I've locked up a 686 with a lock, shooting my monster Ruger loads! I mean the whole gun seized! Hammer wouldn't go back. Cylinder wouldn't turn. You name it.
Did the lock fail? Nope. It appeared to me at the time, the brass expanded enough to lock itself on the recoil shield. As after it cooled, you could see all the brass was scraping there. It's been so long I really don't know if that was really what was up. I still shoot the gun today. All I did was clean it afterwards. It could turn and open after it cooled a bit.
Were you able to disengage the lock therefore letting the gun fire? Did the gun seize completely and let you open the cylinder after? What did you see that made you realize the lock did fail?

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