What causes micro, mini cracks it the mouth of a stainless cylinder? This is at the front where the bullets exit. If you didn't inspect it closely you would think it was a scratch but I can see the heat line going from the edge of the corner of the crack into the chamber. I can't say much right now. More info later.
Have you ever seen this? The forcing cone looks ok.
It's not a s&w.

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Plant garlic in the fall

Based on that sparse info, I suspect either bad heat treatment or a considerable overload.