The difference between single and double action is that single
action has the hammer back much further than double action. The
question is - where is the binding coming from ?
I doubt its related to the cylinder, since it seems to work fine in
double action. One test would be to open the cylinder, and then
single action the gun repeatedly, to see if the problem reoccurs.
You need two hands - the second hand has to hold the cylinder
release thumbpiece in its rearward position. If it binds in this test,
perhaps there is something in the lockword that the hammer is
rubbing against, towards the rear of its single-action travel.
I suppose its also possible that there is something on either the
toe of the hammer, or the rear part of the trigger - dirt or a bit of
metal, or whatever. During single-action pull, this could be causing
the trigger to retreat too far, and that would feel like binding.
Either way, I would remove the side plate, and at least clean the
lockwork with copious amounts of borecleaner. That will wash out
lots of stuff.
Mike Priwer