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

[stonehog]

IMG_3759.pngIMG_3756.pngIMG_3757.pngIMG_3758.png

[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]

[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!

[BluntForceTrauma]

barrels, for example, are correctly dimensioned for this.

[DeNinny]

barrels, for example, are correctly dimensioned for this.

I would agree with that with the added provision that you stay under/at max pressure, which according to Hornady is 29.1 gr for the 123gr ELD-M.

[Klem]

I would agree with that with the added provision that you stay under/at max pressure, which according to Hornady is 29.1 gr for the 123gr ELD-M.

A grain or two either side of SAAMI probably doesn't make a difference to the OOB danger. If the base of a case is unsupported then even a half-load (25K lbs) would blow a hole in brass.

[DeNinny]

A grain or two either side of SAAMI probably doesn't make a difference to the OOB danger. If the base of a case is unsupported then even a half-load (25K lbs) would blow a hole in brass.

Not disagreeing, but if the failure is happening while still chambered, i.e. it is not OOB at the time of failure, there is still some chamber wall support as soon as the case expands to the diameter of the chamber at its widest point, giving it the necessary support to withstand failure...up until the point that the round is at max (or failure) pressure.

And also, assuming the failure is during OOB, remember the pressure is dissipating with time, so, for sure, by the time the bolt is OOB, the pressure is definitely lower than max. This might very well be 25K, but if you started with a heavier load, then it might be 26K instead of 25K at a lower starting load.

Edit: To me, the more you play around near the documented max load, you are simply increasing the risk of a kaboom relative to working with loads below max. It's a spectrum of risk.

[lazyengineer]

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

And there it is. Look at that Lapua on the left, look at the Hornady on the right. Then go look at OPs kaboom. THen go back and look at that Lapua case on the left again. Lapua = KaBoom; Hornady doesn't.