Firearms are built to a load and a service life. Lets say the service life is "one". Safety factors are applied to compensate for variances in alloys, manufacture. Lets say the safety factor for bolt lugs is "two", that is there is twice as much metal needed for a load of "one". That doubling of material would make the structure twice as strong as needed, when new, if the metal and manufacturing processes were perfect, but given stress cycles, the material degrades. That safety factor of two is there so the structure before it completes its life time of "one", with loads of "one".
Also, loads over "one" will deteriorate the structure faster than loads of one or less than one. And this post shows an analysis indicating that:
Fatigue 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
Note: poster shows a diagram in which given over pressures it is reasonable to expect the material to fail at 650 rounds.
I participated in a discussion on another forum, and one of the posters wrote this:
About Cartridge Maximum Pressures
http://thefiringline.com/forums/showthread.php?t=579451
I
don't think it is any coincidence that the SAAMI proof loads to max average load ratio is similar to ratio of threshold of plastic deformation to the fatigue failure stress at a number of cycles approximately the max number of times a gun might be fired.
That's right, I am saying there are signs of intelligent design at SAAMI.
10 or 12 years ago some guy working in Boeing structures drew this plot for me in relation to Rem700 bolt lugs. I don't think he copyrighted it. I know I have not.
10,000 stresses at 70% of yield stress reaches fatigue.
SAAMI proof loads for centerfire rifle are between 1.3 and 1.4 times the max average working pressure = 77% ~ 71%.
When Stoner designed his AR15, he sized the bolt lugs for loads of 50,000 psia (because CUP was thought to be PSI) and he had to pass an endurance test of 6000 rounds. What you find today, is AR15 bolts tend to start cracking lugs around 10,000 rounds. There are bolts shot peened, made of more expensive alloys, and they have a reputation of lasting 30,000 rounds. But they do crack. The Army incidentally has been bumping up the pressures of the 5.56, it went from 50 kpsia to 52 kpsia, I think 54 at one time. Now they have a load which they won't publish the pressures, but it is rumored to be 65,000+ psia, which is too close for comfort to the proof pressures of 70,000 psia. One day we will find out the increase in cracked bolt in Army rifles. But don't worry, you the tax payer will pick up the tab.
Reloaders don't really know the pressures in their barrels, published data is a guide, but your actual pressures will be different. Something also to understand, is that humans don't understand exponential processes. The slope of the curve for smokeless powder is exponential. Pressures do rise much faster than people can comprehend. Humans think in linear terms.The mathemation Al Barr is in this video to make the point that "
The greatest shortcoming of the human race is our inability to understand the exponential function". This is an hour long video, I don't expect anyone to watch it through.
but, pressures are increasing a lot fast than charge weight. And you don't know the sensitivity. You don't know if an extra grain of powder doubles, triples, or spikes the pressure curve by a factor of ten., or a 100. You don't know. So it is best to stay conservative, if you are a person who has a good sense of risk.
You will encounter or see people who have a poor sense of risk. Indian Larry was one of those. Indian Larry built fantastic custom motorcycles, and for fun, Indian Larry used to stand on the seat of one of his motorcycles, hands out, while the motorcycle was moving. You can find all sorts of pictures. And, Larry did not wear a helmet. The culture is one of bad ass and you can understand, wearing a helmet is for sissies. One day, when demonstrating his trick, Larry fell off and hit his head on the pavement. Larry is not with us anymore. But Larry thought he had control, he thought what he was doing was safe, thought he did not need a helmet, and he was wrong. You will run into a lot of Larry's, they seem sane, but you will find that they have a high risk threshold.
We needed these types as humans spread out across the world. The example I am going to use is Easter Island. Easter Island is 1,300 miles from the closest land mass. It is easy to miss a tiny island from the ocean, a couple of clouds, a rainstorm, and you won't see it. And the Polynesians made it there in open canoes, no compasses, no GPS, no maps. You just have to wonder how many canoes went to the bottom, lead by charismatic, high risk leaders, with winning track records, who had faith they would find something new. And guess what, they ran out of food, water, and everyone on their canoe died. But one of them made it. In fact, those types made to all the islands in the Pacific, which is remarkable. But a lot of them went to the bottom.
You will find, that those who dance on the edge, will eventually trip over into the chasm.
