I had a 629-6 a while back that did exactly the same thing, but in my case it was the forcing cone shaving off a chunk of the copper jacket and wedging it in-between the cylinder face and the forcing cone surface. It would freeze the action up completely (cylinder wouldn't turn, so nothing else moved either!). Once I realized the problem a gentle rap with a nylon hammer on the side of the cylinder would open it....just long enough for me to take pictures of it and box the sucker up for a trip back to the factory (took 'em two tries to get it fixed).

--Neill