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

[DeNinny]

6.5 Grendel isn’t a relatively high pressure round for modern cartridges. It has a much lower Maximum Average Pressure than 5.56x45, .308 Winchester, and other rifle cartridges that have been developed over the last 30 years. Most of your factory ammunition is running in the 47,000-49,500psi range, with MPSMs that can’t exceed 52,000psi. These pressures were more common from the late 1800s-1950s, for reference.

See later discussion on this.

Excessive headspace can be caused by a combination of shoulder location and incorrect (too deep) bolt face depth. The brass grows to fill the void, since cartridge brass is so soft (Alloy 260 ~70/30 Copper/Zinc). Brass is just a high pressure gasket designed to hold the primer, propellant, and projectile together for repeating firearms for easy storage, transport, and stacking in feed devices. The chamber pressure immediately swells it into the shape of the chamber and bolt, from which it rebounds some back between its original shape and the obturated shape. Different brass manufacturers have their proprietary processes that create stronger brass, or easier to mass-produce brass. Lapua brass is known around the world for decades to have the strongest construction, last for the most reloads, and have the smallest SDs in weight, volume, neck thickness, flash hole geometry, etc.

Completely understood on all this. It matches my understanding of the science. I completely understand the reputation of Lapua too.

So during many excess headspace scenarios, the vast majority of the pressure is contained well within the chamber, but as the brass finishes its rearward growth, it stretches past the point of full chamber support, and there is enough residual pressure to blow out the now-exposed portion of brass.

This is where I have issue. Based on what I already posted, the M16 style feed cone is already unsupported chamber. Moreover the feed cone itself is unsupported and this is entirely in line with Bill Alexander's statements that he had to redesign the feed cone for the grendel round and make it shallower than the standard M16 feed cone. As such, I don't see a need to bring excessive headspace into the failure equation. In my own analysis I already accounted for the total overall unsupported depth which includes a deep bolt face and the added depth of an M16 style feed cone compared to an Alexander Arms one.

All that said, if you could quantify how much excessive head space would add to the total exposed area than what is already accounted for, I could understand the magnitude of its significance more. Your science has convinced me to put it on the table again, but I can't take the other failures off the table.

Since the extractor is still intact, that steers us towards excess headspace and not an over-pressure event. When you have case failures in the AR-15, it’s very common to have extractors destroyed. (A sidebar anecdote on this is that Hk placed a pin inside the barrel extension in the extractor clock location to help prevent this from happening on the Hk416.)

Since the case has a blowout, this is unequivocally an over-pressure event. But to be clear, I mean this simply that the pressure exceeded that of the brass to physically deform it. And this could happen in multiple ways, as mentioned earlier.

I think the pressure was simply not enough to break the extractor but that doesn't necessarily have to define the failure as excess headspace. And note that I am viewing excess headspace itself as an over-pressure event based on my simple definition above. I'm simply seeing this as an over-pressure event that was to the point of rupturing the brass, but it wasn't to the point of breaking the extractor. That is all. And I don't want to bias my judgement from assuming it could only have been due to excess headspace. Again I have other failure models that can explain this besides headspace.

I don’t think we can say that Lapua brass definitely played a role in this. The only possibility there would be this particular piece of brass, not Lapua as a whole, and even that’s a stretch.

As stated, this is why I said it is a "weak brass" issue as opposed to a Lapua specific issue. But also I'm not ruling out the possibility that Lapua brass could have a design issue here when it is used in conjunction with M16 feed cone style barrels. This is based on Bill Alexander's words himself...that he redesigned the feed cone around Lapua brass.

So you could argue that it is an M16 feed cone issue rather than a Lapua brass issue, but to me, by admission of Bill Alexander himself, the issues of both are intertwined.

Lapua 6.5 Grendel brass fleet experience at this time is in the millions of samples in production-who knows how many times those have been reloaded. Bill’s initial work was one of the only smaller companies that actually did pyramid testing on his rifles not just for reliability, but for accuracy and durability. In addition to that, he did high sand/dust testing regularly, flying out to a large training center in New Mexico where the surface is covered in sharp rocks, sand, and dust. If any of you recall the Future Weapons episode on 6.5 Grendel with “Mack”, it was filmed there. That was all done with Lapua brass loaded cartridge by Alexander Arms. 6.5 Grendel wasn’t some small project with minimal testing, but an unusually-extensive and professionally-executed developmental program for a non-big-named company at the time.

So Lapua brass isn’t a new, unproven, or anecdotal report-based “it works for me” component in this equation. It was already well-established in the 2000s due to extensive design, testing, and proven performance in large production. That has only grown exponentially since then. Janne Pohjoispaa of Lapua, who was instrumental in the development of .338 Lapua Magnum and 6.5x47 Lapua (superior design compared to 6.5Creedmoor), was the lead engineer in developing the finer details of 6.5 Grendel brass, working in conjunction with Bill Alexander. These are not only formally school-trained engineers specific to firearms and ammunition, but both had many years of experience prior to this developing cartridges for military and industry requirements. Bill worked on the UK’s answer to the NATO PDW requirement with a cartridge called the .224 Boz, was trained within UK MoD how to do depot-level inspection and maintenance overhauls on AR-15s in UK service (by Eugene Stoner and other senior engineers), did .50 Beowulf, 5.45x39 in the AR-15 (.221 Genghis), and then 6.5 Grendel.
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Many of the reference rifles used in velocity testing, in conjunction with pressure test breeches, were Alexander Arms AR-15s. For example, Hornady’s sample rifles include 14.5” and 18” AA AR-15 carbines. I have personally confirmed this with Hornady’s engineers, who went and looked in their armory to verify those test rifles for the 8th, 9th, and subsequent editions of their load manuals.

I have no reason not to take your word on any of this. I'll point out though that if Bill Alexander were involved in all of this, then I'm pretty sure that he would make sure all the rifles involved had the Alexander Arms feed cone barrels. You stated that much of the testing was with Alexander Arms AR-15s, so again all this great data is based on mostly barrels that addressed the issue that Bill Alexander himself stated was an issue.

Furthermore, this is all why I think the feed cone design should have been explicitly specified. By not specifying it, then ignorant barrel manufacturers are going to run into the issue that Bill already knew!

I already addressed the hypothesis of too much propellant earlier. 29.x grains of AA2520 under a 123gr cup and core bullet is not the max load with a chamber with his measurements.

Hornady’s published loads for AA2520 are limited by the Monolithic 120gr GMX, which is longer than some of the 140gr cup and core bullets. That limits case capacity due to intrusion, and also resists being driven into the lands more than cup and core bullets. His stated load is well under max, which is likely why we didn’t see the extractor blow out, primer intact, case head looking ok other than the failure in the side of the brass.

Again, I'm simply stating the more powder and the closer to the max load, the higher the likelihood of a kaboom. In particular if the case has more unsupported brass than normal. You can't just rule it out because it's not considered "max". Other conditions might compromise the brass, such as being less supported, to take a load that is fine to a load that kabooms. I don't view things digitally and absolute here. There is a spectrum of the likelihood of a failure. More powder, more kaboom and more chance of kaboom. Less powder, less kaboom and chance of kaboom. It's that simple to me.

Everything really points to excessive headspace. No other variable is needed or indicated from what I’m seeing here, not having the components in my hands.

I'm still leaving the same failure models on the table here. I'll add excessive headspace back to it, but again I don't see any numerical quantification why it is more significant than the feed cone depth issue which again Bill Alexander deemed VERY important in the grendel round development whilst using Lapua brass in the process.