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

[DeNinny]

Extraction can’t physically transpire until the projectile has passed the gas port, otherwise there is no way for gas to flow back into the Stoner expansion system and cause carrier inertia.

No carrier inertia, no bolt movement other than thrust.

Extraction caused this case to fail. I know with absolute certainty the extractor ripped the case open because of the extractor shelf mark seen on the case head, clocked perfectly with the case shear.

This means the front of the case was happily stuck to the chamber wall still, under significant residual pressure.

The slower-burning AA2520 propellant hit the port, and the port is probably cut too large.

This allowed too much gas to flow into the expansion chamber too early in the cycle, causing the bolt to rotate and unlock, while the extractor held tightly to the case rim for dear life (O-ring installed, otherwise it would likely have just slipped off and caused a FTExtract).

The power of the excess gas was so significant, it drove carrier inertia so quickly that it had sufficient force to rip the rim off partially.

We wasted about 8 pages talking about brass strength and case webs basically, when the answer was right in front of us, had we been able to see the case head like that.

We are aligned on the extraction happening after the bullet passes the gas port. That's what I mean by well after the bullet has left the mouth.

Also I understand your model, but the other failure model is also supported by this evidence that I posted earlier...

The case head expanded into the extractor and the blowout occurred while still in battery and it started at either the sharp edge of the extractor or it started at the score line where the unsupported wall starts.

Also remember...by the time the extractor is actually extracting, all the pressure is already released from the casing. The brass is no longer subject to the severe pressures as it was when the bullet was just leaving the mouth.

Also...note that there is zero extractor damage to the other two failed casings. To me these two earlier failures (but not blowouts) are an indication of a pressure issue at the head. The pressure is pushing the limits of the brass and there were two signs of this before the blowout.

I'm sorry but there are holes in the extraction failure model, and there are other models that still support the failure as shown in the picture.