American makers proof test their guns in house as standard practice. Second hand U/O, pump and semi automatic actions and barrels are very strong because they don't have to be reduced in diameter to accommodate another barrel horizontally. The steels in modern shotguns is better and usually chrome lined to prevent rust. Quality double barrelled side by side guns must be cut down to a minimum for weight and handling properties. When they are neglected, they rust inside the bore and the rules of proof protect the buyer. I saw an American double barrelled shotgun with very thick barrels which would probably last many bore enlargements, but they were bad handlers, too heavy and would not sell well. You were better off with an under and over. Chrome lining prevents the barrel from "shooting in" and may not give good patterns, Screw in chokes are not good for patterns either. In fact, ball trap shooters might be better with none-chrome lined barrels without screw in chokes to give even patterns at long range,