Howdy
I thought I would chime in on this one, as many of you know I have some strong opinions on this matter.
I have no first hand knowledge of how 'cowboys' (let's be clear here, the Colt Single Action Army was an expensive gun, and not a whole lot of 'cowboys' would have been able to afford one) loaded their revolvers, whether they loaded five or six.
I can't put my finger on it right now, but I have read of an incident where Wyatt Earp's revolver slipped from his pocket while he was gambling, and it fired when it hit the floor.
I don't know one end of a horse from the other, but I do know it is common practice to flip one stirrup up onto the saddle while cinching the saddle, then the stirrup is lowered so the rider can mount the saddle. Elmer Keith related an incident where a friend of his was saddling his horse, and the stirrup fell onto the hammer of his holstered Colt. The Colt discharged, the bullet struck the man in the leg, and he died of blood loss.
Ruger completely redesigned their line of revolvers in the early 1970s to include transfer bars after loosing more than one expensive lawsuit from shooters injured by revolvers that had discharged when dropped. The revolvers had been loaded with six rounds.
The only historical document I can point to is this reprint of a manual on inspection and operation of Army revolvers and Gatling Guns, that was originally published in 1875.
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Yes, this manual indeed says to load all six chambers, and lower the hammer to the 'safety notch'.
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Now, let's get to what I do know. I have posted this photo many times. These are the lockwork parts of a 2nd Generation Colt. The upper arrow is pointing to the so called 'safety cock' notch on the hammer, the lower arrow is pointing to the sear at the tip of the trigger. For those of you who have not been inside a Colt, when the hammer is cocked to the 'safety position' the sear rests in the 'safety cock' notch. Look how thin the sear is. It would not take much of a blow to the hammer spur to shear off the tip of the trigger, allowing the hammer to fall with the force of the blow onto the primer of a cartridge. This could easily be enough force to fire a cartridge.
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Now let's look at the parts of a 1st Generation Colt. This one happens to be a Bisley Model that shipped in 1909. Again, look how thin the sear is. Again, it would not take much force to shear it off.
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While we're at it, let's take a look at the parts inside one of the old Three Screw Rugers, that predated the redesign which included transfer bars. Again, look how thin the sear is and how it would not take too much force to shear it off. Which is why Ruger lost the lawsuits.
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Now let's look at Smith and Wesson single action revolvers.
I have a number of antique Smith and Wesson revolvers, but these are the only ones with a rebounding hammer.
Top to bottom in this photo is a New Model Number Three that shipped in 1882. Directly below it is a 38 Single Action 3rd Model. I do not have a shipping date for it, but I do know it was refinished at the factory in 1937.
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This is a 32 Single Action, with box, that shipped in 1887. As stated on the box it has a rebounding hammer.
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Let's take a look inside the New Model Number Three. In this photo the hammer is at full cock.
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This is the position of the hammer at the moment a cartridge fires. The sear is trying to rock the hammer back, and I am holding the hammer all the way forward with my thumb.
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The hammer is in the half cock position in this photo. The hammer must be in this position in order to reload. Notice the slot at the top of the hammer has cleared the step at the bottom of the latch. The latch cannot be opened to load or unload the revolver unless the hammer is in this position.
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This is the most important photo for this discussion. The hammer has rebounded, rotated back slightly by the camming action of the sear. This is the normal position of the hammer. Look at how tiny the notch is on the hammer where the sear is resting. Also, look at how tiny the sear itself is. I have fired this revolver many times. I would not dream of leaving the hammer in this position with a live round under the hammer. It would not take much of a blow to the hammer to shear off that tiny interface between the sear and the hammer, allowing the revolver to discharge.
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I bought this 2nd Generation Colt a few years ago. It left the factory in 1973. It came with the box and the original owner's manual.
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I took a couple of photos of the owner's manual.
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I zoomed in on the paragraph about the 'Safe Position"
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So, you can call it a myth if you want, and I have no real information about what was done in the 19th Century, but I have been shooting single action revolvers for over fifty years and know a fair amount about them.