I believe that most of the bbls with a longitudinally welded seams were made of wrought iron, not steel. This practise remained in use longer for the heavier walled rifle bbls than for shotgun bbls. These bbls of wrought iron required a wall of enough thickness that building a practical double gun for "Shooting Flying was a virtual impossibility until better bbl construction came along, in the form of the "Twist" bbl.
Those steel bbls which have failed with a long crack are generraly presumbed to be from a flaw in the steel. This is something which doesn't occur often but has happened. The problem comes from a air bubble or slag in the ingot, which upon being rolled into a bar is stretched out into a long narrow seam. This seam then has to be in the bar in such location that it falls entirely within the wall of the bbls, thus undetected in the making. This explains why it occurs so seldom, most steel bars with seams were cut into in the process of making or were in the part bored out. Only those which fell entirely within the walls, & also managed to pass proof, remained to present a problem later on.
