Weak Point In A Ruger Single Action?

This may be it. The little round rod you see is the Hammer Plunger. It has a spring behind it in the hammer, and a relieved area on the side so it remains captured by the little pin you see in the hammer. This plunger pushes forward on the backside of the Cylinder Latch when you draw the hammer back, which lowers the front end of the latch, allowing the cylinder to turn. The Plunger is pushed upward into the hammer as the hammer drops, and then it pops back to the limit of the relieved area against the little pin when it is at rest.  The plunger broke on a Single Six I had in the mid-1970's, and I saw this repeated on a friend's Single Six a year or two later. It was easy to replace by using a drill bit shank cut to length, and then filing the side where the pin rides.

If this breaks in your revolver you cannot operate the action normally. The cylinder latch is not moved when you try to cock the gun, and it is jammed up as the hand tries to rotate the cylinder. I think you could make the gun work by opening the gate, rotating the cylinder so it is unlatched, closing the gate, and then cocking the hammer. The important thing for you to do if this happens is to turn the gun upside down and catch the bottom of the Hammer Plunger. You will need it when you make your new piece so you can make the length right. I haven't had this problem again for over thirty-five years, but I sure have not forgotten it. A prudent person would probably order a few of these plungers just in case one is needed.