I had a few that did this, the cause was always a bent extractor rod. On the one, an old M&P, the trigger pull was different in DA for each chamber, and the rod was bent and was tying up the cylinder. This also puts more stress on the hand and will, over time , increase wear on the ratchet and hand.
I can't for the life of me figure out how an otherwise minty revolver ends up with a bent extractor rod, other than some nimrod doing the "Hollywood flip", trying to open the cylinder with the thumb latch not all the way back, a rod unscrewing during shooting, tying the gun up and someone forcing the cylinder open, or firing some over-spec nuclear heat handloads.
I even have a Ruger Service Six that came to me with a bent extractor rod, I just tapped it straight with a brass hammer and it works fine. With a Ruger Six a bent rod is less crippling than it is with a S&W, it does not tend to affect timing, but it will in a S&W if it's bent bad enough.
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