The pictures tell me one thing clearly -- this was caused by excess pressure. Look at the 12 O'Clock chamber: The brass base has been melted and welded to the chamber walls. That's caused by pressure that's far too high. And it tells us that the problem originated here, though it might have spread to the right-hand chamber.

Look at the left-hand chamber: That's caused by pressure from the center chamber leaking in and crushing the casing.

The question is: What caused the excess pressure?

There are typically three culprits: Too much powder, a bullet seated too deeply; and a bullet crimped too tightly. Each of these can cause a catastrophic failure.

What if the mark on the bullet was caused not by the frame, but by the edge of the seating die? That would mean the bullet was seated far too deeply.

tk

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NRA instructor, SIG armorer