Most modern sxs and o/u makers use the monobloc barrel making process described above. The barrels are not welded onto the chamber but inserted into the breech, soldered in and then the chamber is cut. The process is also known as sleeving.Most side by sides are built different than other guns. The chamber area is actually a seperate piece of steel that is welded to the barrels. When 2 3/4" guns are the chamber is very near the barrel welds. When extending the length of a chamber, the lengthened chamber can extend past the reciever block and actually extend into the welded on barrels. Not a safe practice. This advise came from a top gunsmith, and former president of the custom gunmakers guild, so I assume it to be correct.