Is this why in western novels the cowboy always loaded 5 rounds in a 6 shooter.
Kind of but not really.
This is a photo of the lock parts of a Colt. The upper arrow is pointing to the 'safety cock' notch on the hammer. The lower arrow is pointing to the tip of the trigger called the sear. When the hammer is pulled back so the sear pops into the 'safety cock' notch, the firing pin is retracted and pulled back slightly into the frame, away from the primer of a live round under the hammer. Notice how thin the sear is. If the revolver should fall so that it lands on the hammer spur, chances are excellent that the sear will shear off, or the overhanging lip of the 'safety cock' notch will break off. If that happens, the hammer will be driven forcefully forward and there is an excellent chance the the firing pin will fire a live round under the hammer. That is why anybody who is familiar with a Colt, or similar revolver never keeps a live round under the hammer, even if the sear is in the 'safety cock' notch.
To answer the obvious next question, yes, it has happened many times. Which is why Ruger changed the design of their single action revolvers back in the 1970s to include a transfer bar. The hammer of a Ruger never touches the frame mounted firing pin. The hammer is shaped so that there is a void in the area of the firing pin. These are the lock parts of a Ruger Vaquero. The long thin vertical piece attached to the trigger is the transfer bar. When the hammer is cocked, the trigger rocks back slightly, raising the transfer bar up so that its tip is partially covering the firing pin. Then when the hammer falls, it strikes the transfer bar, which transfers the hammer blow to the frame mounted firing pin.
In this photo the hammer is cocked and the transfer bar is raised up, ready to transfer the hammer blow to the frame mounted firing pin. After the hammer falls, when the trigger is released its spring rocks the trigger forward, pulling the transfer bar down. The upper face of the hammer remains at rest against the frame, but there is now a space where the relief is on the hammer so that it cannot contact the firing pin. This makes all modern Ruger revolvers completely safe to carry fully loaded with a live round under the hammer. Even if it falls on the hammer, nothing will happen. The earlier Three Screw Rugers did not have a transfer bar, the action parts were similar to a Colt, and if the revolver fell on its hammer there was an excellent chance it would discharge.
Recently Uberti changed the design of all their single action revolvers. Now there is a retractable firing pin in the hammer. Normally, the firing pin is retracted slightly so that it does not reach forward far enough to contact the primer of a round under the hammer. When the trigger is pulled, it pushes an actuator bar up inside the hammer that extends the firing pin forward enough to fire a cartridge. When the trigger is released a spring pulls the firing pin back slightly again. These most recent Uberti revolvers are said to be safe to carry fully loaded.
The mechanism inside a Smith and Wesson revolver is very different. This is the lockwork of an old S&W revolver that left the factory in 1917. At this time there were no hammer blocks of any type inside a S&W revolver. Any modern double action revolver has to have a mechanism to withdraw the firing pin from the primer of a fired round, or the cylinder cannot be opened. One of the two arrows at the bottom of the photo is pointing to a bump on top of the rebound slide. There is a strong spring inside the rebound slide that pushes it forward when the trigger is released. This causes the bump to rock the hammer back slightly. The upper arrow is showing the space between the hammer and the frame. Every time the trigger is released, the hammer rocks back like this.
If the revolver should fall onto its hammer, the lower part of the hammer could shear off at the red line I drew, or the bump in the rebound slide could be crushed. The rebound slide is hollow after all. If either of these things happened the hammer could jump forward with enough force to fire a round under the hammer.
This is the type of hammer block that failed when the sailor died in 1944. There was another type previous to this type, but this type was installed inside S&W revolvers for many years. The hammer block is a piece of spring steel. It is peened to the side plate and sits in a slot in the side plate. Difficult to see in this photo, but at the top of the hammer block there is a rectangular tab that is extending straight out towards us. The normal position of the hammer block keeps the tab at its top positioned in the space between the hammer and the frame, so if the revolver should fall on its hammer, and if the bottom portion of the hammer should shear, or the bump on top of the recoil slide should be crushed, or if the stud the hammer rotates on breaks, the hammer block will prevent the hammer from riding forward far enough to fire a round under the hammer. Notice there is a tab extending out to the side on the hammer block. There is a ramp on the hand. When the hammer is cocked, or the trigger pulled the hand rises and the ramp engages the tab on the side of the hammer block shoving the hammer block down into the slot in the side plate. This withdraws the tab at the top of the hammer block from between the hammer and the frame and allows the hammer to fall all the way. When the trigger is released, the hand retracts and the spring action of the hammer block returns it to the raised position, blocking the hammer again.
Smith and Wesson had a large contract with the government at the time, supplying 38 caliber revolvers, mostly Victory Models, to government. When the accident happened the Army threatened to cancel S&W's contract unless they quickly came up with corrective action. It was determined that cosmoline inside the revolver had hardened and prevented the hammer block from springing back to the 'safe' position. So when the revolver fell on its hammer, the hammer block was in the retracted position and did not prevent the hammer from jumping forward when something broke inside. S&W set up some test fixtures and repeatedly dropped revolvers onto their hammers and a fair number of them failed. The engineers were called in and within a week a completely new style of hammer block was designed. The long, thin, slanted part is the new hammer block. A pin was added to the rebound slide. The new hammer block rides in a slot in the side plate very similar to the slot in the side plate with the old hammer block. I have positioned the hammer block in this photo the way it sits when the revolver is at rest. A tab at the top of the hammer block is sitting between the hammer and the frame. If something should break, the hammer block will prevent the hammer from moving forward enough to fire a cartridge. When the trigger is pulled or the hammer cocked, the rebound slide moves back and the pin drags the hammer block down in its slot, removing it from between the hammer and the frame. When the trigger is released, the rebound slide moves forward again, pushing the hammer block up between the hammer and the frame again. Notice there is actually a small amount of space between the hammer block and the hammer. The hammer block is actually a redundant safety device, it only truly blocks the hammer if something inside fails. Normally the hammer never touches the hammer block. This same style of hammer block is still used inside every S&W revolver manufactured today. It makes a modern S&W revolver completely safe to carry fully loaded.
Notice the difference in function between a transfer bar and a hammer block. Each does what its name implies. A transfer bar transfers the blow of the hammer to the frame mounted firing pin. The hammer block blocks the hammer from falling all the way unless it is pulled out of the way.
It should also be said, that the fragility of the parts in the old Colts made a discharge much more likely if the gun happened to fall onto its hammer. A cowboy could even be shot in the leg if he was standing next to his horse and a stirrup should happen to fall on the hammer. The parts inside the old S&W revolvers were not as fragile as the parts in side a Colt. A discharge was less likely to happen if a S&W revolver fell onto its hammer, even before hammer blocks were installed. You can bet that law enforcement officers regularly carried their S&W revolvers fully loaded and did not worry much about an accidental discharge. The death of the sailor in 1944 was a freak accident. Everything went wrong in a cascade of events. Even though the accident occurred on shipboard, I have never seen any information about how far the revolver fell in that incident. Did it fall from hip level, or from an upper deck. I do not know the answer to that.
By the way Colt double action revolvers had a different mechanism called the Positive Lock that also prevented an accidental discharge if dropped. I have photos, but that is another topic for another time.