Howdy
Picture is worth a thousand words department:
Here is a photo of the interior of a 44 Handejector 4th model. Although not the same model as the Model 1917, this photo illustrates the way all S&W revolvers have retracted the firing pin ever since 1905.
This is the normal 'at rest' position of all the parts on the inside of a Smith. The two arrows at the bottom are pointing to the hump at the top of the rebound slide and the hump at the bottom of the hammer. The spring inside the rebound slide has pushed it and the trigger forward, and its hump has wedged the hammer back, withdrawing the firing pin into the frame. The arrow at the top shows the gap between the hammer and the frame. The long slanted piece is the modern hammer block incorporated in all Smiths ever since WWII. Notice the hammer is not actually touching the hammer block, the block is redundant and is a fail safe part. If the gun were to fall onto its hammer spur, and if the hump at the bottom of the hammer should shear off, or if the stud the hammer rotates on is bent or broken, the hammer block will prevent the hammer from moving forward enough to allow the firing pin to contact a primer.
In this photo I am holding the trigger back as if the gun has just discharged. I have removed the hammer block for clarity. In this view, the small swivel bar of the trigger has forced the rebound slide back and the hump at its top is no longer in contact with the hump at the bottom of the hammer. The hammer spring has pushed the hammer all the way forward, with it coming to rest against the frame. The firing pin is protruding through the frame and if there were a live cartridge under it the primer would have fired. If the hammer block were in position the pin in the rebound slide would have pulled it diagonally down its slot in the side plate and it would be clear of the hammer.
Although the parts inside a Model 1917 are shaped slightly differently, the principle is the same, and this sequence happens every time the trigger is pulled in a Smith, either double action or single action. With the Model 1917 in question it is impossible to determine exactly what is wrong without removing the side plate for a look see, but the likely culprits are a broken hammer 'hump', or an altered rebound slide. If the stud the hammer rotates on is broken I doubt the hammer would be able to rotate properly, but I suppose it is a possibility.
*****
Regarding the safety of carrying a S&W fully loaded with six rounds:
The modern design with the modern hammer block is absolutely safe to carry fully loaded.
This photo shows an earlier style of hammer block. It is a piece of spring steel peened in place in a slot in the side plate. At its top is a right angle extension that blocks the hammer the same way the newer design does. With this design, the 'at rest' position of the hammer block kept the right angle tab in position, blocking the hammer. The arrow points to a tab on the hammer block, and the other arrow points to a ramp on the pawl. When the trigger was pulled or pushed back by the rebound slide, the ramp on the rising pawl engaged the tab on the hammer block, flexing it and withdrawing it into its slot, creating clearance for the hammer to fall all the way. Even though this design was inside S&W revolvers for decades, eventually it was determined that improperly removed heavy grease or cosmoline, could cause the hammer block to remain in the withdrawn position, allowing the gun to discharge if the hammer spur received a heavy blow.
The Model 1917 was built before there was any hammer block at all in S&W revolvers. It was believed at that time that the rebounding nature of the hammer was enough to keep the gun from firing if the hammer spur received a heavy blow.
I hasten to add that like most early 20th Century double action revolver designs, even without a hammer block the rebounding hammer of the S&W Handejectors was still far safer than the designs of the old single action revolvers of the 19th Century. Here are the parts of the lockwork of a Colt Single Action Army. The two arrows point to the tip of the sear on the trigger and the so called 'safety notch' on the hammer. Notice how thin the parts are. Dropping the gun on the hammer spur was almost guaranteed to shear off either the sear or the overhanging lip of the 'safety' notch, causing the revolver to discharge. This was a well known fact and anybody who regularly used a Colt knew to always keep an empty chamber under the hammer. In contrast, failures with the S&W design were very rare. It was the accidental death of a sailor from the discharge of a S&W dropped from the superstructure of a warship that caused Smith to come up with the modern hammer block design.
Regarding fully loading the Model 1917 on the battlefield, as John Wayne once said, 'if you think you are going to need all six, then load all six'.
