That pretty well states it.
As far as rimfire semi-autos are concerned, I stick with the manufacturer's recommendations. Ruger has clearly stated in the manual for the Mark II and Mark III that as long as you put the cross pin back in the bolt when you reassemble the gun, dry firing is fine. They also state that if you forget, dry firing the gun even once may ruin the barrel. Probably an exaggeration, but I ain't going there. Ruger also states in the manuals for their Vaqueros that dry firing is fine. More on that in a bit.
With centerfire revolvers, most of it gets down to geometry. Think of it this way. If a car slams into a bridge abutment, when the car stops, the forces acting on the people inside try to keep them moving forward. The old bit about a body in motion will try to keep moving if not acted on by a separate force. The exact same thing happens when a hammer drops, except the effect is actually much more extreme. When the car crashes, there is a crumple zone that slows down the impact and absorbs a great deal of the shock. Not so in a firearm, when the hammer slams into the frame, it comes to a sudden, violent stop. The G forces of deceleration are actually much greater than in the car crash.
Take a look at these photos.
The first photo is of the hammer of a S&W Model 1917. It's not so much the fact that the pin is mounted to the hammer, its the shape that is important. Notice how the firing pin (S&W calls it the Hammer Nose) tapers down sharply to the final diameter, and then becomes cylindrical for the final length. The firing pin is most susceptible to breakage right where I drew the red line. When the hammer slams to a stop, the entire firing pin is just like the person in the car crash. It wants to keep moving forward. The rivet holding the firing pin in place restrains it, just like a seat belt. But the firing pin is made of hardened steel, and stress points can be generated by sharp transitions. The G forces are so extreme, that the tip of the firing pin can break off right at the red line and keep going forward. No, it is not going to happen the first time, or not the second, but if you dry fire it enough, that tip might just decide to keep flying forward. Fortunately, S&W firing pins are easy to replace.
The next photo is of a 2nd Gen Colt hammer and firing pin. Same story. There is a better transition to the cylindrical shape at the business end of the pin, but if it is going to break, the blue line indicates where it is most likely to separate.
Ruger Vaquero and Blackhawk firing pins are shaped like a hockey puck with a small projection where the actual firing pin is. They are mounted in the frame, but they still get slammed forward every time the hammer falls, and if there is no primer to cushion them, they come to a violent stop a microsecond or so later. But their shape seems to be more robust, and I have never heard of one breaking.
This is the hammer from a S&W Schofield revolver made in 1875. Look at that firing pin. It is typical of S&W firing pins of the time. It is an integral part of the hammer, machined into the shape, it is not a separate part. There is no way I would dry fire a gun with a hammer design like this. The square, sharp shoulder where the firing pin meets the rest of the hammer is a perfect place for Stress Risers to form, a perfect place for the firing pin to separate from the hammer, and it would be really tough to find a replacement.
Now all of this begs the question, why is it OK to drop the hammer on a live round or a snap cap?
A live primer or a snap cap works exactly the same as an air bag in a modern car. It cushions the blow. The firing pin contacts a primer or a snap cap before the hammer comes to that slamming stop. The primer begins deforming, and a snap cap starts yielding to the firing pin and forma s protective cushion, just like the airbag, but on a much smaller scale. So that when the hammer slams to a stop, the primer or snap cap is supporting the tip of the firing pin and preventing its impulse to continue moving forward, just like the air bag.
One more comment about snap caps. Use a real snap cap, don't rely on the old false wisdom of using a spent case with a fired primer. After a few impacts, a spent primer will be pounded into a new shape so that it can no longer cushion a firing pin. The firing pin will come to that slamming stop before it contacts the reshaped spent primer. So the spent primer will not protect the firing pin like a proper snap cap will.
One last comment, nothing is foolproof. The firing pin in that Colt is not the original. The original broke one day when I was firing it at the range. I have no idea how many times previous owners may have dry fired the gun, perhaps weakening the pin, just as I have no idea how many times that Schofield hammer may have been dry fired. I do know that one day my Colt broke its pin, and I had never dry fired it.
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