What he said.Maybe , just maybe the little foot of the loading gate spring has poped out of place. That will wonkey the loading gate all up. With the cyl. out you should be able to push it back in with a screwdriver or somesuch.
Also, you can leave the cyl. out while troubleshooting but put the pin back in all the way. That will keep the transfer bar from sticking under the firing pin preventing it from being cocked.