting a double barreled elephant gun, pulling both triggers at the same time. The 50 cal kaboom is another example of risky click bait behavior on YouTube. Makes ya go huh...??
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Old May 4, 2021, 08:26 AM   #14
ghbucky
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I got a picture of the breech of one of these rifles of gunbroker:


Compare that with the breech plug for a T/C break open BP rifle:


Kind of startling to realize a BP breech plug has what appears to be more beef to it than a .50 BMG rifle.
Attached Images
File Type: png serbu breech.png (492.3 KB, 166 views)
File Type: png Screenshot 2021-05-04 092424.png (101.4 KB, 167 views)
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Old May 4, 2021, 09:25 AM   #15
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Quote:
Double Naught Spy:

My comment about Mark Serbu's inspection wasn't to imply that a creator/builder is unbiased.

I stated what was mentioned on THR...just in case some people were unaware of it.
I understand and didn't think you had. My intention was to express that I definitely didn't think it could be unbiased, but only because he is the designer, manufacturer, and potential litigant in a civil case and has a HUGE vested interest is what information comes out from his analysis, for his company, his company's reputation, and his reputation. There is potentially a lot riding on this for Mark Serbu.

Anything short of him saying the rifle was at fault will have to be viewed as being a potentially biased, even if he is 100% accurate, because of his position in the situation. Of course, we can't know if what he says is accurate and unbiased until an unbiased party does the analysis. In other words, his analysis will do little for the gun community knowing one way or the other what happened.
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Old May 4, 2021, 10:43 AM   #16
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Quote:
Originally Posted by Double Naught Spy
Anything short of him saying the rifle was at fault will have to be viewed as being a potentially biased, even if he is 100% accurate, because of his position in the situation. Of course, we can't know if what he says is accurate and unbiased until an unbiased party does the analysis. In other words, his analysis will do little for the gun community knowing one way or the other what happened.
What will be more informative is what he does with that model going forward. Off the top of my head, he has three choices ... and none of them look good.

If he concludes to his own satisfaction that there's nothing wrong with the design and manufacture, then I guess he just keeps on selling them -- perhaps with a disclaimer not to shoot SLAP rounds with a muzzle brake installed. What happens, though, if he determines (privately) that the design is deficient? In that event, his choices are:

1) Keep selling it with no changes, but with a disclaimer as above;

2) Stop selling it altogether, which would effectively be a tacit admission that the design was deficient; or

3) Change the design to strengthen the breech cap and locking lugs, which would also be a tacit admission that the original design was deficient.

Tough call.
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Old May 4, 2021, 02:04 PM   #17
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It is, because we have seen what can happen when it fails. JohnKSa and I have discussed this off site and failure is at the weakest point which appears to be the threads means the end cap gets blown into your face and the 'ears' on the lower aren't sufficient to stop the cap and themselves become dangerous projectiles. So failure force is directed AT the shooter. That is a very bad direction for that to occur.
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Old May 4, 2021, 02:33 PM   #18
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Serbu RN-50 explodes

Quote:
Originally Posted by Double Naught Spy View Post
It is, because we have seen what can happen when it fails. JohnKSa and I have discussed this off site and failure is at the weakest point which appears to be the threads means the end cap gets blown into your face and the 'ears' on the lower aren't sufficient to stop the cap and themselves become dangerous projectiles. So failure force is directed AT the shooter. That is a very bad direction for that to occur.

This is the root of the problem. Good firearm design dictates that in an over pressure situation, the weakest part of the gun will fail first, and the gun should be designed so that the weakest part of the gun will fail in a manner that does not direct energy towards the shooter.

Hence, some guns have “vent holes” designed to direct a failure elsewhere. Revolvers, the cylinder blows upwards, not backwards. The 1911 (and almost all tilt barrel designs) the slide comes off forwards, and cannot blow backwards before some other part of the gun fails. Mausers have the tri-lug safety.

The problem with the Serbu design is that the rear cap failed before some other part of the barrel failed. It should be the reverse. That barrel should split before that rear cap ever has a chance of coming off. And the ears on the receiver of the Serbu were clearly insufficient to deflect the cap in the event that there was enough pressure to blow it backwards.

Yes it appears that the shooter had either an ammo problem or broke a safety rule by shooting SLAP rounds through that configuration of gun.

But even with that, the gun should have been designed to deflect the energy of a catastrophic failure in a different direction, which it was not. That’s the biggest issue in my mind.


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Old May 4, 2021, 03:21 PM   #19
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I am sure you are making some sort of point, but since the gun did fail at the weakest part, the cap, the failure results in direct injury to the shooter, that is pretty much the root of the problem, I would say.
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Old May 4, 2021, 03:53 PM   #20
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Quote:
Originally Posted by Double Naught Spy
I am sure you are making some sort of point, but since the gun did fail at the weakest part, the cap, the failure results in direct injury to the shooter, that is pretty much the root of the problem, I would say.
If "you" referred to Radny97, the post immediately before yours, I think that was his point: the gun did fial at the weakest point, but the breech cap should not have been the weakest part. The weakest part should have been father forward, and oriented such that in the event of a kaboom the pressure would be vented away from the shooter, not directly into his face.
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Old May 4, 2021, 05:43 PM   #21
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Right, we have all now stated that the gun failed at the weakest point and that the location is bad. That the location should be elsewhere is rather obvious.

The root of the problem is where the gun failed.
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Old May 4, 2021, 09:02 PM   #22
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Serbu RN-50 explodes

Quote:
Originally Posted by Double Naught Spy View Post
I am sure you are making some sort of point, but since the gun did fail at the weakest part, the cap, the failure results in direct injury to the shooter, that is pretty much the root of the problem, I would say.

Double Naught Spy, Not sure why you have it out for me on this thread and others. I’m merely agreeing with you and expanding on your point.

Anyways, I think Serbu, who I believe is a good guy and is horrified at this awful accident, needs to go back to the drawing board with this gun design. It’s not designed in a way that protects the shooter in the event of a catastrophic failure.


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Old May 5, 2021, 02:00 AM   #23
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What might be interesting is Varmint Al can do finite element analysis.

I believe Pro-Engineer software is capable of finite element analysis.

These tools model force or energy flowing through solids.Pretty cool stuff.
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