Something else to check is to load the cylinder with it out of the gun. If you have access to depth mics or calipers with a depth probe, measure the case position on each chamber. I have a hunch that either one of your chamber is tight enough to not allow a round to seat in fully or one recess for the cartridge rim is just a bit shallow. If so, that one round standing proud is probably dragging on the recoil shield just enough to cause your problem. BTW, the difference may only be 0.003 to 0.005 inch, so you'll have to excercise a lot of care when you measure it. I would also suggest that you use an average of 3 to five sets of measurements to minimize the error.
BTW, use a magnifying glass and check the recoil shield, and the front and rear of cylinder for any drag marks that look fresh. Fact is that marks can provide a great clue to what's going on. BTW, you can use machinists bluing (a blue dye for marking steel for scribing) to put a freach coat of "paint" on the areas in question. It dries to a film that marks up very easily and is easy to remove with the proper solvent, and also can be removed with mineral spirits with some vigerous scrubbing. BTW, I suspect that Nitro solvent would probably clean it up quite well. Personally, the brand I prefer is Dykem Steel Blue, it's quite smelly when drying but it leaves a hard film that won't rub off on your fingers. Note, in spite of it being called Steel Blue, it's just a dye, NOT a bluing compound.
