There is such a thing called metal fatique. And depending on the thickness of metal, the force by which it bends, the fatique life of a part can be very short, or infinite.
Generally speaking gun parts do not have an infinite fatique life, but the design limits I do not know. I do know that as a competitive shooter, dry firing is important, and if a firing pin breaks, well you gotta replace it.
As for your pistols, a Ruger Mk II 22lr has quite a robust firing pin. It is designed so it does not bump or damage the breech (some old rimfires hit the breech wall), and I really doubt that you will dry fire it enough to bust it.
Same basic opinion on your Ruger Single six. I have a Ruger SuperBlackhawk, bought it in 1982, I have dry fired it thousands upon thousands of times, no problem.
I have dry fired Smith and Wesson revolvers some ungodly number of times. No problem.
My Charter Arms Bulldog will bust a firing pin faster than you can say Boo!.
I do not dry fire that pistol.
My M1894 Marlin busted its firing pin. I don't dry fire that anymore.
Mauser rifles can be dry fired, well I have no idea the service life of a Mauser firing pin. I purchased a match Mauser, the guy told me he must have dry fired it 50,000 times, and it is on its second barrel.
M1903 Springfields should never be dry fired. Absolutely rotten firing pin design. I have had at least three strikers break, couple collars break, two or three cocking pieces broke. Had a firing pin break during a rifle match. The firing pin was sticking out through the bolt face as I loaded the rifle. I was lucky that a primer did not go off as I pushed the bolt forward. I would have had a rifle bolt through my forehead.
M70 Winchester firing pins are robust. Pre 64 firing pins seem to last forever, but I have know people who had Classic M70 firing pins fail around the cocking piece.
M1 Garand and M1a firing pins will break. Takes time, buy a couple and stock up.
The FN/FAL firing pin is fragile, I broke one, and I do not recommend dry firing that weapon.
A AR15 can be dry fired thousands of times, and the firing pin is cheap.
Most Service rifles are robust. Lee Enfields, Arisaka's, Mosin Nagants, M1917 can be dry fired till the cows come home.
I have not broken a K31 firing pin, but I don't dry fire as it is a two piece design, and based on my M1903 experience, I don't trust them.