I, along with a lot of firearms and personal defense enthusiasts, carry a 1911 type semi-automatic pistol as my primary sidearm.
Like most people who do, I carry it "condition one"; which means I carry the gun with a round in the chamber, the hammer cocked, and the safety on.
A frequent question those of us who carry in that way is something along the lines of "Why are you carrying it with the hammer cocked, isn't that dangerous or ready to go off or something?".
It’s a common mistake. Many people unfamiliar with guns assume that carrying a cocked weapon with a safety on is unsafe; mostly because in Hollywood, when someone is about to shoot some other person they almost always visibly cock the hammer (or rack the slide).
It is assumed by people who don't know any better (even some gun owners) that carrying with the hammer down is normal, and safe, however for almost all guns where condition one carry is possible, condition one is in fact the safest way to carry that weapon with a chambered round.
"Condition two", where there is a round in the chamber, but the hammer is down; is in fact the most dangerous way to carry a 1911 type pistol (even more dangerous than having the hammer cocked, but leaving the thumb safety off - sometimes called condition 0).
The only safer way to carry a 1911 (than condition one), is "condition three", with no round in the chamber; which to my mind requires too much time, and too many motions in order to bring the weapon into action for it to be used effectively in self defense.
Well, above all else, there's the human factor. Carrying a 1911 in condition two, requires chambering a cartridge, then manually decocking the hammer onto a live round. The most vigilant person in the world could have a slippery thumb that day, and negligently discharge.
In general, the less often you cock and decock (also the less often you load and unload, or holster and unholster), the safer you will be.
However, it is mechanically unsafe as well. First, a bit about the 1911. A 1911 is a single action pistol, meaning that you must manually cock the hammer before firing the first shot. This is as opposed to a double action pistol where the trigger will both cock the hammer, and release it to shoot.
Classic 1911s have two safeties: one a manual thumb operated safety, and two, safety in the back of the grip of the gun, that prevents the trigger from being pulled unless the gun is firmly gripped. "Series 80" type 1911s (and some others), have an additional internal safety that acts directly on the firing pin.
Many people unfamiliar with guns assume that the firing pin is on the hammer. This is not correct for most modern firearms; and in fact was generally only correct for older revolver designs from the 1890s through 1970s.
In a 1911 (and many other automatic pistol designs), the firing pin moves back and forth in a tube cut into the slide; and isn't long enough to actually ignite the primer if the hammer end of the firing pin is simply pushed flush with the frame. The firing pin actually must be struck with significant force, for the inertia of that strike causes the pin to fly forward (beyond flush with the frame), and strike the primer, igniting it.
When the hammer of a classic 1911 is resting on the firing pin and there is a round in the chamber, as in condition 2, there is nothing mechanically preventing the hammer from impacting the firing pin, except the fact that the hammer is in a resting position. If you then drop the weapon, the force imparted to the slide and/or hammer may be (alone, or in combination with impact and rebound forces) enough to cause the hammer to impact the firing pin and ignite the primer.
This is why condition two carry is considered mechanically unsafe.
1911 models with a Series 80, or other type of firing pin safety actuated by either the trigger or grip safety, add in an additional mechanical lock on the firing pin so that unless the trigger or safety are compressed, it is not possible for the firing pin to travel forward and ignite a primer (presuming the firing pin safety is functioning properly).
When a properly functioning 1911 is cocked, the hammer cannot fall unless the trigger is pulled, AND the grip safety is compressed. This is a direct mechanical restraint on the hammer. When the safety is on, there is an additional mechanical lock preventing an accidental discharge.
Even if for some reason the hammer fell without a trigger pull, the hammer would generally fall onto the second notch (the half cock notch), and not impact the firing pin. It is for all intents and purposes impossible for a 1911 with a properly functioning safety engaged, to fire.
No matter how you drop it, a 1911 in condition 1 (round chambered, safety on) will not fire; unless you drop it straight on the hammer hard enough to break both the safety and the sear, or straight on the muzzle hard enough for the inertia firing pin to strike the primer with enough force to ignite it (either would take a 30 foot drop into concrete. It’s been tested). If the pistol in question has a firing pin safety, it simply will not fire period (at least not with an 80 foot drop onto concrete it won’t. It’s been tested).
The only way to make a properly functioning 1911 fire from condition 1, is intentionally (or negligently); and this is why condition one carry is considered the only safe way to carry a 1911 while still having a round in the chamber.