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

[LRRPF52]

I disagree because it could have easily dissipated. Remember that the pressure was also dissipating at the bullet end. And again, as soon as the blowout started. Pressure is going to drop like a rock.


Or, there was enough pressure to blow the case wall. And it started relieving there and also through it's normal path. With two escape routes, the mouth and the blowout, the pressure dissipated VERY quickly.


Earlier case failures (2) are supporting the failure model because their deformation shape and location of score line are indicative of the unsupported head area expanding to match that of the feed cone shape (to a degree). Unfortunately the third failure, the blowout one, expanded even further, to the point of blowing out.

Again, all the pictures are supporting this. To a "T". And the intact extractor is irrelevant here. It played no role in the model I'm explaining. And with the two cases NOT blowing out, of course it would stay intact!


I have seen them. This failure is just one that happened at a lower pressure than those others.

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

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

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

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:



The existing structures outside of the barrel shank, tennon threaded into the extension, extension, and bolt are not designed to handle escaping gases of a 47,000psi or greater peak event.

They will direct the explosion away from the shooter’s face though, which is one of the nice features of the AR-15, as opposed to several other designs.

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

[LRRPF52]

Again, the chamber wall is supporting the other theoretical end, so there is no blown out brass there. That's why there is a clean tear there that matches the consistent score line position. This matches theory. The blowout and brass damage is only occurring at the unsupported area which is at the feed cone. This is what I see in the pics.

There’s nothing theoretical about the brass being supported in that location. It either was or it wasn’t.

If the feed cone was the factor, then we should see circumferential and relatively-even failure with a total case head separation.

Instead, we just happen to see it fail aligned perfectly with axial extractor force.

We also don’t see evidence of catastrophic failure of the surrounding pressure containment structures and the non-pressure-bearing structures that hold them.

I would suggest more study of AR-15 catastrophic malfunctions, brass yield strength, and what those look like.

[DeNinny]

Yes, where the feed cone transitions to the supportive chamber walls is the area I'm thinking of. Rather than a hard angle, it should have a radius. If failure occurred there in battery, there should be evidence of the transitional radius on the upper half of the brass that was supported

I get this and understand your point. Two things:

1) We are not looking at the zoomed in level of the failure. At that scale, there might be more "jankiness" like you are suggesting, which indicates a less clean break over a more rounded surface. From the bird's eye view, the hard angle looks like a sharp edge. Keep zooming in and it will start looking like a radiused "hump".

2) We do not yet know how radiused the OP's barrel was inside. Was the feed cone transition to chamber wall a sharp turn, or was it a smooth curve, or something in between?

[Klem]

Guys,

We have a couple of posters deciding they know for sure.

1. Suggesting it is "text book" adds faux credibility to an assumption.
   
3. Lapua does not have a 'tendency' to do this when it kabooms. Because it does not have a tendency to kaboom.
   
5. Quickload is not "useless" and works for me in this calibre. The pressures/velocities may be off sometimes, but close enough to be useful and no reason to throw the baby out with the bathwater - it is a computer simulation with a lot of nuanced utility. The opportunity cost are manufacturer load tables, the books, and this forum. I don't trust any source and prefer to weigh them all up (I don't have the books).
   

Seeing we are going there...I am leaning towards a closed-breech overpressure situation from too much headspace. Too much headspace can be either from a breech that is too fat, and/or cases that have been sized too small.

The fact that only three cases have the ballooned effect on the case body suggests that there was a range of headspace going into that breech. The OP also confirmed a range of headspaces, which suggests a Normal distribution bell-shaped curve. One tail of that distribution curve will have a few cases with generous headspace. This marries with only three cases ballooning. And one had so much headspace that it exploded in the least supported spot at the time. The weakest spot in battery is where there is no lug; where the extractor is.

If it were early unlocking I imagine the extractor claw would have sheared off rather than have the strength to rip that case open. But I am not completely sure here...maybe it is the extractor claw from Hell.

That said, the early unlocking theory also sounds possible.; Slow powder/short gas system = higher pressure on unlocking. A few cases with too much headspace are getting expanded too much and while hot and stressed are then being dragged out with residual pressure expanding them even further as they travel towards the wider breech mouth. One failed in this state.

All that said, I'm not going to lobby for any one possibility at this stage.

[Zeneffect]

I agree, still too many unknowns to say for 100% certainty however I'll stick to early unlock as the culprit based on powder, general configuration, and barrel length combine with my own perceptions of how the brass looks. I bet it was nothing but huge fireballs while shooting before any failure occurred, and that should have been an indication that things probably are sub-optimal.

[DeNinny]

There’s nothing theoretical about the brass being supported in that location. It either was or it wasn’t.

The design of the feed cone and chamber is both theoretical and real. The brass is supported in the chamber area and it is unsupported in the feed cone area. Reality will ALWAYS match theory. If it doesn't, you have one of them wrong.

If the feed cone was the factor, then we should see circumferential and relatively-even failure with a total case head separation.

See my earlier explanation of how the extractor area is literally more open than the other areas with lugs. It literally takes up less space than the lugs. To repeat, that area allowed the brass to expand the most for the blowout round. As such, it failed there first. I would never assume a relatively even failure when the extractor is physically different than the lugs and the metal is stretching to the point of touching them.

Instead, we just happen to see it fail aligned perfectly with axial extractor force.

This is completely explainable by my repeated explanation of it. It's aligning to the location of the extractor. And the blowout location flap is aligning perfectly with the feed cone score line.

We also don’t see evidence of catastrophic failure of the surrounding pressure containment structures and the non-pressure-bearing structures that hold them.

I would suggest more study of AR-15 catastrophic malfunctions, brass yield strength, and what those look like.

We don't see any more failure because the pressure dissipated before it did damage to anything else (besides blowing out the OPs magazine).

The catastrophe occurred at the blowout and it didn't need to be any more catastrophic than this, other than minor damage to the mag and such.

I have studied all of those things and know what they look like. Hence, all my points throughout this thread.

[Zeneffect]

Playing devils advocate here... if it were an in battery blowout, would the resulting minimal damage be attributed to the short barrel length providing a quicker drop in pressure as bullet exits muzzle in a relatively quicker time?

Conversely, I'd we extrapolate this failure to a 20 inch barrel, would we expect to see more damage due to slower pressure drop off from the barrel length?

[LRRPF52]

That is a hot, compressed load.

QL is predicting your load is 64K lbs (Grendel SAAMI is 52K lbs). 106% case full is crunching powder when you seat the bullet. Pressures increase exponentially when powders are being compressed, and also less predictable as you change the burn characteristics. 2520 is a bulky, slow powder for Grendel out of a short barrel. The case becomes full before the powder becomes compressed.

I'd be loading no more than 27.8 which is exactly 100% full and right on the SAAMI pressure limit for 2520.

What does the case look like?

QL told you that this load would generate 64,000psi. That’s 12,000-14,000psi more than all of the available actual pressure test data from calibrated 6.5 Grendel test breeches.

Do you at least understand why I say QuickLoad isn’t a valid tool here?

I’m literally looking at all of the available pressure test data with multiple 123gr bullets, most of which have more case intrusion than the 123gr ELD-M, similar jacket thicknesses and hardnesses.



If you’re comfortable with a program that kicks out those kinds of numbers, it really defies the general attention-to-detail I’ve come to expect from your posts.

I don’t know what the issue is with fixation with QL. It’s just wrong with the numbers it generates for 6.5 Grendel, by huge margins.

This is the reasoning behind why I call out QL the way I do. It isn’t a personal slight on anyone if they happen to be involved with it professionally.

[DeNinny]

Guys,

We have a couple of posters deciding they know for sure.

For the record, I'm simply at high confidence over my failure model(s). High confidence, but not for sure. And it's because the more pictures that are shown and discussed, the more my failure model(s) are supported by it all.

If I'm ever going to post something 100% for sure, it's going to be a law of science or engineering and I have a book to back it up. But that's it. This failure simply doesn't have enough data and it never will have enough data to outright prove 100%.

Seeing we are going there...I am leaning towards a closed-breech overpressure situation

Just want to point out there is some commonality on all our positions here. I agree with this part of your model 100%.

[Zeneffect]

A mechanical engineer, a software engineer, and a materials engineer join a forum. Someone says "what's up?"

13 pages of discussion to define what up actually is commences.


Awesome.

[DeNinny]

Playing devils advocate here... if it were an in battery blowout, would the resulting minimal damage be attributed to the short barrel length providing a quicker drop in pressure as bullet exits muzzle in a relatively quicker time?

Conversely, I'd we extrapolate this failure to a 20 inch barrel, would we expect to see more damage due to slower pressure drop off from the barrel length?

All other things equal, I see this as entirely plausible (Edit: no comment on the damage part, though). To clarify, though, I see it as pressure drop off from the gas port distance as opposed to the barrel length. But I nitpick because it's basically the same thing.

The gas port is a point of depressurization. Prior to reaching it, there is higher pressure. Upon reaching it, pressure starts to drop more than if it would without a gas port.

So absolutely the sooner the bullet reaches the gas port, the sooner the pressure starts dropping. Shorter gas port, earlier pressure relief. (All other things equal.)

[DeNinny]

A mechanical engineer, a software engineer, and a materials engineer join a forum. Someone says "what's up?"

13 pages of discussion to define what up actually is commences.


Awesome.

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.

[LRRPF52]

Playing devils advocate here... if it were an in battery blowout, would the resulting minimal damage be attributed to the short barrel length providing a quicker drop in pressure as bullet exits muzzle in a relatively quicker time?

Conversely, I'd we extrapolate this failure to a 20 inch barrel, would we expect to see more damage due to slower pressure drop off from the barrel length?

No. Chamber pressure doesn’t know whether it’s hitting peak pressure in a 10.5” or 28” barrel.

Curve ball: Some will point to gas port location, which is one factor I’m pointing to in this failure, but my perspective is that gas port location led to early extraction while the case was obturated/trying to stay sealed to the chamber walls. Gas port isn’t enough to lower pressures significantly though, as plug dwell time is still present.

Muzzle velocity differences between gas shut-off and gas open are minuscule, sometimes in single digits. The near-miraculous energy imparted on the projectile takes place very quickly due to the efficient converting of the propellant mass inside the chamber. The resultant expanding gas is what propels the projectile down the barrel. Slower-burning ball powders have been shown to have significantly higher port pressure though, as well as higher muzzle velocity. This is one of the main early lessons that the AR-15 experienced with the change from stick to ball powder in the early batches of M193 from 1959-1965.

What was one of the main problems encountered? Excess cyclic rate, partial and total case head separations, frequent malfunctions in rifles with tight and corroded chambers, using the lightweight Edgewater spring guide for the buffer. Adding the heavy rifle buffer and chroming the chambers were 2 of the main things that fixed the problem, along with maintaining better consistency on reamers. The 1963 production Colt 602s were made with reamers that were allowed to run ragged, under-spec, so chambers got too small.

Brass trying to adhere to the chamber while the extractor is trying to rip it out at excess carrier velocity....torn rims, partial and full case head separations.

This is why I do what I do with 10.5”, 11.5”, 12”, etc. CLGS AR-15s. I use proven methods and components to slow down the cyclic rate, number one being use barrels with gas ports cut within spec (smaller).

Next is to use adjustable gas carriers that choke the gas before it can flow unrestrained into the expansion chamber.

If I didn’t do these things, I would expect to see brass being stretched and case head separations from too fast carrier velocity.

I also remove the O-rings from any bolts that come my way.

Those were a band-aid from Crane to fix excess carrier velocity problems with 14.5” suppressed M4A1s and suppressed 10.5” CQBRs, especially the initial batch of SOCOM barrel profile M4A1 “upgrades” that had tight chambers out-of-spec.

[Zeneffect]

I was thinking failure occurred after peak pressure regardless of the scenario, which is where barrel length would dictate available (diminishing) pressure before equalizing to atmosphere post projectile exit. Gas port is relatively small in comparison, like having a nail in your tire. Slow leak so there is some pressure drop but if I drill a 3" hole all of a sudden pressure drops much faster.

[lazyengineer]

[doublepost somehow]

[lazyengineer]

A mechanical engineer, a software engineer, and a materials engineer join a forum. Someone says "what's up?"

13 pages of discussion to define what up actually is commences.


Awesome.

heh - not too far off. We're well past the point of "hey, that makes sense, you have convinced me", and pretty much all just talking at each other, at this stage.

To be honest, I'm curious what the OP (Stonehog) has decided to take away from all of this, and his intended path forward. His gun/ammo/whatever Kaboomed. What's his plan for not having another? Obviously, I've been clear about my own opinion. Others have theirs. But of all the people talking; none of them have actually Kaboomed a Grendel (er.. that I know of at least). OP has. What have you concluded as your own path forward from here, to try and avoid this happening again?

[LRRPF52]

Short answer to that is gas port location absolutely has an impact on......

Cyclic rate

Cyclic rate has major impact on.....

Early extraction or correct window for extraction relative to carrier velocity and bolt unlocking.

20” RLGS is one of the most well-behaved gas system/barrel length configurations.

CLGS anything is one of the more difficult to deal with, always has been since the XM177, XM177E1 (10” guns early-mid 1960s), pretty well worked-out with the 11.5” XM177E2.

Alexander Arms early catalog of options included 10.5” 6.5 Grendels.

Now imagine a school-trained engineer from MoD, who got specific guest instruction from Eugene Stoner himself, who also was charged with repairing/depot-level overhaul on UK SOF Deimaco Colt Commando carbines.

When doing his due diligence on the short CLGS models, he builds full-auto test apparatus/guns, pressure test breech, and develops his own proof loads along the way.

You think that maybe he worked out these details? Gas port diameters, chamber specs, has to work with standard carrier weight and buffers so when people slap them on their lowers, everything works.

These are the kinds of thinking and background that went into 6.5 Grendel development. It wasn’t just 28” barrel target rifles, 24”, and 20”, but a wide range of barrel lengths and gas port locations.

This was all coordinated with Saber Defense, Les Baer, Precision Firearms, and anyone who wanted to license reamers before making their own rifles and ammo.

So there is a very professionally-developed TDP for every barrel length and gas port location combination that AA did along the way.

The short barrels and CLGS can be made to run well without any of these issues.

Port size is one of the biggest factors in getting that right, along with FSB or gas block seating on the journal.

The rest of the gun starts off with the assumption of a TDP spec, which very few AR-15s adhere to.

[Klem]

If you’re comfortable with a program that kicks out those kinds of numbers, it really defies the general attention-to-detail I’ve come to expect from your posts.

I don’t know what the issue is with fixation with QL. It’s just wrong with the numbers it generates for 6.5 Grendel, by huge margins.

This is the reasoning behind why I call out QL the way I do. It isn’t a personal slight on anyone if they happen to be involved with it professionally.

Like Lapua, QL does not have a tendency to catastrophically fail. It's not a zero-sum game of QL works/does not work, and nothing in between. If you can dial down the absolutist rhetoric then it will be closer to the truth. Some, like me use it knowing it is not exact in some predictions, but it others it is; it's been a great help.

Lets' not start on QL here or this thread will turn into a life story. For now we can agree to disagree.

[Zeneffect]

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.

[LRRPF52]

All other things equal, I see this as entirely plausible (Edit: no comment on the damage part, though). To clarify, though, I see it as pressure drop off from the gas port distance as opposed to the barrel length. But I nitpick because it's basically the same thing.

The gas port is a point of depressurization. Prior to reaching it, there is higher pressure. Upon reaching it, pressure starts to drop more than if it would without a gas port.

So absolutely the sooner the bullet reaches the gas port, the sooner the pressure starts dropping. Shorter gas port, earlier pressure relief. (All other things equal.)

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.

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.

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.