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

[LRRPF52]

So I forgot that you had some data here regarding the feed cone specs. And it's very helpful in this analysis. But it's not perfect. So I have to make some assumptions which I will clarify along the way. Here goes...

To start, I've been wanting an approximate size of the feed cone depth because we are all mostly in agreement that this is the unsupported area of the case and the failure occurred at an unsupported area. Also remember that Bill Alexander himself stated that he had to reduce it for the grendel round. And per the note above..."the edges are radiused .0625"...I think this is a good starting approximation of the standard M16 feed cone depth which supposedly is the same for an Odinworks barrel. I'll note that it is not perfect, but I'm pretty sure the feed cone depth will be approximately close to this dimension.

Next, I want to compare this to the feed cone depth of an Alexander Arms barrel. But unfortunately, it was stated that this is proprietary information, so we will have to make an assumption here. And remember that Bill stated that this change was critical to the design because of the high pressure in the grendel round. So if it was this important, I think we could assume that he made at least a 30% improvement to reducing the size of the feed cone depth. So taking 70% of 0.0625", I am getting 0.7 x 0.0625" = 0.04375”.

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.

At this point I want to mention that the excessive head space issue is in the wrong direction with respect to exposing the brass to unsupported wall area. Excessive head space is at the shoulder end of the case and allows the round to go more into the chamber rather than out of it. As such, I'm not accounting for this in my math and moreover I don't think it is a viable failure model...but I'm open to being convinced otherwise.

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.

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. 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.)

Now as to the brass itself. It definitely plays a role here and cannot be dismissed as well. With more unsupported brass exposed, weaker brass will tend to fail more often, and stronger brass will tend to fail less. To this point, the first question I have is...would the OP have had the same failure with Hornady or another brass besides Lapua? To this issue, it is still undetermined. But it is entirely possible. I know some folks are saying no way it was due to Lapua brass, but I'm sorry that's not being very objective without numerical evidence. We have 3 threads on this forum alone that are of Lapua grendel brass failing. We have Bill Alexander saying that he designed the grendel working with Lapua brass and he outright states that he had to change the feed cone design to limit the amount of unsupported brass with it. We also have visual proof that Lapua has less overall brass around the case head. (And the only counterarguments to this are non-numerical claims..."Lapua is the gold standard of brass"..."I've used Lapua brass for years without problems"...etc.)

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.

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.

Another aspect of the inherent design soundness of 6.5 Grendel brass was how smoothly the SAAMI certification process went. Normally with SAAMI, if they find problems with your design, they will kick it back, because even among the large ammunition and firearms manufacturers, mistakes are made during the design process, schematic drawings, pressure ratings, angles, radii, wall and neck thicknesses, alloys, etc. 6.5 Grendel development was unusually stacked with very experienced designers from 2 of the most well-respected firearms and cartridge engineering cultures.

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.

And finally, we have the OP using Lapua brass with an Odin barrel which has the M16 feed cone design that Bill Alexander intentionally redesigned for the grendel round.

So with all that said, all my earlier failure models are on the table and I'm in alignment with lazyengineer that it's a tolerance stacking issue...the barrel feed cone was too long, the brass was too weak*, there was too much powder, and (possibly) the bolt face was too deep. All these combined led to the failure.

*And note that by brass being too weak, I'm open to the fact that there may have been a defect in it and/or other factors besides that it was simply Lapua brass.

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.

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.