TexasCHLforum.com

The "half-cock" position on an external hammer is, indeed, a safety feature. On the 1911, it's actually a secondary notch machined into the hammer. If the sear happens to slip (and the trigger is not being pulled), the sear will end up catching in the half-cock notch, stopping the motion of the hammer. It's designed to add one more barrier between the user and a true accidental discharge (as a result of parts failure). While the 1911 can be carried with the hammer at this position, that's not the primary function of it - the 1911 was designed to be carried with the hammer back, thumb safety on.

It's not limited to semi-autos, though, nor is it limited to this one safety feature.

Old single action revolvers had a half-cock notch. While it could also function as a "safety," that was not the main purpose of the notch, due to the design (it was far safer to carry the hammer completely down, with no round under the hammer). Rather, placing the hammer in the half-cock notch allowed the hammer to disengage from the cylinder mechanism, letting the cylinder be turned by hand for loading/unloading.

The familiar lever-action rifles from Marlin and Winchester, however, are intended to be carried with either an empty chamber or a hammer at half-cock over a loaded chamber. With the hammer at half-cock, the rifle would break before it would fire, but a simple drawing back of the hammer to full-cock readies the rifle to shoot (and is more quiet than working the action).

Then you look at the Winchester 1897, an external-hammer shotgun. Not only does placing the hammer at half-cock render the shotgun "safe," it also locks the action so that you can't accidentally yank on the slide and empty the chamber. With that hammer at half-cock, the hammer itself is the only part that can be moved (unless you hit the slide release). Once again, this is to allow a hunter to safely carry the shotgun "fully loaded," and only have to draw the hammer back to fire.

Hope this helps.