The cylinder on my well used 15-2 can be turned clockwise (looking from the rear) when at rest. With the hammer cocked, the cylinder will not turn. It will not turn after the hammer is dropped, but before the trigger is reset. Once the trigger resets the cylinder can be turned clockwise. So worn cylinder stop, stop spring or cylinder notches?
Sounds like the cylinder stop is hanging up when the trigger returns. I'd suggest a really good internal cleaning first. If that doesn't do it, I'd suggest a new stop spring, properly installed (no kinks, in line with the hole in the cylinder stop.) first. Then a new stop.
By any chance was it recently taken apart? If so, I suspect the stop spring isn't in there correctly.
When the trigger returns the stop pops up into the notches, but with just a little pressure clockwise on the cylinder, the stop is pushed down. Pressure in the counter clockwise direction results in no movement. I suspect you are correct about the spring.
thanks