Got a family heirloom, a 1939 M&P K frame in .38 special. Worn but looks good internally. SA carry up and timing is very ncie. Problem seems to be DA trigger pull binds just before the nose clears the frame. It will follow through but it takes some effort.
I watched with the plate off and suspect the trigger is binding on the edge of the cylinder stop as I can't locate any other catch point.
Anyone have suggestions on this? My son fixed a similar situation with his M1917 by stoning the lip of the cylinder stop.
But I'm not that brave!

Be careful not to cycle the action under mainspring pressure with the sideplate off.

If it is the ball (top) of the cylinder stop hanging up on the edge of the cylinder stop notch, the bevel on the cylinder stop is probably worn to the point that the stop is not camming down out of the way before the cylinder begins to turn. That would be just one possible explanation. If this is the case, installation and fitting of a new cylinder stop should solve the problem.

Diagnostics is best done in person with the gun in hand, and there are several possibilities with respect to your description of the issue......so perhaps a trip to your local, trusted gunsmith for an evaluation is in order?

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Ret. LE, FA Instr, S&W Armorer