The "empty chamber under the hammer" is a carry-over from the old single action revolvers, whose actions allowed the firing pin to rest on the primer when the hammer was down. Almost any blow to the back of the hammer (a falling stirrup, when saddling a horse) would cause a discharge. Ruger re-designed their single actions because of this. Modern double actions have either a transfer bar (Ruger) or a hammer block (S&W) which prevent hammer/firing pin contact with the primer until the trigger is pulled. To discharge the guns with these features would need a blow sufficient to collapse the frame. Strangely enough, the "empty chamber" thing has persisted. A friend in NC, back in the late 70s, took a job as a corrections officer at a state prison. They were advised that officers carrying revolvers were required to keep an empty chamber under the hammer, for safety. They were issued M19s - go figure.