I had a similar problem today. I loaded six round into my Colt King Cobra, locked the cylinder and stepped up to fire. As I pulled the trigger, the cylinder turned slightly and then hung up. After gently opening the action, I discovered that the round on the right side of the frame had a primer that was not fully seated, and thus could not pass the gap. I am glad I did not try to pull the trigger again risking crushing the expose primer. I used to measure each pistol round, like I do rifle, but for expediency I started measuring every five. I can't believe I missed it, but loading in volume, I guess it happens.