FWIW, I have lived through the history of the cracked barrels on .357 mag K Frames. It is my considered opinion that the total number of those occurences are a relatively small percentage of the total number of K Frames manufactured and sold in this caliber. That doesn't help a bit if you have had the experience of a cracked barrel, because it has certainly happened in a notable number of them.
I was issued a Model 19 in 1971. I carried it for about 15 years or more. Since I was a firearms instructor in my agency's annual recruit school, and since I was responsible for the quarterly district firearms qualifications in my home district during all those years, I have been present when a great number of these Model 19's were fired a great number of rounds. I was also privy to the number of these revolvers that passed through the armorer's hands either because of trouble or just to be refreshed before being re-issued to another officer. I would have been very aware of any issues of cracked barrels.
Since our issue ammo in those early days was either Remington or more likely Winchester rounds bought by low bid, we were issued the old lubaloy coated 158 grain semi wadcutter product. All they were good for was seriously leading the barrel of your gun! Most officers would keep them and use them for trading stock to get the new fangled 125 grain (or other) semi-jacketed hollowpoints. If you were good friends with the LGS, you could sometimes trade two or three boxes of that issue ammo for one box of the hotter hollowpoints. Many of the above mentioned revolvers had quite a large number of these rounds fired through them, as well as untold rounds of 148 grain wadcutters.
I have never personally seen or examined one of these K Framed .357 mag revolvers with a cracked barrel. I have seen pictures of them, and I have talked to a very few who have actually seen or experienced this happening. As I said, I know it happens, and I have read several studies about this happening and it's cause. It can happen in a revolver that has never fired the hot lightweight bullets. All the guns I ever saw and owned showed evidence of flame cutting underneath the top strap at the end of the forcing cone end of the barrel. My copies today have that in evidence more or less. That phenomenon apparently will begin to appear and get worse, but only to a certain point, where it apparently ceases to get worse. I have never known this to cause any problem.
The concensus was, and I tend to agree with it, that the barrels that cracked (or at least some of them) cracked because of an over torqueing of the barrel when it was installed in the revolver in order to get it "clocked" correctly so that the front sight was standing straight up and centered with the rear sight. Different installers on different days with different batches of barrels over a period of several years could certainly be a part of this problem. Who knows for sure? I don't.
But I do know that, while this was a known problem, there are a great many more copies of this revolver that did not and have not experienced this crack in the forcing cone than there have been those that did. It's a **** shoot! But because it is a known occurrence, it behooves us to be aware of it and check any copy very carefully before we lay down our hard earned money to buy it. There can be many things wrong with a gun, especially if it has been in Bubba's care for a period of time. Some of these guns have been fired with ammo that has been overloaded in the quest some have to get the last fps of velocity out of them. That causes or contributes to all kinds of problems. But of all those, perhaps the most obvious that we can easily detect is the cracked forcing cone. We should always remember to check this area on any copy we find for sale. Same thing applies to the aluminum framed .38 Specials like the Model 12. There we should be looking for a cracked frame around the forcing cone end of the barrel. But we should look for these things in our inspection of any revolver we a thinking of buying. It's just common sense!
I suspect (without having seen this mark that the OP mentions) that it is not likely a crack, nor even a crack beginning. I think the crack is either there or it is not, based on my experience over time. I also agree with OldChief's comments above. I sure hope I'm right. I understand the concern. But I'd shoot normal ammo through the gun (not the really hot ones) and keep a close eye on it. I don't bet on nuthin', but if I did, I'd bet that that mark does not get any worse. But that's just me, as I said, FWIW. You gotta decide for yourself. Good luck, Sir!
