The phrase "going off half cocked" didn't just spring out of nowhere.
First rule, VERY IMPORTANT RULE,
NEVER PUT 100% faith in a mechanical safety. 99% sure, but always be aware that any mechanism CAN FAIL. Use of the mechanism, the way it was designed to be used, ALONG WITH PROPER SAFE GUN HANDLING is what keeps you safe.
Generally speaking with pistols, half cock is the least safe way to carry or store the gun with the chamber loaded.
This is for two reasons, one of which is the small but real risk when lowering the hammer from full cock to half cock. The other reason is because of the way the mechanisms work.
Modern DA/SA semi autos generally have inertia type firing pins. This means that it is a floating firing pin and is shorter than the distance it has to travel (and so doesn't contact the primer) and there is usually also a spring, pushing the firing pin to the rear.
This is a pretty good system but can allow the gun to fire unintentionally under certain extreme and rare conditions.
A firing pin block mechanically binds the fring pin in place until the trigger is pulled (which moves the block out of the way, freeing the firing pin), at which time the firing pin will move just like a gun with no firing pin block.
SO, this is one of the risks in lowering the hammer, (if the gun doesn't have a decocker), you have to pull the trigger, deactivating the firing pin block, so you can lower the hammer. IF you slip off the hammer, the gun fires.
Now, if the gun has a decocker, or a safety that also decocks the hammer you have a couple of other points you need to be aware of. First, how does your gun drop the hammer when you decock it??
Some guns, like Sigs drop the hammer onto a safety notch that is short of the firing pin. This sort of looks like the hammer is half cocked but it is really something different.
Some other designs lock the firing pin and drop the hammer down onto the firing pin. And some others interpose a block between the hammer and firing pin and drop the hammer down on that.
Now, it may be a one in a million thing, but people do win the lottery, and if its your "unlucky day" and things go wrong in just the right way for bad things to happen, these systems can fail and the gun could fire when you decock it. Always ALWAYS make sure the gun is pointed in a safe direction WITH a safe backstop before you drop the hammer with a safety or decock lever.
Now, the reason a hammer at half cock over a loaded chamber is worse idea than the hammer down over a loaded chamber is again, inertia.
To fire, something has to strike the rear of the firing pin, (and the pin has to be free to move) to drive it forward with enough force to set off the primer. The hammer does this being spring driven and "having a running start".
With the hammer all the way down, it may be touching the end of the firing pin, but it is also in firm contact with the slide and possibly also the frame. There is no running start possible. Any blow to the hammer has its force transferred to the slide or frame, and not the firing pin. IF the hammer is at half cock, and an impact causes it to fall, it gets a "running start" and does strike the firing pin. Possibly hard enough to fire the gun. That's why half cock is less safe than hammer down.
Another question I have is: If its not a great idea to carry "half-cocked" why do the decockers on my HK USP 45 and SIG P226 (I think the 226 does anyway) both drop the hammer to half cocked position?
They don't drop the hammer to a half cock position, it only looks like they do. The purpose of the half cock position comes down to us from the days when all firearms required the hammer to be manually cocked. It is there to catch the hammer if you slip off it before it locks back cocked.
The half cock is in the approximate middle of the hammer arc, to catch the hammer because from beyond that point the hammer can fall with enough force to fire the gun. If you slip off the hammer before you have pulled it back to the half cock position, it shouldn't have enough force to fire the gun. (it doesn't fall far or fast enough - not enough "running start")
Not certain about the HK but I have a couple Sigs and the spot the decocker drops the hammer onto is not half cock, its position is more like 1/4 cock, or less, its just off the firing pin a little bit short of touching it. To get the hammer to go all the way down, you have to pull the trigger.
Pulling the trigger with the hammer in the "decock notch" fires the Sig DA, and actually takes a bit less trigger effort than firing the gun DA with the hammer all the way down.
If the gun you want to carry doesn't decock, and you want to carry hammer down on a live round, you have to carefully pull the trigger and ride the hammer down, there's no other option. Not 100% safe but millions of people have done it millions of times without incident.
Of course, like I said, people do win the bad luck lottery sometimes, and it only takes once to end or ruin a life, and it will be entirely your fault if it does happen.
Want to do it as safe as practical? Stick your finger between the hammer and the firing pin, THEN pull the trigger and ride the hammer down onto your finger. once its down against your finger, let go of the trigger, siide your finger out and ride it down the rest of the way. This way, even if you slip where the hammer has enough of a running start to fire the gun, it will hit your finger (hopefully teaching you a painful lesson), the softness of your finger will not transfer enough energy to the firing pin to fire the gun.
My personal thought is that its best to use a gun that allows you the carry it the way you want to, and if you feel a need to lower the hammer (which is waay better than half cock) you are best off with a gun that lets you do that without needing to pull the trigger.
Lots of guns will do that, lots won't. If they won't, we can work around that, and not shoot ourselves or anyone else, but one needs be vigilant to be safe.
