I would second the warning in the earlier post about cycling the action under spring tension when the sideplate is off. This can result in damage.

If the front barrel cylinder gap is zero, then you probably have considerable end shake. (either on the cylinder, or on the yoke, or both)

End shake on cylinder is the condition where the cylinder can move front to back when the cylinder is closed. The movement of the cylinder forward can tie the gun up for a variety of reasons. After replacing the sideplate, (without the screws is ok) can you close the action and while holding the cylinder to the rear, try pulling the hammer back. See if it works normally while holding the cylinder to the rear.
End shake on the yoke is due to improper fitting or wear on the yoke button, at it's interface with the yoke screw. When the yoke is properly fitted to the yoke screw, and it is closed, there will be no noticeable movement of the yoke forward or aft. This check is best done with the yoke installed in the frame without the cylinder assembly, and the yoke screw fully screwed in. The yoke should rotate in and out of the frame with minimal resistance, and should have no movement front to back.

If you have a set of feeler gauges, the measurement between the breechface and the rear of the cylinder should be .008" - .010". If the cylinder assembly has developed severe endshake, there are cylinder shims available that will restore the proper gauge. (move the cylinder back)

As others have said, start with a good cleaning first, especially the cylinder assembly and the extractor, and then check the gauge on the cylinder. Excessive end shake may be the problem.

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Ret. LE, FA Instr, S&W Armorer