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

[DeNinny]

As the extractor pulled rearward with extreme force, the case wall thinned out and succumbed to the residual chamber pressure, even after primary extraction had occurred. The bolt is rotating almost immediately, so now shear forces are introduced as well, not just axial forces. Normally the rotational shearing force will contribute to a complete head separation and also break the extractor lip off of the extractor shaft.

This one all held together quite well. Normally if you have a case failure while totally in battery, the bolt gets welded or impinged to the extension by molten brass residue, and the gases will split the carrier in half.

Because primary extraction was in progress, it allowed the gases to vent out the receiver and down the magazine well, blowing out the magazine.

One of the most overlooked aspects of the AR-15 or any rotating bolt lock-up is the rotational force during an event like this, or even on guns that seem to run, but are excessively-gassed.

You can expect extractor failure with extractors getting beat up like that under tension from out-of-time actions.

It was a major problem for 14.5” M4A1s suppressed with the KAC can, as well as 10.5” guns-all of which are 5.56 NATO of course.

It led to the development of the LMT Enhanced Bolt Carrier Group, which elongated the cam helix in the carrier, and created a pre-vent gas relief in the gas path inside the carrier key/expansion chamber junction.

If you dropped that carrier into the OP’s gun, I doubt this would have ever happened.

I understand all of what you are saying. And none of it refutes the blowout failure model.

That tear or flap is much more likely to occur from the high pressure of a blowout as opposed to an early extraction which is definitely at a lower pressure by the time it happens.

[LRRPF52]

Sorry for resolution. You can look at OP's first post of the picture and you'll see the same thing there, without arrows.

I'm sorry but the flap could easily occur from the blowout itself, when the pressure is much higher than during extraction. Because of this, I think the blowout has a higher likelihood of creating that flap instead of the extractor pulling on it.

It is 100% impossible for that flap to be present while in-battery, even in a sloppy, out-of-spec chamber. Again, if you had a case failure in battery, where do the back-end gases go?

At those pressure levels, they will destroy the extractor, usually split the carrier, bow-out the upper, bow-out the lower, send fragments from the carrier and bolt in different directions, with magazine blow-out being the least of your worries.

This event was after that possibility, with enough carrier inertia to have cleared that much of the case head so the brass could fold back from the initial failure point.

Has nothing to do with headspace or case web support by that point because it’s partially extracted.

Lapua brass likely saved his firearm from further damage.

[LRRPF52]

I understand all of what you are saying. And none of it refutes the blowout failure model.

That tear or flap is much more likely to occur from the high pressure of a blowout as opposed to an early extraction which is definitely at a lower pressure by the time it happens.

I think it absolutely refutes the case failure model at peak pressure. That’s the whole point of what I’m saying.

Our first indicator of this was the intact extractor, but all the jumping to conclusions about Lapua brass derailed the thread.

[DeNinny]

It is 100% impossible for that flap to be present while in-battery, even in a sloppy, out-of-spec chamber. Again, if you had a case failure in battery, where do the back-end gases go?

That flap occurred within the unsupported area. Within the feed cone itself. And it 100% has the room for it, according to my measurements. The feed cone is wide open space for it. And while in battery, it looks like the other half of the flap bent right into the bolt itself and right over the extractor.

At those pressure levels, they will destroy the extractor, usually split the carrier, bow-out the upper, bow-out the lower, send fragments from the carrier and bolt in different directions, with magazine blow-out being the least of your worries.

Sorry, but the pressure quickly dissipated as the bullet got farther away from the case mouth. And it dissipated at the blowout. As soon as the blowout started, all the pressure started releasing.

And as OP stated, the pressure at that end ended up blowing out his magazine.

I'm very sorry but the more we discuss this, the more likely I see the blowout model occurring instead of the extraction model.

[DeNinny]

I think it absolutely refutes the case failure model at peak pressure. That’s the whole point of what I’m saying.

Our first indicator of this was the intact extractor, but all the jumping to conclusions about Lapua brass derailed the thread.

Based on your logic, my arguments absolutely refute the extraction model then. Again, all the pictures are supporting exactly what I'm explaining, and you think the same thing for your explanations. We are going in a circle now.

As stated, let's just agree to disagree.

[LRRPF52]

That flap occurred within the unsupported area. Within the feed cone itself. And it 100% has the room for it, according to my measurements. The feed cone is wide open space for it. And while in battery, it looks like the other half of the flap bent right into the bolt itself and right over the extractor.


Sorry, but the pressure quickly dissipated as the bullet got farther away from the case mouth. And it dissipated at the blowout. As soon as the blowout started, all the pressure started releasing.

And as OP stated, the pressure at that end ended up blowing out his magazine.

I'm very sorry but the more we discuss this, the more likely I see the blowout model occurring instead of the extraction model.

At those pressure levels, far more damage would have happened.

There had to be some pressure relieved, but enough residual pressure to allow gas escaping from the case to participate in the partial case head separation and flow around the bolt.

Earlier case failure isn’t supported by the way everything is presenting, the intact extractor being the biggest piece of evidence.

If you have seen in-battery/over-pressure catastrophic failures, you would see the distinction.

[lazyengineer]

That flap occurred within the unsupported area. Within the feed cone itself. And it 100% has the room for it, according to my measurements. The feed cone is wide open space for it. And while in battery, it looks like the other half of the flap bent right into the bolt itself and right over the extractor.


Sorry, but the pressure quickly dissipated as the bullet got farther away from the case mouth. And it dissipated at the blowout. As soon as the blowout started, all the pressure started releasing.

And as OP stated, the pressure at that end ended up blowing out his magazine.

I'm very sorry but the more we discuss this, the more likely I see the blowout model occurring instead of the extraction model.

Pretty much how I see it on all points made today. Unfortunately, this has become a case of just having to agree to disagree I guess.

[lazyengineer]

At those pressure levels, far more damage would have happened.

There had to be some pressure relieved, but enough residual pressure to allow gas escaping from the case to participate in the partial case head separation and flow around the bolt.

Earlier case failure isn’t supported by the way everything is presenting, the intact extractor being the biggest piece of evidence.

If you have seen in-battery/over-pressure catastrophic failures, you would see the distinction.

I don't think this was an over-pressure catastrophic failure. I think it was an in-pressure brass failure. Extractor survived because bolt was locked and it couldn't move much, and the relatively lower pressure than most kaboom's vented into the star-chamber and blew out between the opening between the lug-clearance passage, and then down the mag-well. Notice how Lapua Kabooms don't result in much rifle damage; where'as most AR Kaboom's are due to massive overpressure events like barrel obstruction or powder error, and tend to grenade, split the BCG, etc. Lapua Kaboom's, tend to not, because they're in-pressure case-failures that vent the lower pressure, and split much lower in the case body in a manner inconsistent with most other kabooms, because they can fail while still locked (in some BBL's).

[Zeneffect]

If unsupported area is radiused into supported via feed cone, wouldn't we see some evidence of the brass following the radius rather than be straight to a fissure with only the lower half exhibiting extrusion? I can see a tear causing such a clean slice, but an in battery blowout should be a bit jankier on both sides of the failure point.

It's an interesting thought.... both sides of this, where the only difference really is 1 microsecond of time.

[LRRPF52]

[ATTACH=CONFIG]19151[/ATTACH]

Ok so I was able to enlarge the photo of the first failed case until it was 37.5" across my monitor. then, per my blue arrows, I measured the unsupported head area (or feed cone depth) at approximately 2.25". Scaling this down to the actual casing size I get an estimate of 0.0915" for the unsupported head area. Here is my math:

(2.25"/37.5") x 1.5245" = 0.0915"

Based on the failure score line location, this is my estimate of both the feed cone depth and the unsupported case wall depth. This is fairly close, bigger actually, than what I estimated from Bill Alexander's statements about the M16 style feed cone design.

So now taking 30% of this, I get .3 x 0.0915 = 0.027". <- This is the estimated additional feed cone depth for a standard M16 feed cone from an Alexander Arms feed cone, assuming that at least 30% improvement was achieved when Bill Alexander redesigned it.

This still puts this failure model as more significant than the bolt face depth out of spec model which was estimated at 0.005". Instead of 4x difference from my earlier estimation, it is now 5x different.

As such, the feed cone depth issue is still a viable failure model. This shouldn't be a surprise since Bill Alexander acknowledged it was an issue to begin with.

None of that approach is applicable though, since you don’t know how far the case was extracted already when the partial head separation occurred.

The side wall growth with that line is indicative of residual pressure on the case walls with a case that is already being extracted, not one that’s in battery.

This is again a textbook sign of early extraction, bad timing of the system.

[DeNinny]

At those pressure levels, far more damage would have happened.

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.

There had to be some pressure relieved, but enough residual pressure to allow gas escaping from the case to participate in the partial case head separation and flow around the bolt.

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 failure isn’t supported by the way everything is presenting, the intact extractor being the biggest piece of evidence.

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!

If you have seen in-battery/over-pressure catastrophic failures, you would see the distinction.

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

[DeNinny]

Pretty much how I see it on all points made today. Unfortunately, this has become a case of just having to agree to disagree I guess.

Thank you and I wholeheartedly agree.

[DeNinny]

None of that approach is applicable though, since you don’t know how far the case was extracted already when the partial head separation occurred.

It's applicable because my model is an in battery blowout. I never said the case was being extracted because it doesn't need to be. Not even partially. It already has unsupported sidewall without having to move! And this is something Bill Alexander himself acknowledged and addressed in his grendel barrel design. He shrank the overall size of the feed cone so there's more supported wall as compared to the M16 feed cone.

We are literally staring at a failure that Bill Alexander himself designed around. Because he knew it was an issue.

The side wall growth with that line is indicative of residual pressure on the case walls with a case that is already being extracted, not one that’s in battery.

I disagree. It's exactly indicative of the point where the feed cone starts inside the chamber. While in battery, the brass is literally bending and forming the score line where the chamber wall ends and the feed cone starts (at that end).

This is again a textbook sign of early extraction, bad timing of the system.

This is again a textbook example of a case head expanding into the unsupported wall area of the barrel which is the feed cone. Two rounds expanded and deformed without blowing out. The third one did.

[DeNinny]

Notice how Lapua Kabooms don't result in much rifle damage; where'as most AR Kaboom's are due to massive overpressure events like barrel obstruction or powder error, and tend to grenade, split the BCG, etc. Lapua Kaboom's, tend to not, because they're in-pressure case-failures that vent the lower pressure, and split much lower in the case body in a manner inconsistent with most other kabooms, because they can fail while still locked (in some BBL's).

To me this is indicative of a more brittle (albeit harder) alloy. It takes less pressure to create the kaboom, especially if there is a weak point. With a more elastic and malleable alloy at the same pressure, the case head expands like a balloon but it doesn't burst before the bullet leaves the mouth. So in order to get this type of alloy to burst, it takes MORE pressure. Relatively speaking.

*Disclaimer Alert* All of what I said is metallurgy theory only.

[LRRPF52]

If unsupported area is radiused into supported via feed cone, wouldn't we see some evidence of the brass following the radius rather than be straight to a fissure with only the lower half exhibiting extrusion? I can see a tear causing such a clean slice, but an in battery blowout should be a bit jankier on both sides of the failure point.

It's an interesting thought.... both sides of this, where the only difference really is 1 microsecond of time.

This is the main fact that showed up with subsequent pictures today.

What appears to be the point of the failure node just coincidentally aligns with axial extractor force.

The brass wall ahead of anything related to the extractor position just knew to initiate its failure there, then split the case outwards circumferentially in both directions away from that point radially.

How did it know to initiate the failure node there, and why didn’t it evenly separate the case head?

There was obviously enough internal force to exert gas pressure through the failure node, because it blew out and bowed-out the magazine, blew the floor plate, follower, and spring out.

This is why I’m suspecting early extraction caused by a combo of CLGS, AA2520 hitting the gas port with sufficient port pressure and volume.

An in-battery case failure is bad ju-ju for the action, normally accompanied by damaged-beyond-repair critical pressure containment and receiver parts.

If the action was built with exotic alloys and held together, we should then see the case remnants taking the brunt of the force, namely:

* Case head extrusion over the extractor shelf with mangled presentation (We see the opposite, with otherwise normal case head appearance.)
* Case extrusion into the ejector channel (None appears to be present on the case in question, while I see some very slight ejector channel shadowing on one of the other pieces of brass that had no issue, proving that photographic resolution provided has the ability to show that.)
* Fragments of brass distributed on the bolt face, and inside the extension.
* Primer is often blown or cratered substantially.
* Bolt face receives pitting from propellant blast circumferentially around the firing pin aperture.

A normal action will usually see the carrier and upper receiver split apart or at least blow-out, since high pressure was directed into non pressure-containment parts.

47,000psi is still 23.5 tons of peak pressure.

A peak pressure failure event is not indicated by these pieces of evidence posted by the OP so far.

[DeNinny]

If unsupported area is radiused into supported via feed cone, wouldn't we see some evidence of the brass following the radius rather than be straight to a fissure with only the lower half exhibiting extrusion? I can see a tear causing such a clean slice, but an in battery blowout should be a bit jankier on both sides of the failure point.

It's an interesting thought.... both sides of this, where the only difference really is 1 microsecond of time.

Remember that the other half is supported by the chamber wall, so only half becomes "janky". The half that is unsupported which again is under the feed cone area.

[Zeneffect]

52, I think we are on the same page. I was only raising the question as a blowout would deformation on both sides of the failure point. It should radius outwards slightly like a bell lip where this doesn't seem evident. The only place where we deviate in thought is initial data analysis and what constitutes as valid theoretical data for this scenario.

The feed cone is hard lined to the supporting chamber with no radius? In battery failure would follow that radius and would produce the bell lip would it not?

[DeNinny]

A peak pressure failure event is not indicated by these pieces of evidence posted by the OP so far.

It is entirely indicative because again, the pressure was dissipating at both ends...at the blowout and at the case mouth. Also the overall size of the main blowout was large enough to dissipate a lot of pressure at once.

Also it didn't necessarily have to be an outright peak pressure. If the metal had a weak point it could have failed earlier than said "peak" pressure.

[DeNinny]

52, I think we are on the same page. I was only raising the question as a blowout would deformation on both sides of the failure point. It should radius outwards slightly like a bell lip where this doesn't seem evident. The only place where we deviate in thought is initial data analysis and what constitutes as valid theoretical data for this scenerio.

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.

[Zeneffect]

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. The top half should have a slight flare to it where it failed, not a straight shear.