Howdy
You know the old saying it's not the fall that hurts, it's the short stop at the end?
It's the same story with firing pins.
I used to work for a company that made tiny electronic components. We tested them for vibration. Vibration is expressed in Gs, the same way gravity is. One of the old engineers told me one day that when an object is dropped onto a hard floor, the deceleration (the opposite of acceleration) forces approach infinity. That's why stuff breaks when you drop it on the floor. But if the blow is cushioned by something, the deceleration is much more survivable. It is also true that the smaller the component, the more Gs it can take. Remember those pictures of the astronauts being tested at 10 Gs or so? Tiny mechanical components can survive Gs in the hundreds or even thousands. But they usually don't survive infinite Gs.
Let's look at a Colt firing pin for a moment.
When the hammer slams to a stop on the frame, it is like dropping it onto concrete. Deceleration is way up in the thousands of Gs. Sorry, it's a little bit out of focus, but see that little tiny tip at the end? When the hammer slams to a stop against the frame, the firing pin wants to keep right on going. The only thing holding it in place is the tiny rivet that runs through it. If there is any invisible crack or tiny irregularity, it is right at the base of that tiny section that the tip will want to break off and keep right on going. This is a 2nd Gen Colt, but modern 3rd Gen Colts have a similar firing pin. First Gen Colts had a much more robust conical firing pin, but that is besides the point.
If the firing pin strikes a live primer or a snap cap, the deceleration forces on the tip of the pin get reduced dramatically. The tip is actually cushioned by the primer as it gets dented, slowing the firing pin down. For this reason, I never dry fire anything with a firing pin shaped like this unless there is something underneath, like a live primer or a snap cap to cushion it. Nope, spent primers in spent brass is no good. Within a few strikes, the primer takes on the shape of the firing pin tip and completely loses its ability to cushion the tip of the firing pin.
I don't drop the hammer with firing pins that look like this without something under the hammer for the same reason. That one in front is a little bit over 100 years old and I don't want to find out how much stress it can take. True, it is simple to replace a firing pin like this, but why take the risk?
I would never dream of dropping this hammer onto an empty chamber. That hammer is 130 years old, and the firing pin is integral to the hammer. If it breaks, there are no more replacement parts.
Rugers firing pins are a different story. They are not long and thin. Most of them are hockey puck shaped, with the actual pin protruding a bit from the hockey puck. That was part of Bill Ruger's genius, replacing old fashioned parts with modern engineered parts that were more robust. It says right in the manual of every Vaquero that there is no problem dry firing them.
Contrary to popular opinion, there is no problem dry firing some of the better made rimfire pistols. There is a small pin that sits across the bolt in a Ruger Mark I or Mark II pistol. This pin limits the travel of the firing pin. With this pin in place, the firing pin never touches the breech face of the barrel. It always stops a few thousandths short. In the owner's manual to my Ruger Mark II pistols it clearly says to be sure to replace the pin after field stripping the gun. The manual warns that forgetting to put the pin in can ruin the gun. But no problem if the pin is in there. Same with my Single Sixes. The firing pin never touches the cylinder. There is always a slight air gap, so the pin never strikes the cylinder.
I have a few K22s. I'll have to take a good look and see if the firing pin ever touches the cylinder. I'll bet not.
I did buy an old High Standard pistol a few years ago. It was over 50 years old, and there was evidence of firing pin strikes on the barrel. But it still fires all the ammo I put into it.