If you can only see the pattern in one place, it's possible it's not a genuine Damascus type barrel. A lot of the Belgian knock-offs of high-priced English shotguns just etched a pattern on the outside of the barrel to look like Damascus-twist steel, which would eventually wear off from repeated cleaning. If you look down the inside of the barrels, and there's no pattern evident there, that's what you have. Ironically, the cheaper solid steel barrels actually hold up better than the Damascus as shooters, because they lack the microscopic pores in the steel that let rust and corrosion get a foothold to weaken the metal.