My M617-2 occasionally will not permit the trigger to travel far enough to release the hammer. It might happen once in forty or fifty times. The trigger travels most of the way back, then simply will go no further. Sometimes an stronger squeeze will make it go, sometimes not. I have a Wolff reduced-power rebound spring in the gun, and it has the original factory trigger stop rod inside the rebound spring. I am wondering whether the trigger stop rod may be involved with this. I notice that many S&W revolvers do not have this rod.
Any thoughts?

Thanks.

That trigger stop rod is orientation dependent and if you install it backwards what you have observed CAN happen. The first time that I tuned the trigger in my 610-3 I had the same issue when I put it back together and finally solved the problem by flipping the trigger stop rod end for end. It's been a few years and my memory aint the best but IIRC the end that is very slightly pointed goes against the pin in the frame for the rebound slide. I would suggest that you make sure to note the current orientation after removing your rebound slide and then look at how you had in installed. Then flip it exactly end for end and then post the results and the correct orientation here. Because I frankly don't trust my memory on this and really should write it down, which I will do when you report on the successful orientation. BTW, I can't confirm the correct orientation because after that first problem I've started "double ending" all my trigger stop rods so orientation cannot be wrong.

These stop rods are standard in the N frame and normally only seen in the smaller frames when that particular revolver has been through the Performance Center. So, it's presence is a fairly good indicator that your 617 was a Performance Center model or was sent in for a PC trigger tuning. Personally, I have a private hunch that these rods reduce buckling of the rebound spring in the bore of the rebound slide and as a result produce the most stage free triggers you can achieve. However, it's just a hunch based on how stage free the triggers on my N frames are when compared to my K and L frame revolvers that lack this rod.

Scooter, I already removed the rod before reading your post, so too late to observe the orientation. But as you said, one end is rounded and the other is flat. I did a search on this forum and found some info that indicated the stop was mostly for single action precision shooting, to limit trigger movement after breaking the shot. Apparently, many double action shooters remove it. Here is the thread:

Best way to fit a trigger stop rod?

On my gun, it did not seem to limit trigger travel to a significant degree. Perhaps because I had it installed wrong.
I doubt that the rebound spring can buckle much at all, since it is a close fit in the rebound slide hole and would have no room to buckle. Since the rod is much smaller than the ID of the spring, I don't think it could limit buckling at all (assuming that buckling were possible).
Thanks for your reply. The orientation issue may be the problem.

Postscript: After reading the above thread more carefully, I noticed that the orientation issue is addressed. The rounded end of the rod should point towards the trigger, according to the poster.

Hard to diagnose from what you've described. If you open the cylinder and pull the thumbpiece back, do you have a significantly different trigger pull? If so, the cylinder may be binding, which could mean something as simple as fouling on the cylinder face or under the extractor. You may want to also inspect the ratchets of the ejector to see if they have burrs or any other deformation that might be causing the hand to bind when it is rotating the cylinder.

If you have access to the Miculek trigger job video he goes over a number of things that could be a solution to the problem you're having.
Gun Video Trigger Job: Complete Action Tuning Smith Wesson Revolvers

HTH

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Scooter, I shot 75 rounds through the gun today after reversing the orientation of the rod. Rounded side towards trigger, flat side towards rear of gun. The problem did not reappear. Thanks for your help!
Andy