I would expect the expansion to be in the cylinder bolt locking notches, or at least one. The question I have is, what the heck was your local gunsmith measuring?
If the factory has determined that any part of your cylinder has exceeded yield, and thusly, stretched and is permanently deformed, that cylinder is toast. Have you ever heard of fatigue life? Pressure vessels have fatigue lives, they expand, contract with every pressure cycle, and few things don't have finite fatigue life cycles. There is even something called SN curves with provide designers a means for predicting fatigue lifetime based on stress cycles. I have seen one fatigue cycle curve where the material under test had been deliberately stressed beyond yield. And it failed much sooner than the materials which had not stressed beyond yield.
Ever had a fire extinguisher refilled? I have, and found it very interesting. Fire Extinguishers are pressure vessels and I found an example of someone killed by a bursting fire extinguisher. And they just don't fill your fire extinguisher, it is tested before hand.
My fire extinguisher was placed inside this thick as heck steel chamber. The chamber and the fire extinguisher were both filled with water.
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The fire extinguisher was then pressurized to some standard, which expanded it. The amount of water displaced by the expanded fire extinguisher was measured. And then, the amount of contraction was measured once the pressure inside the extinguisher went to ambient. If the fire extinguisher expanded too much, they would refuse to fill it, they had standards. If the extinguisher did not contract enough, after the pressure was removed, they would refuse to fill it.
This is a pressure release disc, in case a fire extinguisher blows when under pressure
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and they had enough fire extinguishers blow, that the paint was removed from these things on the wall.
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Anyway, if S&W says the cylinder is toast, find a new cylinder. Don't be this guy
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or this guy
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or this guy
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or this guy
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These cylinders fail unpredictably, and no matter how much you think the pistol is worth, your eyes, face, hand, etc, are worth more. You can buy extra guns, you cannot replace an eyeball.
I wish I had the picture a CMP range officer showed me of a firing pin from a striker fired pistol in a shooter's eyeball. It went through the shooter's safety glasses. I have not been back since then, but the whole Organization was so spooked they were wanting shooters to wear multiple shooting glasses. They did not know how many layers of shooting glasses it took to stop a firing pin that blew out due to over pressure, but obviously one was not enough, so two sounded good. Maybe they got to three or four! They were just having a knee jerk reaction.
This guy was trying to explain why a Ruger 44 Magnum cylinder could blow within 650 over pressure loads.
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
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