Have you checked the fired casings for evidence of swelling or distortion of the rear of the casing, in the area of the firing pin impact? Will a fired casing from the gun stand straight up when placed on it's base on a flat surface?

And the cylinder rotates freely/open and closes normally when fully loaded with live rounds? (not during firing, just rotation when loaded)

Sounds like the fired casings are dragging on the breechface for some reason.

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Ret. LE, FA Instr, S&W Armorer