Chuck;
Not really a lot of info, have just seen a few photos & descriptions over the years of bbls with seams. I seem to recall someone posting one here on the board some time back. Burrard showed pics of several in his book & a few from other random sources. Fortunately between machining & proofing the majority never make it out the door, but a few have. These generally do not show up until after some years of use.
ItalianSxS;
The mat'l factor of most importance to a bbl maker is Tensile Strength. This is normally stated as a two part factor, namely Yield & Ultimate. Any time force is applied the mat'l will be stretched to some extent. The Yield point is where the elastic limit is exceded & upon release of the force the mat'l does not return to it's original configuration. Ultimate is of course the point at which it seperates/breaks/bursts etc. The higher the yield point in relation to the ultimate the less likely one is to have a bulge. The humourus part is that "Old Timers" eulogized their Damascus bbls saying they were more apt to "Bulge" while steel ones would burst. In either case a bulge simply shows too high a prerssure applied for the steel to properly resist. If this bulge occurs as a "Ring" bulge it is almost a given that a localized pressure spike was the cause, which was not the fault of the gun itself. "If" the pressure of the shell exceded the yield point of the steel, but not the ultimate then the entire bbl would have swollen. The pressure is of course in the form of a curve reaching it's max within the chamber itself & falling off quite rapidly to a more uniform pressure for the remainder of the bbl, gradually falling as it goes toward the muzzle. An "Overloaded" shell will thus burst the weakest point in the chamber.
Now consider this, say you load 1 1/8oz shot (12ga) ahead of a sufficient charge of BP to give it 1200 fps MV & it does so at a max pressure of 6K psi. We then load that same 1 1/8oz ahead of a smokeless charge & reach the same 1200 fps MV but at 10K psi. Now first we could say the smokeless load "Stressed the bbls more than the black", But Note, this was only in the chamber. As both loads pushed the same wt of shot with the same force the same amount of work was done. With a possibility of only an extremely small margin of error we could say the "Average Bbl Pressure" for both loads were the same. Since the chamber pressure for the smokeless was considerbly higher, we can thus conclude the presure spread out over the rest of the bbl was higher in the BP load. For a number of years after the introduction of smokeless powder the proof house used a supplementary proof for the "nitro" proof. Unfortunately as I have read on such things over a long period of time I do not now remember the source, but this was explained by the fact, that at that point in time the proofers did not have suitable powders to proof the entire bbl, without exceeding a desirable chamber pressure. The major portion of the bbl was proofed by the "Black Powder" proof with the chamber area being done by the "Nitro Proof". 99.98% of bursts forward of the chamber will be found to "Not" be caused by an overload or a defect of the gun itself. Those other few will be a major flaw in the bbl metal & can & "DO" occur in all types of guns, both old & new.
Not to be forgotten is the possibility of the shot charge itself becoming the obstruction from an improperly ignited load. This has been discussed here several times so will not go further into it unless someone has missed it & desires to. It is something to keep in mind though when loading to extremely low pressures with slow burning powders in particular.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra