Chances are very good that it's an unscrewing ejector rod.
DO NOT try to force it open by tapping on the cylinder with a hammer!
You can bend the ejector rod, damage the locking seat on the end of the ejector rod, or damage the locking stud.
To remedy this situation, you need...
1. A business card.
2. Several match sticks or popscicle sticks.
Things are a made a little easier because the Model 13 doesn't have a shrouded ejector rod.
Hold the revolver so that you can look at it from the side. Just above the trigger, you'll see a small stud that pushes up into the cylinder. This is the cylinder bolt. When you pull the trigger partially, you'll see that it drops down.
Take the business card, and with the trigger slightly pulled to move the cylinder bolt out of the way, and slip the business card between the frame and the cylinder. This will block the cylinder bolt and all you to "freewheel," or turn, the cylinder.
Next, take the popscicle stick or match stick and wedge it between the knurled end on the ejector rod and the barrel (you can also grasp the ejector rod with padded pliers, but I find wedging the rod to be easier).
Now, watching the ejector rod to make sure that it doesn't move, turn the cylinder counter clockwise to screw the rod back into place.
DO NOT try to force the cylinder at this point. Right now your only concern is to get the cylinder open.
Once you do that, you need to tighten the rod.
Take three, or even six, fired shells and put them in the cylinder. This will protect the extractor star and the alignment splines from being damaged when you torque on the ejector rod.
Grasp the cylinder firmly with one hand, and with padded pliers (use leather, or rubber, less slip) firmly torque the ejector rod into place.
This should take care of your problem, at least in the short term.
If it happens again, you'll need to disassemble the cylinder, thoroughly clean it, and use a little of the WEAKEST loctite, the non-permanent stuff, on the threads to keep the rod screwed into place.
