Nice snow fall, 30*F, no wind. Shot my original Colt Navy. After a couple of rounds I realized that the hand wasn't stopping the cylinder from going CCW. Thought the hand spring had sprung (i.e. become a former spring, an ex-spring, a no-more-spring). Had one unfired chamber and unscrewed that nipple and pushed the ball out.
So, time for the annual strip and clean. And there, behold, was an obviously rather old fragment of cap deep down in the innards. It had probably been around for some time, doing the tourist tours of the interior.
Hand spring was OK, but had somehow been obstructed by that fragment.
When all was cleaned and greased, the gun was good to go again.