We teach the kids in the hunters safety course "Safety, a Mechanical Device That's Been Know to Fail". So, in my opinion, none. Your reference to the hammer over an empty chamber is in regards to the old Colt single action Army. That's because it had no safety at all, therefore, when the hammer was at rest and the chamber beneath it was loaded, a sharp rap on the hammer would set the loaded round off. Modern double action revolvers are probably the safest of the bunch today, with their hammer block safety mechanisms.