At a local Bullseye match I talked to two Master Class shooters who shot an amazing number of rounds through revolvers. One was a 10 times PPC National Champion. The PPC Champ estimated he had 750,000 rounds through a S&W K frame, the other shooter, about 250,000 through his revolver. The predominant 38 Special round fired was a 148 HBWC with 2.7 grains Bullseye. However, the PPC Champ said there was a time when he had to shoot 125 grain bullets that made major. That would crack the forcing cone of a K frame barrel. The L frame revolver barrel did not have that problem. Barrels seemed to last forever as long as you are shooting low pressure loads with lead bullets.
But, both of the shooters had to replace extractor stars, cylinder hands. These wear probably due to the load imparted on the extractor star to the cylinder hand at the moment of cartridge ignition. I got the idea that extractor stars are fairly soft steel anyway. Colt revolvers got out of time much quicker than S&W, which more or less validated what I had heard from other shooters. Colt revolvers, the hammer falls just at lockup, S&W fall just after lockup.The Colt is out of time sooner with the same amount of wear. Makes sense to me. The PPC shooter had a firing pin break. Hey, stuff happens.
I don't know the lifetime of frames and cylinders but all structures are designed to a finite number of cycles of use. How revolvers were designed previous to CAD/CAM software, I don't know. Load paths would have been determined, simple shear calculations would have been done, rudimentary safety factors, maybe estimates of fatigue lifetimes based on very limited information. That is what I think they did. It is possible manufacturer's did less. Today you can model everything and see dynamically what is going on during ignition, get an idea of predicted load, and build up, or take material away, from parts. Modern cylinders are still designed to a load (pressure times surface area) , the frame would be designed to support the load imparted by the case head (pressure times surface area), but the ability to model and see stress concentrations is incredibly advanced compared to the 1990's. And we had spread sheets in the mid 1990's, but it was late stone age in terms of software. Maybe early iron age. The legacy revolvers that date back to the early 1900's or pre WW2, that was the stone age.
This is worth reading to have an idea of fatigue lifetime:
Fatique Life of 4140 steel
http://castboolits.gunloads.com/showthread.php?150409-Ruger-om-44-convertible&highlight=convertible
Just a few thoughts on this. For Background I am a mechanical engineer with a heavy background in failure and fatigue.
I wonder if I could request a high quality photo of the fracture zone of the cylinder? I am specifically interested in the grain structure of the bolt notches.
I put fort the following.
1) Firearms in general (the type we plebeians can get our mits one) are not designed for infinite fatigue life.
2) The Factors of safety used in firearms design are in line with low end of fatigue requirements (usually less than 10,000 cycles).
3) One of the funny things about fatigue is that each time you push the material past its original design point, you lower its expected life.
4) I am looking at this as an older gun with an unknown number of rounds through it. but based on its age a substantial round count seems likely.
5) When these firearms are designed it is generally preferable for something else to go before the cylinder lets go and takes the top strap. Generally this takes the form of the gun wearing loose or the barrel wearing out. But they are designed to handle X rounds at standard pressures.
6) I see alot of folks calculate the strengths of Rugers, but these calculations are only ever performing an evaluation on a straight static pressure basis. This is wrong when trying to determine if a load is safe.
I attached a couple of marked up figures for your perusal
It is very interesting to see a reasonable blowup period of around 650 rounds if you fire over pressure rounds. Reloaders should understand that when they over load a structure beyond its design limits, the lifetime of that structure is dramatically reduced. When ever you read about some "Gunsmith" chambering a WW1 era Mauser in a modern belted magnum, you ought to know, the action was not designed for that load. Typically that belted magnum is operating above the proof pressures of the vintage action. "Gunsmiths" and owners of these claim that nothing bad has happened, but if you can get coherent information out of them, you find that maybe these rifles have fired 200 rounds since the conversion. I predict, get these characters to shoot enough rounds to burn out the barrel, sometime on that path will be a catastrophic receiver failure.
Basically, the conclusion of this rant, is if you want your revolver to last, shoot light loads. Light loads will stress the mechanism less. Expect to replace springs, screws may fall out, on aluminum frame revolvers, the frame will crack in time. Aluminum has a finite fatigue life, the lighter the load used, the longer the frame will last. If you ever shoot hundreds of thousands of rounds, the ammunition cost will be several times the price of the revolver, so, replacing the revolver won't seem too costly.