After the internals have been cleaned and you have verified the integrity of the cylinder stop spring, if the cylinder stop comes up to early, either the bevel and point of the stop is worn, or improperly fitted.....or the hook on the trigger is worn/altered.

If the tip of the bevel on the cylinder stop is too short, as it interfaces with the hook on the front of the trigger (as the trigger is pulled back), the stop returns up too quickly, and the ball of the stop re-enters the stop slot before it can rotate out of the way. It can also demonstrate this stoppage when the stop does not get removed from the cylinder stop slot in time....or is "late". This can also be due to a worn tip on the bevel of the stop, or wear or alterations to the trigger hook.

The "simple" fix is to fit a new stop. You can let out the existing stop by moving it back slightly (toward the trigger) on the stem, but the fix will be a temporary one at best. Fitting a new stop is not as simple as it seems, as the ball of the stop has to verified to fit, or modified to fit the smallest cylinder stop slot, (there are minor differences in width of the cyl stop slots) and then the bevel on the stop itself has to be fit to the trigger hook so that the stop moves up and down, and back and forth, with the proper timing. If the hook on the trigger has been damaged or altered, a new trigger may be required.

I have also seen this happen when there was a missing extractor alignment pin (or pins) under the the extractor. Does it also do this when loaded with empty casings? Severe end shake on the cylinder can also be a potential contributor, as can damage to the hand or ratchets.

Since there are potentially multiple issues involved, I would suggest taking it to a local, trained and trusted gunsmith familiar with S&W revolvers. S&W Customer Service would also be an option......1-800-331-0852

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Ret. LE, FA Instr, S&W Armorer