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

[DeNinny]

If the bullet slid forward and was into the lands during chambering, could that have created enough pressure to blow the casing like that?

[Klem]

If the bullet slid forward and was into the lands during chambering, could that have created enough pressure to blow the casing like that?

Extremely unlikely - bullets jammed in the lands raise the pressure/velocity, but nowhere near blowing out the base of a case. Some bench-rest shooters deliberately load their rounds long so they jam into the lands as the bolt is closed.

[LR1955]

Stone,

The split in the photo is lower than typical for case separation (too much headspace), or incipient case separation (gradual work hardening from too much repetitive stretching).

In your case the split is right down at the end where the brass is thickest, and only on one side. This suggests headspace was correct and the round was in battery. The brass expanded as normal and was supported by the chamber, but kept expanding and failed at the weakest point of support at the time - the extractor. There is no lug at the extractor. Looks like it was in battery and an over-pressure situation took the path of least resistance, out past the missing lug and back into the receiver. Then, continuing path of least resistance down the magwell.

Unsurprising the bolt is now out of spec with the No Go gauge. For brass to split at its thickest part implies a huge hammer blow to the bolt face and lugs before pressure release. The barrel extension will have copped the same thump. Once stressed, steel can become brittle and fracture, but in some situations compression from a detonation can actually make it stronger. In this case, my money would be on the event having stressed the extension. In an AR that means the barrel has to go as well. That is expensive, so up to you where you go from here. I would pull, weigh, and measure every single round from that batch.

How does the bolt look?

Klem:

I have had a couple of complete head separations with M-1A's and all that happened was the head ejected and the rest of the case stuck in the chamber. Never anything coming near to blowing out the magazine.

Yes, it is headspace separation but I contend it happened with the cartridge far enough out of battery to channel its gas downward.

Why his and my AR's blowing the magazine out and damaging parts of the bolt while aside from the PITA of removing a stuck cartridge in the M-1A's, no damage at all?

LR-55

[Klem]

Klem:

I have had a couple of complete head separations with M-1A's and all that happened was the head ejected and the rest of the case stuck in the chamber. Never anything coming near to blowing out the magazine.

Yes, it is headspace separation but I contend it happened with the cartridge far enough out of battery to channel its gas downward.

Why his and my AR's blowing the magazine out and damaging parts of the bolt while aside from the PITA of removing a stuck cartridge in the M-1A's, no damage at all?

LR-55

55,

Yes, I think you are right. It makes more sense it was not completely in battery.

It must have been close to battery however, to not simply pop the bullet out the neck (path of least resistance). Also, 29.7 grains would not cause a grenade effect. 62K is manageable in a bolt gun and even in an AR long before blowing the case up like this.

[lazyengineer]

Please post a photo of the case head. An OOB will have a lighter primer strike. I contend it is not OOB. A over-pressure will have extrusion into ejector. I contend that wont be there either - but look forward to the photo. A kaboom that is neither in Grendel, will be due to insufficient brass to contain the normal pressure in the gap between body head and chamber walls, and blow right the at the seem between the case body and head.

Here is a related thread on brass issues from earlier. Is the rifle overgassed? In the thread, another poster getting the dreaded belted brass behavior in his suggested his Odin bbl had too generous chamfer. Meaning more brass exposure to hold pressure.

https://www.65grendel.com/forum/showthre...zyengineer

I continue to suspect there is a tolerance stacking issue between Lapua, some Bbls, and overgassing, resulting in blowout at the unsupported seem. Thr most experienced Grendel shooters don't seem to experience this blowout - but others apparently do.

[montana]

LR1955
(I have had a couple of complete head separations with M-1A's and all that happened was the head ejected and the rest of the case stuck in the chamber. Never anything coming near to blowing out the magazine.)



Yup, I had the same thing happen with an HK 91..I didn't know it even happened until the next round failed to feed into the chamber..

[DeNinny]

I'm on board with lazyengineer here. Seems like the round was in battery but the bolt opened before all the pressure dissipated. This pressure caused the casing failure at the head. OP stated that other rounds showed the "ring" phenomena at the head. So it was happening regularly.

[stonehog]

This was new unfired Lapua brass. It was sized (neck only - no shoulder set back as it was already well back). My rifle's fired cartridges measured with the B350 Hornady HS gauge come out around 1.221". I just checked a few of the unfired cases, and they vary quite a bit from 1.2045 to 1.220 - everywhere from 17 thousandths to 1 undersize. Hmmm... I am learning. At this point - I will have to treat all new brass as "need to pre-fire with reduced load to blow out shoulder to spec"... This has not been a part of my general practice. As far as bullet length - I measured the case to lands max with the Hornady COL gauge at 2.310 and CBTO at 1.736, so I don't think this was a bullet getting stuck in the land as my COL on these were 2.250 and CBTO at 1.678.

[stonehog]

The bolt looked fine. The lugs looked normal, too. When I put a new bolt in, it passed headspace - wouldn't seat on the no-go gauge. I ordered a bullet puller today, so will do that on the remaining rounds. How many folks load full up loads on new brass? Isn't that what all manufacturers do? I may be getting too careful if I start pre-firing all new brass, I fear. Better than blowing my face off, but...

To be fair - the rifle handled this well - as far as I can tell it vented gas in a safe direction down through the mag, and minimized impact to the receivers and BCG. That said - I'll test fire it a bit with other "safer loads" before I decide anything on the barrel. This is my nicest grendel as far as accuracy and reliability so far...

[DeNinny]

The bolt looked fine. The lugs looked normal, too. When I put a new bolt in, it passed headspace - wouldn't seat on the no-go gauge. I ordered a bullet puller today, so will do that on the remaining rounds. How many folks load full up loads on new brass? Isn't that what all manufacturers do? I may be getting too careful if I start pre-firing all new brass, I fear. Better than blowing my face off, but...

To be fair - the rifle handled this well - as far as I can tell it vented gas in a safe direction down through the mag, and minimized impact to the receivers and BCG. That said - I'll test fire it a bit with other "safer loads" before I decide anything on the barrel. This is my nicest grendel as far as accuracy and reliability so far...

I wouldn't have thought brand new brass needs an initial safe load either. I never worked with Lapua brass but I've used new Starline brass with no issues going to a proven load. But I do full length size it because that's what Starline says to do before using it. Does Lapua have similar requirements?

Also, have you ever fired that load with used Lapua brass? Maybe it's just too hot of a load regardless of the brass age or initial conditions.

[stonehog]

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[lazyengineer]

Thank you. No extrusion evidence of gross overpressure. Solid primer strike with a deep impact, which you can only get in an AR15 with a fully rotated bolt. Brass blowout and peel back right at the seem between the web and the body. That brass failed because that brass wasn't supported by a steel chamber wall there. Meaning the chamber cut was too short (too much bevel), the brass head wasn't thick enough to cover that spot with strong enough solid brass head at that depth, or the bolt started unlocking and moving exposing that spot of the brass while still under pressure. Or a tolance stack of all 3 of those combine juuust enough, to kaboom. The correlation this seems to occur only with certain brands, is an item of note.

Section one of your lot of casings and another brand like Hornady, and take a hard look at the head, web, and transition between the two. And any different in how easy each was to cut.

You do you - I've seen this enough times now that when I pick up Lapua Grendel brass - I either scrap it, or run it as a mild fire/forget field load

[Klem]

Stone,

I have seen flatter primers from hotter loads, yet those primers look normal. Maybe a bit flat, but not seamless to the case or cratered.

The blow-out hole is right at the thickest part of the case/web. Not forward where it thins out so it's either a manufacturer fault, or it was almost in battery - supported everywhere else except the base.

I'll say it again, I've never heard (until now, from one person) of any concerns with Lapua cases. Lapua is universally believed to be the best brass available in the world for civilian competition. I have used it for years and so have my fellow competitors.

As for prepping any brass including Lapua for first firing: I throw a few into a rifle gauge to check headspace, then size them all so the bullet will be held by the same tension. I also put a slight bevel on the mouth with a hand reamer. But that is it, no baby loads to start, and nor should this be necessary - unless you have a out of spec chamber. SAAMI permits brass to be as short as 1.213" (base to datum line on shoulder), and chambers to be as long as 1.2301". That means SAAMI is comfortable with up to 0.0171" case stretch for at least one shot. No doubt there's a bit of safety margin built into that figure as well.

I would measure a few non-split spent cases with those loaded/unfired rounds to see what the stretch is in your chamber. How do they compare against SAAMI's 0.0171" max? Lapua brass ought to take this 0.0171 stretch, and a little more for safety.

[Dt219]

Is it possible to post a picture of the barrel extension and barrel ? I had a lilja that in my opinion and finally liljas opinion that was chamfered too deep and didn’t support the brass. I didn’t have a case blow out thank goodness but it sure stressed the hell out of it. They refunded the barrel

[StoneHendge]

Do we know what type of barrel yet? My blown case experience (Starline) was with a McGowan 6mm Grendel that didn't fully support the case.

[BluntForceTrauma]

Attached is some info from a Bill Alexander post on Sniper's Hide in 2020 about the 6.5 Grendel case.

[DeNinny]

Attached is some info from a Bill Alexander post on Sniper's Hide in 2020 about the 6.5 Grendel case.

Seems to me that he's acknowledging that the grendel chamber design intentionally has more wiggle room at the rear of it.

I'd be interested in the differences in the web shape and thickness between Hornady, Starline, and Lapua brass. In particular at the failure point. Maybe there is enough of a difference to make one more prone to this failure than another.

[DeNinny]

... some of the other rounds near this one had a slight line developing around the case head. I will post pics later if I can get a good close-up.

Based on all the great posts in this thread, I'm thinking your 12" just doesn't dissipate the pressure to the 5700 psi spec (mentioned in the Bill Alexander quote screenshot). And this was impacting all your brass, but it was fine at the start. And as your rifle heated up more and more, the issue became exacerbated, and as that line or ring near the head started forming, it got worse and worse with each shot. Add to this the thermal expansion of your chamber itself. Eventually...kablooie...at round #41.

Edit: Also, you might want to start at minimum load and work up to 28.5 gr. Out of an abundance of caution and such.

[BluntForceTrauma]

There is a photo out there of sectioned cases. See attached.

[DeNinny]

There is a photo out there of sectioned cases. See attached.

This is so awesome! I just joined this forum and this thread alone made it worth it! I only wish that pic had Starline in it too. Anyway, the comment is absolutely correct. All other things equal, a sharper right angle like the Lapua brass has from base to sidewall is going to be theoretically more prone to failure than the smoother curve like the Hornady brass.

I would definitely not load Lapua to max pressures based on this pic. And Hornady FTW!