Vern, what is the purpose of what you are calling the half cock and what I'm calling the safety notch? I'm really curious about this.[/quote]
Traditional gunlocks -- including the Colt SAA and old-style Blackhawks -- had a "half cock notch" which was
supposed to be a safety. In the case of SAAs and similar guns, the hammer spur is too exposed -- it can easily be struck a blow, and the leverage is such that the gun can be broken even by a light blow there.
The tumbler (which in these guns is the lower part of the hammer) has notches cut in it. The hammer is held back by the sear -- a chisel-shaped piece of metal which engages the notches. In the case of the full-cock notch, the face of the notch is vertical, so the sear can slip out under pressure.
In the case of the half-cock notch, the notch is undercut, so the sear cannot slip out, even if the trigger is pulled.
For the SAA and old Blackhawk, you need to be able to rotate the cylinder to load. But a revolver
must keep the cylinder fully under control. There is, however one point in the cocking cycle when the cylinder stop is momentarily disengaged -- so there is another undercut notch on the tumbler. This notch, the loading notch, corresponds to that point in the cocking cycle when the cylinder stop is disengaged. Like the half cock notch, the loading notch is undercut. You cannot pull the trigger with the hammer in the loading notch.
And why is it dangerous? Remember, nobody is suggesting a live round should be underneath so just put that thought out of your mind. Is there some danger in having the revolver hammer back one notch from the pin over an empty?
If you carry the gun on half cock when loaded, you still run the risk of breaking the half cock notch or sear nose -- and you may not notice it. It doesn't take much of a blow on the hammer spur to do that. And that leaves you with a broken and dangerous gun -- because the broken sear can slip when you try to bring the gun to full cock.