I had a partial case head separation using a 1.8oz buffer

[DeNinny]

If the brass yields in-battery, it will happen more as a result of peak pressure, not residual pressure post-exit of the projectile.

To me you are claiming the effect is the cause here. "Peak" pressure is completely over as soon as either 1) the bullet leaves the mouth or 2) the blowout occurs.

The blowout occurred because pressure was at some level inside the casing and it simply reached the necessary force to rupture the brass. This is MY definition of "peak" pressure. The laws of chemistry and materials science dictate this.

The reason a metal deforms is because force (or pressure) is applied to it. And all materials, metals/alloys in particular, go through a phase of temporary deformation. And then if enough force is continually applied, the metal will reach a point of permanent deformation. At this point, it will never restore itself to original shape. And then if even more force is applied, the metal eventually ruptures. And then once that rupture occurs, the metal wall itself is no longer holding back all that pressure so it immediately dissipates. The pressure will not ever "peak" after this.

And in this failure, as soon as that rupture occurred, at whatever peak pressure it took, the pressure dropped. This is how the pressure will respond in any fully pressurized vessel that is pushed to the point that a weak point in it permanently deforms and ruptures.

So with all this, the pressure was not "residual" as you stated. The pressure was rising with the cartridge in battery, it simply reached the point of stretching the brass into the feed cone, at which point it started permanently bending to the shape of the feed cone. And then finally, the force (pressure) reached the point of tearing it. And since a bend is a weak point in the alloy, the tear started there. This is all from the pressure within the case that started building as soon as the primer fired. And it kept building until the brass ruptured.

And remember, immediately after the tear started, pressure started dropping rapidly. Plus again, the bullet leaving the mouth dropped it a lot too. So there definitely didn't need to be all this other extra pressure to do all that damage as you are assuming would happen.

The pictures are evidence that the brass ruptured in battery as per all my other points. Above is the chemistry and material science theory of how it happened and is all 100% consistent with what I see in the pictures. Every blowout will happen the same, and yet the peak pressure at which it happened doesn't have to be exactly the same.

To me you are assuming all blowouts have to have nearly the same damage. The science to me says otherwise. You have to analyze each scenario in depth to flush out those differences.

The farther the projectile moves from the chamber, the lower the pressure is.

Agreement!!!

If the brass yields in-battery, you will see substantial damage to the action.

Not necessarily per all my points above.

I would expect to see this with a case yielding in-battery:

...

When extractors appear to stay intact with an in-battery case failure, they look like this:

...

To me these are all just higher pressure failures. The OP's failure just happened at a lower pressure which still happened in battery. Clearly and lucky for him!

But what we have seen is not indicative of a case failure in-battery.

This is exactly what the pictures support and as I have described. IMHO.

[lazyengineer]

I had a manager that once told me that "arguing with an engineer is like wrestling in the mud with a pig - the engineer ends up happy and you just end up dirty."

(They say this about lawyers too.)

For the record I'm a chemical engineer with my BS from UC Berkeley. Graduated in the 90s.

Same. One thing they taught us early in Engineering is #1 goal is for the bridge to not fall down. #2 is under budget/ better mileage aspects / pleasant aesthetics. "Nobody remembers the bridge that came in under budget. Everybody remembers the one that falls down".

At present, I see one parts vendor for my bridge involved in 3 of the last 3 catastrophic failures in my industry with 3 different agencies with 3 different projects, with that being the part that failed every time. And with a seemingly higher frequency of a warning sign deformation behavior. That's a huge red-alert, just so you know. And then I see a difference in construction philosophy that happens to show up at the location of the failures. The other vendors in the same usage aren't failing and aren't so readily showing this strange deformation warning sign? Yea, that vendor is gone.

[LRRPF52]

I went back to the OP and I still think this is early unlock.

An additional detail dawned on me when researching the barrel itself... I can't seem to find it sold and not include an adjustable gas block. This seems to reinforce my suspicion of early unlock since the port diameter may be oversized with an expectation that the gas system would be appropriately adjusted.

I started looking up Odin Works 12” 6.5 Grendel barrels as well, and also noticed that they do come with their adjustable gas block:

I’m not finding any fault with them just for clarity.

That port doesn’t look particularly large, but I obviously don’t have it in-hand.




I do know that with the 12” Grendels I have built, which have at least many hundreds of rounds through them so far, 99% suppressed, they used .068” gas ports if I recall, whatever we spec’d with the Group Buy through Faxon. I still dial the gas down on the Bootleg carriers (no affiliation), mainly because I’m shooting suppressed.

For the few rounds I shot in 2018 unsuppressed, I didn’t notice any issues with the brass, and cyclic rate felt about as in the happy space window as I’ve felt with an AR-15 in a long time.

Extra power action/recoil springs played a role in that, which would also fight early extraction.

Now I’m wondering what action spring and carrier weight the OP used.

[Klem]

Ninny,

Your theory makes sense, but it is not definitive.

Another is, on early unlocking there was still enough pressure in a weakened, superheated case to rupture along the path of least support at the time. This assumes the post-peak/dwell-time pressure was enough to rupture an unsupported case. Expansion from too much headspace may also have been involved to weaken the case, but that is another assumption.

[Zeneffect]

The farther the projectile moves from the chamber, the lower the pressure is.

I disagree. This is only an observational result of expanding volume after loss of potential chemical energy (powder burn) and thermal loss from said volume expansion. Peak pressure can occur (as measured not yeild) well after the bullet has left the case mouth, and can occur more than once. This phenomenon is very well documented in calibrated test breeches (the double explosion is an example)

So yes but not really but yes. There are ways to make the general consensus to be not true. You would be dummy for doing it but it's possible.

After looking over the gas block, I want one in .936

[DeNinny]

The projectile passing the gas port and uncorking from the muzzle are dramatically different. Gas port hardly provides any relative pressure relief to muzzle velocity as I mentioned before.

My analysis was simply comparing the effect of the gas port distances only. Had nothing to do with uncorking from the muzzle. To which I completely agree. It's absolutely going to be much higher whilst uncorking.

The reason is simple and I'll explain it differently than how I did before. Within the casing the gas from powder combustion is enclosed in a small volume. The case itself. The bullet is holding it all in until it uncorks. Once that uncorking occurs. The contained gas now can expand into the greater volume of the barrel and it will continue to expand down the barrel. All the while the pressure is dropping.

Then once it reaches the gas port it will clearly be at a lower pressure than at the mouth.

We have always been at 100% alignment here.

This isn’t me theorizing, but actual test results Bill A. did when developing Grendel, as well as numerous barrel length tests conducted by Rifle Shooter.

I asked him about it when we were discussing MLGS vs RLGS on 18” guns, the main question being whether 18” RLGS would have more velocity.

I appreciate all this real world insight. I would love to discuss the chemistry, physics, materials science, and metallurgy with Bill! You are lucky!

Gas port does bleed enough energy to get the carrier moving though. The closer the port is to the chamber, the more energy imparted into the carrier, which is controlled with port sizes. Smaller port sizes will choke the flow of gas back into the carrier to try to keep it relative to what a 20” RLGS will provide with its much larger port, but lower port pressure.

And all I was saying is that once the gas reaches the port, it now has two paths to go - continue down the barrel and return back to the gas key. And the second path back to the gas key is what makes the pressure drop because again the added volume of the gas tube itself. As such the gas pushing the bullet is slightly lower than it was if the gas port wasn't there.

[LRRPF52]

To me you are claiming the effect is the cause here. "Peak" pressure is completely over as soon as either 1) the bullet leaves the mouth or 2) the blowout occurs.

The blowout occurred because pressure was at some level inside the casing and it simply reached the necessary force to rupture the brass. This is MY definition of "peak" pressure. The laws of chemistry and materials science dictate this.

The reason a metal deforms is because force (or pressure) is applied to it. And all materials, metals/alloys in particular, go through a phase of temporary deformation. And then if enough force is continually applied, the metal will reach a point of permanent deformation. At this point, it will never restore itself to original shape. And then if even more force is applied, the metal eventually ruptures. And then once that rupture occurs, the metal wall itself is no longer holding back all that pressure so it immediately dissipates. The pressure will not ever "peak" after this.

And in this failure, as soon as that rupture occurred, at whatever peak pressure it took, the pressure dropped. This is how the pressure will respond in any fully pressurized vessel that is pushed to the point that a weak point permanently deformed.

So with all this, the pressure was not "residual" as you stated. The pressure was rising with the cartridge in battery, it simply reached the point of stretching the brass into the feed cone, at which point it started permanently bending to the shape of the feed cone. And then finally, the force (pressure) reached the point of tearing it. And since a bend is a weak point in the alloy, the tear started there. This is all from the pressure within the case that started building as soon as the primer fired. And it kept building until the brass ruptured.

And remember, immediately after the tear started, pressure started dropping rapidly. Plus again, the bullet leaving the mouth dropped it a lot too. So there definitely didn't need to be all this other extra pressure to do all that damage as you are assuming would happen.

The pictures are evidence that the brass ruptured in battery as per all my other points. Above is the chemistry and material science theory of how it happened and is all 100% consistent with what I see in the pictures. Every blowout will happen the same, and yet the peak pressure at which it happened doesn't have to be exactly the same.

To me you are assuming all blowouts have to have nearly the same damage. The science to me says otherwise. You have to analyze each scenario in depth to flush out those differences.


Agreement!!!


Not necessarily per all my points above.



To me these are all just higher pressure failures. The OP's failure just happened at a lower pressure which still happened in battery. Clearly and lucky for him!


This is exactly what the pictures support and as I have described. IMHO.

Peak pressure happens directly in the middle of the propellant column, per Aberdeen and every other major company that has placed piezoelectric gauges over chambers and measured the variances.

In-battery case failures happen due to peak pressure finding a failure node in the brass or an incorrectly-cut chamber.

That failure event is extremely close to ignition, which means you now have substantial pressures venting into areas of the system they shouldn’t be.

When this happens, you see massive and irreparable damage to components like in the images I posted. You don’t have clean egress of the escaping high pressure gases without noticeable damage to the upper and nearby parts, with a few exceptions:

Exotic alloy bolts, barrel extensions, and beefy billet uppers.

In a partial case head separation, the vast majority of the pressure has already subsided because the projectile passed the gas port already, but there is more than enough bore pressure to push brass around:



Imagine a 6.5 Grendel pmax of 47,000-50,000psi, taking propellant volume (more than 5.56) into consideration, as well as bore volume, then overlay it on this same type of graph.

You can see that CLGS port pressure could easily be 20,000psi or higher at that port location.

Now look at the pressures between the port location and the muzzle.

This is the plug dwell time.

See how there is enough pressure still being exerted on the case (temporary gasket sealing the rear end of the vessel) so that if the case moves rearward even just enough, the brass can yield since it’s no longer being supported?

This is true regardless of the cartridge.

If there was significant axial force being exerted by the extractor in a certain clock location, stressing the brass in that place, that location is more likely to initiate the failure node from a combination of axial strain and internal pressure.

This is exactly what I think we are seeing right here:



And because I was spending so much time discussing the fleet performance of Lapua brass, I totally missed this other critical detail:

Actually - it's a full Odin BCG and bolt - here's the exact item I purchased:

https://www.joeboboutfitters.com/Odin-Wo...tg-bcg.htm

One other interesting bit about the pistol is I put a Strike Industries short pistol buffer tube with a 1.8oz buffer on it:
https://www.strikeindustries.com/si-ar-spre-slick.html

Would a light buffer potentially put extra strain on the case a bit early? It's a carbine gas system.

[DeNinny]

Ninny,

Your theory makes sense, but it is not definitive.

Another is, on early unlocking there was still enough pressure in a weakened, superheated case to rupture along the path of least support at the time. This assumes the post-peak/dwell-time pressure was enough to rupture an unsupported case. Expansion from too much headspace may also have been involved to weaken the case, but that is another assumption.

I am keeping headspace on the table, but since I don't need the science from it to explain my failure model(s) and since I don't think it is dimensionally significant as compared to the feed cone depth issue, I have it very low on my list as a significant contributor to this.

Again, we have mostly commonality and agreement here in theory! Just not significance.

The only part I disagree with 100% is that you will never rupture a case in any scenario in battery where the pressure was lower than an earlier pressure. This is what I was getting at with LRRPF52 earlier. Again, prior to the rupture, pressure was rising and rising, never falling. Then right at the rupture it is at peak value. Right after the rupture, the pressure will immediately start dropping. So there will never be "post-peak/dwell-time" phase pressure to rupture a case in-battery. If it didn't happen at peak pressure, it's not going to happen at any lower pressure after that.

[LRRPF52]

Same. One thing they taught us early in Engineering is #1 goal is for the bridge to not fall down. #2 is under budget/ better mileage aspects / pleasant aesthetics. "Nobody remembers the bridge that came in under budget. Everybody remembers the one that falls down".

At present, I see one parts vendor for my bridge involved in 3 of the last 3 catastrophic failures in my industry with 3 different agencies with 3 different projects, with that being the part that failed every time. And with a seemingly higher frequency of a warning sign deformation behavior. That's a huge red-alert, just so you know. And then I see a difference in construction philosophy that happens to show up at the location of the failures. The other vendors in the same usage aren't failing and aren't so readily showing this strange deformation warning sign? Yea, that vendor is gone.

The fallacy of this argument was already pointed out, but I don’t recall a refute to it.

Sample sizes, assumption of the forum being a comprehensive collection of all failures, and another poster’s search showing up multiple other brass types to his query.

Did you perhaps overlook those like I did the 1.8 ounce buffer?

1.8oz buffer is significantly less weight than your typical 2.9oz carbine buffers.

I would never put that light weight of a buffer in a CLGS .223 Rem, let alone 6.5 Grendel.

OP basically duplicated somewhat one of the main problems they experienced with the Edgewater spring guide in Vietnam, shooting the new M193 with ball propellant, only his chamber is really clean and more likely in-spec.

I apologize for overlooking this critical piece of information that now pretty much nails the coffin shut on early locking with CLGS and 1.8oz buffer.

[Zeneffect]

You realize that is a screenshot of quickload? Regardless it is exactly the demonstration I was speaking of. I did miss the reduced buffer weight.

[LRRPF52]

I am keeping headspace on the table, but since I don't need the science from it to explain my failure model(s) and since I don't think it is dimensionally significant as compared to the feed cone depth issue, I have it very low on my list as a significant contributor to this.

Again, we have mostly commonality and agreement here in theory! Just not significance.

The only part I disagree with 100% is that you will never rupture a case in any scenario in battery where the pressure was lower than an earlier pressure. This is what I was getting at with LRRPF52 earlier. Again, prior to the rupture, pressure was rising and rising, never falling. Then right at the rupture it is at peak value. Right after the rupture, the pressure will immediately start dropping. So there will never be "post-peak/dwell-time" phase pressure to rupture a case in-battery. If it didn't happen at peak pressure, it's not going to happen at any lower pressure after that.

This last paragraph supports the partial extraction model, no others. Pmax happens right after ignition. 47,000-52,000psi venting through that failure point on the brass would not leave brass looking that clean. Notice the lack of eschar/sootiness.

If you’re not convinced after seeing that he used a 1.8oz buffer, I would suggest spending considerable time studying the AR-15 operating system some more.

Carbine buffers: 2.9-3.0oz
H1 buffer: 3.7-3.8oz
H2 buffer: 4.6-4.7oz
H3 buffer: 5.0-5.4oz

I don’t think I’ve heard of a 1.8oz buffer before in the past 35 years of being eyeball-deep in AR-15s.

There’s our culprit. 123gr 6.5 Grendel, especially with AA2520 or any of the other ball powders like BL-©2, CFE223, H335, etc. would have done the same thing with a 1.8oz buffer on the back end of the BCG.

I started out using a Spikes ST-T2 (4oz)in my first 16” Grendel build if I recall.

Guys, my main reason for posting here is to help out other people to get set up, and to troubleshoot any problems they might be having.

It isn’t to argue ad nauseam about distractions from the roots causes of the problems.

Had I seen this earlier, it would have jumped out at me like a lightning bolt.

[DeNinny]

Peak pressure happens directly in the middle of the propellant column, per Aberdeen and every other major company that has placed piezoelectric gauges over chambers and measured the variances.

I don't dispute this. But it is irrelevant to my failure models. What I need to see is this type of data during an actual failure. One similar to OPs would be great. What you are sharing is what happens under normal conditions. That curve will be much different for an in battery case rupture. At the beginning it would look fine. T = 0. And for sure any in-battery case rupture is going to occur between T = 0 and T at peak. And if it happens before T at peak, then it will immediately drop below that point after that.

In-battery case failures happen due to peak pressure finding a failure node in the brass or an incorrectly-cut chamber.

Agreed. The failure node in the brass was at the bend point as it bent to the shape of the feed cone. This was at the extractor location too, where the brass had literally more volume to expand into.

That failure event is extremely close to ignition, which means you now have substantial pressures venting into areas of the system they shouldn’t be.

To be clear, there are two areas only. The rupture point and the mouth of the case. The gas will escape only those two points and the one that happened first will have more gas escape that way.

When this happens, you see massive and irreparable damage to components like in the images I posted. You don’t have clean egress of the escaping high pressure gases without noticeable damage to the upper and nearby parts, with a few exceptions:

When this happens the damage will depend on the pressure that causes the failure like I already explained. You are again assuming the amount of damage dictates one particular failure. That's an assumption but it's possible to just have a blowout in battery with only enough residual pressure to blow out the magazine. Again, each failure case needs to be analyzed individually. there will be similarities AND differences in all of them.

In a partial case head separation, the vast majority of the pressure has already subsided because the projectile passed the gas port already, but there is more than enough bore pressure to push brass around:

...

Imagine a 6.5 Grendel pmax of 47,000-50,000psi, taking propellant volume (more than 5.56) into consideration, as well as bore volume, then overlay it on this same type of graph.

You can see that CLGS port pressure could easily be 20,000psi or higher at that port location.

Now look at the pressures between the port location and the muzzle.

This is the plug dwell time.

See how there is enough pressure still being exerted on the case (temporary gasket sealing the rear end of the vessel) so that if the case moves rearward even just enough, the brass can yield since it’s no longer being supported?

This is true regardless of the cartridge.

Edit: I misunderstood this the first time. I'll simply say again that the brass was already not supported. There doesn't even need to be a reason it shifted. The existing unsupported and weakened spot ruptured at a high "enough" pressure. No shift needed.

If there was significant axial force being exerted by the extractor in a certain clock location, stressing the brass in that place, that location is more likely to initiate the failure node from a combination of axial strain and internal pressure.

This is exactly what I think we are seeing right here:



And because I was spending so much time discussing the fleet performance of Lapua brass, I totally missed this other critical detail:

All this is simply explained by an in-battery case head that is ballooning into the unsupported area of the feed cone and also into the area of the extractor due to the smaller volume of the extractor relative to the lugs.

That's exactly what I see. Again. And goodnight for now!

[LRRPF52]

You realize that is a screenshot of quickload? Regardless it is exactly the demonstration I was speaking of. I did miss the reduced buffer weight.

Yup. I’ve been looking at screenshots of QL like that for illustrating peak and residual pressures for well over a decade.

Both things are true. QL is totally off the mark (by thousands of PSI) on its predicted chamber pressures in 6.5 Grendel.

Barrel length pressure curve models like this are great illustrations of how pressure subsides as volume increases in the containment vessel.

[Zeneffect]

This is exactly the point I was making. Glad we are seeing eye to eye now regarding software. Accuracy of actual data is irrelevant for the purpose of this demonstration, only the theoretical delta being as accurate as "a lot, holy balls, or meh"

We see it's in the "a lot range" with this powder, combined with improper buffer weight contributing to an early unlock situation (probably overgassed for normal buffer weight as well would be my guess)

[lazyengineer]

The fallacy of this argument was already pointed out, but I don’t recall a refute to it.

Sample sizes, assumption of the forum being a comprehensive collection of all failures, and another poster’s search showing up multiple other brass types to his query.

Did you perhaps overlook those like I did the 1.8 ounce buffer?

1.8oz buffer is significantly less weight than your typical 2.9oz carbine buffers.

I would never put that light weight of a buffer in a CLGS .223 Rem, let alone 6.5 Grendel.

OP basically duplicated somewhat one of the main problems they experienced with the Edgewater spring guide in Vietnam, shooting the new M193 with ball propellant, only his chamber is really clean and more likely in-spec.

I apologize for overlooking this critical piece of information that now pretty much nails the coffin shut on early locking with CLGS and 1.8oz buffer.

While I continue to agree to disagree on some things; I will agree that is a really light buffer!

[DeNinny]

This last paragraph supports the partial extraction model, no others.

It supports everything I've stated. We are going in circles. If the case didn't rupture at peak, it will never rupture in battery afterwards.

By the science, there is a specific peak force that ruptures an alloy at a failure node. If that same exact brass has not ruptured by that pressure, then it will NEVER rupture at a pressure lower than that. The reason is that the brass already withstood deformation at a higher pressure. It's not all of a sudden going to get weaker after that. The forces to deform it are simply lower than that of rupturing it. As proven by it NOT rupturing at peak.

Edit: Ok goodnight for real!

[Zeneffect]

Would the brass not get weaker as the walls thin due to stretch and expansion that occurs concurrently during an early unlock, where peak yeild pressure is measurably reduced due to changes in material dimension?

I'm not a mechanical engineer, maybe you can clarify. If there is no failure, then what is considered peak pressure in the formally trained sense as described previously? Just some greater than x equation?

[LRRPF52]

I don't dispute this. But it is irrelevant to my failure models. What I need to see is this type of data during an actual failure. One similar to OPs would be great. What you are sharing is what happens under normal conditions. That curve will be much different for an in battery case rupture. At the beginning it would look fine. T = 0. And for sure any in-battery case rupture is going to occur between T = 0 and T at peak. And if it happens before T at peak, then it will immediately drop below that point after that.


Agreed. The failure node in the brass was at the bend point as it bent to the shape of the feed cone. This was at the extractor location too, where the brass had literally more volume to expand into.



To be clear, there are two areas only. The rupture point and the mouth of the case. The gas will escape only those two points and the one that happened first will have more gas escape that way.


When this happens the damage will depend on the pressure that causes the failure like I already explained. You are again assuming the amount of damage dictates one particular failure. That's an assumption but it's possible to just have a blowout in battery with only enough residual pressure to blow out the magazine. Again, each failure case needs to be analyzed individually. there will be similarities AND differences in all of them.


With all this you are assuming the pressure follows this curve after the rupture. It will unequivocally have a totally different pressure curve once the rupture occurs. See my first points above.



All this is simply explained by an in-battery case head that is ballooning into the unsupported area of the feed cone and also into the area of the extractor due to the smaller volume of the extractor relative to the lugs.

That's exactly what I see. Again. And goodnight for now!

Nope. If the case fails prior to pmax, you still have considerable amounts of powder that has not been converted, and now you just provided a very tiny aperture for it to continue its burn through.

The powder burn wouldn’t magically stop once the case ruptured in the imaginary event you’re describing.

It would continue to be converted in that tight space formed by the case in-battery, the bolt face, barrel extension, which is why you see in-battery failures turn AR-15s into grenade-like devices.

Remember that the projectile would still be static or in a retarded start-pressure position in this model, obstructing the bore now that a new point of relief has been found.

This is why bolts, barrel extensions, carriers, and upper receivers turn into fragments with an in-battery catastrophic failure.

The tightness of the space doesn’t allow clean, well-guided venting of the continued burning propellant.

In the OP’s demonstrated failure, there is no soot residue emanating from the failure point, which means that the propellant was already converted.

If you have ever seen in-battery failures, there is a ton of soot around the chamber and remaining parts fragments.

His brass is squeaky clean, with a very coherent failure node aligned perfectly with the extractor.

The fact he was using a 1.8oz buffer should put all of this to rest.

That is early unlocking all day long, a literal recipe for it.

[Klem]

Ninny,

The feed cone depth is headspace related. You can have correct longitudinal headspace and still have the case rupture because there is too much space around it - the case is too narrow or the chamber too wide. As shooters we more often refer to longitudinal headspace because it is easy to measure. And if horizontal headspace is OK then typically axial headspace is also.

There is still plenty of pressure in the barrel to do mischief as the carrier unlocks and the bullet is still in the barrel. Especially in a short gas system with a slow powder. Yes, the pressure drops as the bullet heads down the barrel but in this scenario it is still 39K lbs at the port, down to 16K lbs the muzzle (Ref: QL)

[Zeneffect]

Post pmax, you have increased volume which will slow the burn rate of the powder. This is why 95% burn is indicated much further, I think solid propellants would still be spewed from the case after a rupture proceeding a pmax event and is in the down slope. The squeaky clean cases.... that's something isn't it? Nobody finds that really frigging odd? I would expect more soot regardless of failure cause than is pictured.