I have two single-action revolvers. There are (and were) four cocking positions, only two of which meant anything: half and full cock.

You have to put both percussion and fixed cartridge revolvers at half-cock to load them. Otherwise the cylinder won't rotate. Even with today's safeties (a small bar prevents the hammer from hitting the firing pin) it is still recommended that 1) you don't depend on the safety (on any firearm!), 2) you don't depend upon half-cock as a safety, and 3) you load 1, skip 1, and load 4 in a six-cartridge revolver, so that the hammer, even when the safety is on, rests on an empty chamber.

As a gunsmith once said to me, "Don't trust anything mechanical. Be safe!