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?
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?