Most likely caused by an obstruction. I dont buy in to the rust in the seam theory. When they hammer welded metal togeather it fuses it useing heat and preasure. If done properly there are no pockets in the metal to trap air. You need air for it to rust. Just my opinion. Old rule of thum is if the barrel blows in the first half of the barrel (Breech to center) it is because of an over load. If it blowes in the second half of the barrel (center to muzzle) it is because of an obstruction.
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