Snake45 said:
The "passive safety" on S&Ws is the ramped interface between the top of the rebound slide and the bottom of the hammer. The hammer's not going to hit the firing pin unless the trigger is held back. They also have a sliding safety thingie rattling around in there which is completely redundant. "Back in the day" we used to take those things out, but Mas Ayoob convinced me that removing or altering any "safety device" on a gun that might be used defensively is a bad idea, no matter how silly or useless the thing is.
Snake,
Mas Ayoob should know better.
Actually the rebound slide/hammer foot safety only kicks the hammer/firing back off of the primer. It alone has failed many times because a dropped revolver will often shear the hammer pivot pin and the gun will fire.
That's the reason S&W designed three different style hammer block safeties to work in conjunction with the rebound slide/hammer foot safety, beginning in 1915. The second style was prone to sticking in a dirty gun. Therefore only the rebound slide was left functioning in a 38 M&P Victory Model resulting in the death of a sailor in WW II when the gun dropped and the hammer pin sheared. The final solution is the post war sliding bar hammer block you describe.
Neither the rebound slide or sliding bar are foolproof by themselves, only when they work together. The sliding bar will not slip between the hammer and frame w/o the 'kickback' of the rebound slide, and the kickback of the hammer alone has the failure reality described above.