yhtomit,
The old 1917 revolvers lack the "hammer block" feature that's on all the later S&W .45ACP revolvers (and other modern Smiths, of course).
The hammer block prevents the hammer nose tip from being able to go forward, and into a primer on a loaded cartridge if the gun was dropped.
The 1917 revolvers lack this important safety feature and can fire if the back of the hammer is struck hard.
LOOK AT A MODERN S&W REVOLVER, and you'll see what I mean:
1. Unload the weapon first!
2. Close the unloaded gun
3. Under a bright light, cock the gun. The peer from above, and behind the cocked hammer, into the area in the back of the gun where the hammer usually rests.
4. In this area, notice a blued small rectangular block. This is the HAMMER BLOCK, and it drops down this low when the hammer gets cocked and remains that way when you pull the trigger. Only at these times can the trigger's nose contact the primer!
5. Holding onto the hammer securely, pull the trigger and SLOWLY lower the hammer back down under bright light.
6. Just before the top of the hammer disappears, you'll see the small rectangular face of the HAMMER BLOCK move UP into the "safe" position, once again blocking the hammer from moving forward until the trigger is fully pulled again!
A SECOND LOOK/ANGLE . . . AT AN UNLOADED GUN . . .
1. Cock the hammer, then lower it slowly with your thumb while continuing to hold down the trigger.
2. Continuing to hold down the trigger, turn the gun so you can peer into the side of the cylinder. You'll see the trigger's nose extended into the back of the area where the top cartridge and primer would normally be.
3. Still looking, slowly return the trigger to the out position and you'll see the hammer move away from the firing position and notice that the hammer cannot go back into the firing position.
However, you also won't be able to see the HAMMER BLOCK during this time as it rises to block the hammer from being forced forward again . . . until the trigger again is pulled and cocked while the HAMMER BLOCK drops back down.
Here's a diagram of the 1917:
Now, here's the inside of a nasty J frame I bought once, just before I cleaned it.
Note the hammer block part. You can't see it's rectangular face, but you can see its side that moves up and down in a slot to the right side of the hammer. Don't take your side plate off though, for you can see the small rectangular top of the hammer block fine by using the instructions above. Hope this helps!
Tom